Author Topic: EEVblog #645 - TRS-80 Model I Retro Computer Teardown  (Read 22286 times)

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EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« on: July 29, 2014, 02:39:40 am »
Dave looks inside the most popular microcomputer of the 1970's, the Radio Shack / Tandy TRS-80 Model I
And also a look at the TRS-80 Model 102.
TMS4116 16Kb DRAM:
http://www.datasheetarchive.com/dlmain/Datasheets-37/DSA-731104.pdf

Level II BASIC Reference Manual
http://www.1000bit.it/support/manuali/trs/Level%20II%20BASIC%20Reference%20Manual%20%281979%29%28Radio%20Shack%29.pdf
Service manuals and Schematics: http://www.eevblog.com/files/TRS80model1/

« Last Edit: July 29, 2014, 12:10:04 pm by EEVblog »
 

Offline j3gum

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #1 on: July 29, 2014, 04:05:15 am »
Dave,
Thanks for the TRS-80 tear down!  I really loved watching it.

You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.  Later there was additional software you could add that would do software debounce.  However, I followed advice to use WD-40 on a sheet of paper and insert it between the springs in the key and depress and pull the paper through.  This deposited enough goo on the springs to tame the bounce.  And it did work, believe it or not.

Also, I would love to see the RF output if you put some work on that Z80.  I bet it'd be significantly more noisy.  A simple loop should be enough:
10 goto 10

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Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #2 on: July 29, 2014, 04:10:34 am »
That takes me back!

I started working for Tandy when the Model I was introduced in Australia in 1978. They were looking for someone with an electronics background who could program in BASIC. I'd never heard of BASIC or had any computer programming background so I grabbed a couple of books on BASIC and read them several times before the interview. Luckily the interviewer knew less BASIC than me so I got the job.

I spent the next 7 years working on every subsequent model they introduced. Even got sent to head office for training - 2 weeks in Fort Worth, Texas in winter is quite memorable for all the wrong reasons  ;)
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #3 on: July 29, 2014, 04:11:18 am »
You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.

Yes, I forgot to mention that, it was a major issues.
I didn't get an key bounce at all on this, but the owner thinks it's had the debounce mod done. Doesn't appear to be any hardware mod, so maybe in the ROM?


Quote
Also, I would love to see the RF output if you put some work on that Z80.  I bet it'd be significantly more noisy.  A simple loop should be enough:

I showed that, with and without a program running
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #4 on: July 29, 2014, 04:15:14 am »
I started working for Tandy when the Model I was introduced in Australia in 1978.

I did work experience in high school at the Tandy factory working on the Models III' and others. It was a dream, the Tandy factory was walking/bike distance from home.
Also, I used to drop by the factory showroom all the time, as they had a corner of the store where they would have a huge junk bin of stuff, products they were evaluating and didn't sell etc.
They were called Intertan in Australia.
 

Offline Lightages

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #5 on: July 29, 2014, 04:30:14 am »
I remember a friend of mine had a Model 1 and he wanted to to upgrade to 48K but not pay the silly prices Radio Shack wanted. So he decided to buy sets of 16K DRAM chips. He then piggybacked them and lifted the output enable pin and ran that separately to some logic. He inspected his work, deemed it perfect and powered it on. Nothing.

After asking me to look I discovered he had counted wrong and had lifted a different pin. All the chips cam on at the same time and fried each other. They cost, at that time, around $700 total. He looked at me, made me promise to not tell his wife and he went back next week and blew another $700 on another set. That time he did not make the same mistake.
 

Offline j3gum

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #6 on: July 29, 2014, 04:50:31 am »
Quote
I showed that, with and without a program running

Awesome!  That'll give me an excuse to re-watch it!  Thanks again.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #7 on: July 29, 2014, 04:52:23 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.  Didn't want to burn too many bridges with TEK?
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #8 on: July 29, 2014, 05:02:15 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.

Huh? What does that mean? :-//
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #9 on: July 29, 2014, 05:04:33 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.

Huh? What does that mean? :-//

When you were talking about how slow the controls were the audio jumped so it sounded like you started saying something along the lines of it being slow during processing, but then changed it to say slow during heavy processing.
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #10 on: July 29, 2014, 05:07:42 am »
When you were talking about how slow the controls were the audio jumped so it sounded like you started saying something along the lines of it being slow during processing, but then changed it to say slow during heavy processing.

I don't know, just one of my usual edits. Maybe I misspoke and did another take, that happens. I don't recall exactly. Nothing implied or hidden.
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #11 on: July 29, 2014, 05:14:37 am »
Didn't want to burn too many bridges with TEK?

If I was avoiding "burning bridges" with Tek, I:
a) Wouldn't have mentioned it at all
b) Wouldn't have got into a public twitter debate about it with Tek deliberately copied in
c) Wouldn't have ranted about it again on today's Amp Hour

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Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #12 on: July 29, 2014, 05:19:52 am »
I did work experience in high school at the Tandy factory working on the Models III' and others. It was a dream, the Tandy factory was walking/bike distance from home.
Also, I used to drop by the factory showroom all the time, as they had a corner of the store where they would have a huge junk bin of stuff, products they were evaluating and didn't sell etc.
They were called Intertan in Australia.

At Mt Druitt? The Intertan name started in 1986. prior to that they were just part of Tandy Corp.
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #13 on: July 29, 2014, 05:28:42 am »
At Mt Druitt? The Intertan name started in 1986. prior to that they were just part of Tandy Corp.

Yep. I think it was 1987 I was there, in the service department.
Might of even still had Tandy on the building, I can't recall.
Tandy was always the public brand, but the company behind it was Intertan.
Oz Tandy products had "Manufactured in XXXX for Intertan Australia Ltd" on the back sticker.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #14 on: July 29, 2014, 06:02:32 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?
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Offline free_electron

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #15 on: July 29, 2014, 06:04:28 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?
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Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #16 on: July 29, 2014, 06:22:49 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?

Black and white video,  no colour information so no PAL (or NTSC).
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #17 on: July 29, 2014, 06:30:01 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #18 on: July 29, 2014, 07:13:09 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?

Black and white video,  no colour information so no PAL (or NTSC).

I didn't think about that.  So for just black and white video there is no difference between US and Europe besides the 50/60 thing?
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Offline free_electron

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #19 on: July 29, 2014, 07:18:20 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.

i thought you were letting the liquid crystal flow back to the top of the screen...
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #20 on: July 29, 2014, 07:26:25 am »
i thought you were letting the liquid crystal flow back to the top of the screen...

That would be silly!  :-DD
 

Offline Richard Crowley

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #21 on: July 29, 2014, 07:28:16 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.
Are globes down there in the antipodes made with the South Pole at the top?  For a better view of the planet south of the equator?
 

Offline German_EE

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #22 on: July 29, 2014, 07:31:52 am »
Dave, a big THANK YOU for this video. I had a TRS-80 in 1978 and I typed my thesis on it so I have happy memories of that wonderful keyboard. Some extra points:

1) To save costs the keyboard was memory mapped and the ROMs scanned the keyboard area on a regular basis looking for key presses.

2) There was a PAL version of the Model 1 but I can't remember much of the details apart from Tandy in the UK selling a UHF modulator so that we could use a 625 line TV.

3) At some time Tandy introduced a lower case mod because, as I said, I typed my thesis on it using the Scripsit word processor package. After it was finished I took the cassette tape down to the local Tandy showroom where they printed it on a daisywheel printer for me.

4) That warranty sticker was a serious issue because if it was broken the local Tandy repair center doubled your bill. I don't think they liked people to tinker.
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Offline ggchab

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #23 on: July 29, 2014, 07:37:29 am »
The characters generator could produce upper and lower case. But the video memory had only 7 bit, the missing one (bit 7, I think) was "rebuilt" with a logical gate. It was possible to obtain the lower cases by simply adding a 1 Kbit chip, cutting some traces and adding 2 wires  ;D
 

Offline ggchab

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #24 on: July 29, 2014, 07:49:35 am »
The "Memory size" question when booting up is to define a part of the upper memory that will not be used by the basic. This memory could then be used to store a kind of "Terminate and Stay Resident" (TSR) program (a bit like MS-DOS !!). One example is a small debouncing program for the keyboard.
 

Offline moemoe

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #25 on: July 29, 2014, 08:13:46 am »
Hey, the oscilloscope on the upper right is upside down, turn it around before all the calibration data falls out ;)
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Offline ludzinc

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #26 on: July 29, 2014, 09:27:05 am »
The comment about the 1/2 watt DRAM reminded me of when I repaired a Space Invaders (IPM clone actually).

Board on bench kept driving my 5V 3A PSU into current limit.  Took me a while until I realised that there wasn't any fault with the board - the bench supply just wasn't beefy enough.

IIRC the 5V rail on the space invaders was 30W....  |O
 

Offline VK3DRB

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #27 on: July 29, 2014, 09:36:07 am »
Dick Smith sold a TRS-80 ripoff called the System-80 (Video Genie sold elsewhere). I bought one in 1979 - the biggest load of crap ever. I was ripped off a whole  month's salary when I bought it but I wish that I had saved a little longer and bought the far superior TRS-80 from a reputable dealer.

- The tape drive was very unreliable. Errors, errors and more errors reading.
- Keyboard bounce was far worse than the TRS-80. The machine was unusable.
- Dreadful interference to TV. You could not watch the ABC when the System-80 was turned on.

It was a toy - the only thing of use was I could write trigonometric table on it. Although SARGON was one impressive program written by two husband and wife geniuses - Dan and Kathy Spracklen. I was a member of the TRS-80 Users Group in Melbourne. It was a subgroup formed from the world famous MICOM (Melbourne Microcomputer Club) - a terrific bunch of blokes who like me were pioneers with home computers.

 

Offline made2hack

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #28 on: July 29, 2014, 10:20:37 am »
Great video Dave, Good on You!!!  :clap:

Offline RupertGo

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #29 on: July 29, 2014, 01:00:52 pm »
In those glorious pre-FCC certification days, all that digital RF leakage could be a feature, rather than a bug. If you put a MW/AM radio next to the computer (not necessarily that close!) and wrote some tight timing loops with memory access in assembler, you could play tunes. Which was something, as there was no other way to get sound out of some of those early micros. (Sometimes you could flip bits on the cassette out port... you could still pick that up on the AM radio, though.)

Advanced users could also use the AM pickup technique to debug software, as with a bit of experience you could hear the distinct tones of subroutines or even put in little blips as checkpoints. There was even an early UK computer - 50s or 60s, I think an Elliot Automation model - that had an audio amp and a probe built into the chassis so you could hear what was going on at various bus points. With low clock speeds, there was a lot you could pick up.

BTW - I've always heard that it was a TRS 80 Model 100, the NEC 4-line LCD AA-powered portable, that had the last code Bill Gates ever wrote in it. It was certainly his pet project, and the 8080 Microsoft BASIC for the Altair was the first thing he and Paul Allen wrote when they started Microsoft. A legendary piece of coding.

But a lovely teardown. I really like those horrible brown ceramic decoupling caps, which remind me of the Hong Kong 5 transistor superhet radios I cut my teeth on as a schoolkid in the 70s. Tiny circuit diagrams pasted in the back of the case, earphone sockets and crappy PP3 battery connectors that failed in microseconds, and that 'fill a shotgun with components and fire it at the PCB' manufacturing technique. Happy days.
 

Offline CC58

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #30 on: July 29, 2014, 01:46:28 pm »
You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.


Yes, I forgot to mention that, it was a major issues.
I didn't get an key bounce at all on this, but the owner thinks it's had the debounce mod done. Doesn't appear to be any hardware mod, so maybe in the ROM?

The original keyboard had key bounce problems and you could access the contacts to burnish them to fix it.  Later the ALPS keyboard was introduced that had sealed switches and zero key bounce issues.

I worked as an electronic technician repairing TRS80's in Radio Shack's national repair depot in Fort Worth,TX.  Jerry Ballard was engineer I worked for making test fixtures.  Made a few trips to the ivory towers and saw the new things before they were released such as the Model III and color computer. 
« Last Edit: July 29, 2014, 01:58:36 pm by CC58 »
 

Offline nixfu

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #31 on: July 29, 2014, 01:59:27 pm »
As a ham I had considered saving up for one of those MDO3000s or MDO4000s because a spectrum analyiser would have been nice to have on board a nice modern color scope.

But, holy cow, the performance on that sucks....I think I will stick with an old analog phosphor screen spectrum analyzer that can do real-time updates on the screen.

Sheesh.  You think they would have at least made something as good as what was available 20 years ago.
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #32 on: July 29, 2014, 02:12:03 pm »
As a ham I had considered saving up for one of those MDO3000s or MDO4000s because a spectrum analyiser would have been nice to have on board a nice modern color scope.
But, holy cow, the performance on that sucks....I think I will stick with an old analog phosphor screen spectrum analyzer that can do real-time updates on the screen.

To be fair, it can actually perform faster than a traditional spectrum analyser sweep on the same settings, and it captures the entire spectrum in one hit, something a traditional analyser can't do. It's the almost locking up on the front panel, missed key presses etc that make it incredibly annoying.
 

Offline jcochran

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #33 on: July 29, 2014, 04:48:10 pm »
Brings back old memories. The TRS-80 was the first computer I spent a lot of time programming. Also learned assembly language programming on that little beast.
One project I did was to write my own floating point package in Z-80 assembly. I then tested it on a TRS-80 in parallel with the build in floating point they had for Level II Basic to make sure my routines gave the same answer as theirs. In the process, I found a bug in their floating point. If you still have access to that TRS-80, try the following little program.

10 A=1E19
20 B=1E38
30 PRINT A,B
40 C=A*A
50 PRINT C

What you'll find out is that you're gonna get an overflow error on line 40. The print on line 30 is to prove to you that 1E38 is within the representable range of numbers (the actual limit is about 1.7E38). So the multiplication of 1E19 time 1E19 ought to be perfectly fine, but it isn't.
 

Offline Dave Turner

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #34 on: July 29, 2014, 05:34:49 pm »
Yup, sure brings back memories. The best thing was the manuals contained all the circuit diagrams.
 

Offline Rory

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #35 on: July 29, 2014, 05:42:04 pm »
In the early 80's, a friend wanted to use a TRS-80 in his hamshack, it was (obviously) unusable for all the hash noise so he brought it to me with a pile of ferrites and an aluminum enclosure and we boxed it up with "RF tight" DIN chassis connectors, bypasses, ferrites on the ribbons, etc. And it still spewed RF garbage from the keyboard, power supply and the video monitor cables.  Even toroids on the cables didn't work.  A small e-field sniffer probe found the sources, we even tried mounting the keyboard inside the cabinet with holes punched for the key posts, and it still couldn't be tamed.  It was a total exercise in futility, but a fun project anyway.
 

Online PA0PBZ

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #36 on: July 29, 2014, 07:07:33 pm »
Ah, memories...  Tandy started to roll out a few computer centers in the late 70's/early 80's here in Holland and decided it would be nice to have a technical service place in the center in Amsterdam. So they put out an ad looking for someone to start this, and to make a long story short I was hired  :) Not that I knew a lot about computers back then but who did? I remember that I saw my first floppy disc over there... I fixed a lot, learned a lot and finally moved on after maybe 5 or 6 years.
The Model I could only hold 16K, the other 32K was inside the expansion box. The lower case mod was just another cg piggy-backed on the existing one with the chip select connected somewhere. When one of the RAM chips went bad it got really hot, it was easy to spot with your finger but having the text on the chip burned on your finger tip was not so funny. The biggest problem was when one of the buffer chips went bad so I made a board that connected to the expansion interface and to a second Model I. It then single stepped the broken one till the signals on the bus where different from a working model and halted the system. That way you could easily measure the bus and find the defective chip.
Good times...
Keyboard error: Press F1 to continue.
 

Offline nsayer

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #37 on: July 29, 2014, 10:25:17 pm »
I remember these. We used to call them TRaSh-80s. :) Dave mentioned that moniker as well.

We had a lab at our high school with 20 of these. They had shared 5 MB (yes, MB) Corvus hard disk system. Each machine had a 40+ pin ribbon cable running back to a set of multiplexers daisy chained off the drive itself (a box the size of a toaster oven).

The problem, though, was that the machine wouldn't boot over that expansion bus. You needed to boot from a special floppy with a modified TRS-DOS (or in our case NEWDOS). But to save costs, these machines didn't have floppies.

Instead, there was a cassette based boot loader that could be used to bootstrap. In addition to the ribbon cable mess each had a DIN5 cable for the cassette port running back to a multiplexer hooked up to the ONE floppy machine in the room. It ran a shell script that when you pressed enter would transmit the cassette program to anyone listening. If you had to reset your "workstation" (ha ha), you'd press enter twice (for CASS and Memory Size) and then type SYSTEM and LOGON. You'd then shout "Boot me!" and someone near the master would press ENTER. 30 seconds or so later, you'd get another "*?" from the SYSTEM command and you'd run the load with "/" and log in.

Memories.
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Offline Dinsdale

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #38 on: July 29, 2014, 11:15:02 pm »
My first job in electronics.
FIrst I did burn-in and boxing, load and start tests in the evening, box the working ones the next morning.Then I learned trouble shooting and worked on fixing boards off of the production line. They bought a GenRad  circuit tester and I ran that (boring after a while).  Finally I moved to Tandy Systems Design.  Learned a whole bunch about signal integrity when working on the expansion interface: pretty tough to run RAS and CAS across a foot of ribbon cable!
Thanks, Dave.  Brings back memories.  Radio Shack/Tandy was great for on-the-job training.
« Last Edit: July 29, 2014, 11:23:57 pm by Dinsdale »
This can't be happening.
 

Online GeorgeOfTheJungle

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #39 on: July 29, 2014, 11:37:36 pm »
I had the TRS80 and an Apple ][. Compared to the ][, despite its 0.7 MHz/1.7x clock speed advantage, it was such a pitiful snail, I swear it. Running a program made glitches appear on the screen. The Apple video image was in comparison amazingly sharp and stable and absolutely glitch-free. The TRS80 had 64 characters/line and upper and lower case which was much much better than the silly and only upper case 40 chars/line of the Apple ][ (had to buy a Videx 80 colums card to remedy that). The 2 kilograms "power brick" was rather ugly and ran hot like crazy. It was ludicrous compared to the switching power supply of the Apple ][ that also ran much cooler and made a lovely whinning noise when overloaded...  because it had 8 expansion slots built in. In the trash80 instead you had to buy that cheapo-looks external "expansion interface", and have that flat cable hanging forever between your "keyboard" and the silly thing. Even to get past 16k you had to buy that, while in the ][ you only had to swap some jumper wires and plug in more 4116s. Preferably the beautiful ceramic µpD416Ds made by NEC :-). In short, nearly everything in the trash 80 was trashy, and nearly everything in the Apple II was amazing. I'm not an Apple fanboy but the only thing that was better, apart from the price (a big difference!), was its BASIC, much better than the integer (!) basic that came with the IIs. And the DOS too, much much better than Apple's "toy" DOS.
Even when the experts all agree, they may well be mistaken.
 

Offline jcochran

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #40 on: July 29, 2014, 11:54:11 pm »
I had the TRS80 and an Apple ][. Compared to the ][, despite its 0.7 MHz/1.7x clock speed advantage, it was such a pitiful snail, I swear it.
Yep. The TRS-80 vs Apple ][ was a very nice example of "clock speed doesn't mean everything".
The Z80 instructions were generally 4 clock cycles per byte of memory touched give or take a cycle for instructions taking a large number of bytes.
The 6502 was 1 clock per byte of memory touched plus 1 clock.
So minimum cycle time for a Z80 opcode was 4 clock cycles. Minimum time for a 6502 opcode was 2 clocks. And generally a 6502 at a given frequency was about equivalent to a Z80 running at twice the frequency. So a baseline TRS-80 was about 85% as fast as a baseline Apple.
But if you programmed at the assembly language level, the Z80 was a LOT nicer to use than a 6502. But on the other hand, the Apple ][ came with a built in assembly language monitor that made debugging easier (although that 256 byte limit on the stack for a 6502 was a real PITA).
 

Offline synapsis

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #41 on: July 29, 2014, 11:54:18 pm »
The upside down oscilloscope sets off my OCD something fierce. ;)

I like how the casing for the display has the cutouts in the front (seen from inside) for VHF/UHF knobs.

The Model 3 is the first computer I ever touched. I still remember the keyboard feel.
 

Online GeorgeOfTheJungle

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #42 on: July 30, 2014, 12:40:07 am »

But if you programmed at the assembly language level, the Z80 was a LOT nicer to use than a 6502. But on the other hand, the Apple ][ came with a built in assembly language monitor that made debugging easier (although that 256 byte limit on the stack for a 6502 was a real PITA).


Oh yes, I remember that, mine in fact did not boot to BASIC but straight into the monitor *. To get the BASIC prompt you had to type ctrl-B. To return to the monitor CALL -151. But apart from the cycles per instruction differences that you mention, I think the huge difference I saw in running speed was mainly due to the floating point BASIC that came built into the TRS80. The Apple was so much snappier probably due to its (Steve Woniak's) integer-only BASIC. It didn't last long though, later II Plus models ditched the integer BASIC entirely in favor of microsoft's FP BASIC.

Even when the experts all agree, they may well be mistaken.
 

Offline miguelvp

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #43 on: July 30, 2014, 12:50:33 am »

But if you programmed at the assembly language level, the Z80 was a LOT nicer to use than a 6502. But on the other hand, the Apple ][ came with a built in assembly language monitor that made debugging easier (although that 256 byte limit on the stack for a 6502 was a real PITA).


And don't forget the registers, the Z80 wins hands down compared to the 2 lonely registers + accumulator on the 6502.
 

Offline Dave Turner

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #44 on: July 30, 2014, 12:52:19 am »
It was even more complicated than clock speed and clock cycles. If I recall correctly the 6502 was a 2 phase clock whereas the Z80 was single phase and static thus more easily implemented. In general the Z80's plethora of registers (in comparison) made programming at the assembly language level potentially more efficient, particularly because affordable external memory was relatively slow. Nevertheless the 6502's instruction set was more suited to some types of memory addressing; it all depended on what one was trying to achieve.


 

Offline Tek_TDS220

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #45 on: July 30, 2014, 02:29:51 am »
Yes, it's crusty, but it still runs because it was well-built.  I just repaired a $1000 Samsung TV (4 years old) because they put Chinese caps 5 mm from a hot switching transistor on a board with a lot of room.  I'm suspicious that it was purposely built to fail.
 

Offline BobC

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #46 on: July 30, 2014, 03:05:42 am »
I was an impoverished sailor in the US Navy when the TRS-80 came out.  My local RS would let me use their floor unit for limitless hours, so long as I explained it to customers.  I really should have been on commission.

But when I finally had the money, I got an Apple ][+ with the Language Card so I could use UCSD Pascal.  I wound up attending UCSD in the mid-80's because of it!
 

Offline aadderoos

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #47 on: July 30, 2014, 07:36:54 am »
You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.

Yes, I forgot to mention that, it was a major issues.
I didn't get an key bounce at all on this, but the owner thinks it's had the debounce mod done. Doesn't appear to be any hardware mod, so maybe in the ROM?


If I remember correctly, the earlier models you had to load a little programm to debounce the keys. Later on there was a rom revision that had that build in. You could tell wich version you had by looking at the free memory. The original reported 15572 bytes free, the one with the debounce routine reported 15570 bytes free. (I hope I remember these numbers correctly)

I think the TRS 80 was the most popular one in the Netherlands of the three major ones those days (TRS 80, Commodore PET and Apple 2). Everyone thinks Apple was number one but that was just good marketing from Apple. And the fact Apple is the only one arouind these day helps too.

I myself have build a clone of the TRS 80, a LNW 80. You had to solder the circuitboard yourself, it was also available ready build but at a much greater cost. I think the total cost was around 2500 dutch guilders, which is 1200 Euro's. It had some extra grafics, 480x192 monochrome and 384x192 color but these were unsupported by the software. It had a nice (heavy) metal case, that also had room for the expansion board, also made by LNW (compatible with the Radio Shack one, with improvements.)

This LNW was the thirth computer I owned, before I had a sinclair MK14 (very rare today) and a Nascom 1 (maybe even more rare now), both were kits.
Here is some info on the LNW: http://fjkraan.home.xs4all.nl/comp/lnw/index.html
 

Offline ggchab

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #48 on: July 30, 2014, 08:33:54 am »
I bought my TRS-80 in 1980, in a Tandy shop. It was a wonderful machine and I spent thousands of marvelous hours working with it. Some years later, I bought the expansion unit and a first floppy disk drive. It was only 90K and a big part of the space was used by TRS-DOS that had to be on the floppy disk. But at that time, the remaining space was still big enough to store programs and other files !! And floppy disks were a lot more reliable and faster than cassettes. Cassettes were sometimes a real nightmare and it was easy to loose hours of work  >:(

When I watched Dave's great video, many souvenirs came back to my mind :D
 

Offline tocsa120ls

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #49 on: July 30, 2014, 09:31:16 am »
Yes, it's crusty, but it still runs because it was well-built.   they put Chinese caps 5 mm from a hot switching transistor

I think this is analogous to the "Why is the milk in the back of the store?" debate. The caps have to be close to the FET switch. Imagine the RF you would get when you´d have to route that high power trace around the PCB to the cool side. That´s what 105C caps are for.
And the milk is in the back because old fridges had huge compressors that were located outside of the store. Plus that´s close to the delivery entrance, so the cooling chain is not broken.

And to all of the ppl complaining about the upside down scope, it´s in Australia, so wouldn´t be that the only one right-side up? :D
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Offline amyk

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #50 on: July 30, 2014, 09:48:14 am »
Hey, the oscilloscope on the upper right is upside down, turn it around before all the calibration data falls out ;)
Sorry, the near-symmetry of that screenshot tempted me...
 

Offline VK3DRB

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #51 on: July 30, 2014, 01:37:04 pm »
A few other points...

Loved the tear down of an old friend, the TRS-80.

Tarnished pins is quite common on old chips - a common sight on old digital clock chips. It has to do with the plating on the pins... maybe some had silver content.

The shimmering of the image was NOT due to beating with the video camera. It was a common feature of the TRS-80 and the System-80 due to 50Hz interference superimposing on the video sync signals, causing a slight time variance in the horizontal and vertical sync signals. The varying intensity is however caused by the beating with the camera frame rate. For the System-80, good shielding of the video signal helped a great deal. The monitor on the System-80 could be any TV that was modified (I modified a GAC B/W 12 inch portable - perfect TV for the job).

It is pronounced "ALPS" as in the mountain range, not A.L.P.S. Alps still make great key switches. They also made 3.5 inch diskette drives for IBM PCs. A Japanese company, they go all the way back to 1948.

Billy Gates wrote that BASIC interpreter I believe... one of the last pieces of hands-on hacking work he ever did. People forget Gates made a lot of money out of that Microsoft BASIC, well before the IBM made the PC.

I am sure the TRS-80 booted up with "TRS80>" as the prompt... must be a different ROM. Dick Smith's System-80 came up with "Ready>". The System 80 had 16K of RAM and if I recall upper and lower case characters up to 80 columns across. In some ways better than the TRASH 80, but the build quality was atrocious - absolutely terrible in comparison to the TRS-80. If someone has a System-80 they should give it to Dave to do comparative EMI measurements. The spectrum analyser would not only slow, it would have heart failure :).

Thanks Dave once again for the teardown and the excellent quality videos! Its like being there.

PS: One of Dave's CROs in the background was on the shelf upside down and another was right side up. Not sure what the go there is.
 

Online GeorgeOfTheJungle

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #52 on: July 30, 2014, 01:52:38 pm »

PS: One of Dave's CROs in the background was on the shelf upside down and another was right side up. Not sure what the go there is.


Yes Dave, seen from Spain everything but the scope on the right is upside down!
Even when the experts all agree, they may well be mistaken.
 

Offline RupertGo

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #53 on: July 30, 2014, 01:55:59 pm »
I'm sure the upside-down CRO is just there so we in the Northern Hemisphere can read the screen properly.

There was a big 6502-v-Z80 division at my school, which boiled down to Atom/BBC/Apple/Commodore owners versus Spectrum/TRS/CPM/the rest. It was a posh school and I was a pauper (story of my life) so there was a lot of pressure to write smarter code to 'prove' your point. "The Z80's got more registers!" "Yeah, but the 6502's got zero page!" "You can't write re-entrant code with zero page!" "Your mother writes in COBOL" (etc).

The very best thing, though, as exemplified by the TRS-80 teardown, was that you could actually understand everything in your computer down to gate level. Even the processors were manageable - the 6502 has 3500 transistors, the Z80 8500 - if you dug down. These days, I think the biggest processor has somewhere over 4 billion transistors, and I can't count that high...
 

Offline jlmoon

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #54 on: July 30, 2014, 03:31:12 pm »
I think it would be fun (just for grins!) to build a TRS-80 clone with FPGA's and some interface goodies.  One could even use the original Level II rom set to get that "Crusty" vintage effect..  ;D
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Offline jcochran

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #55 on: July 30, 2014, 05:55:38 pm »
I think it would be fun (just for grins!) to build a TRS-80 clone with FPGA's and some interface goodies.  One could even use the original Level II rom set to get that "Crusty" vintage effect..  ;D
If you want to do that, you could start at worse places than http://opencores.org/project,t80,overview or http://zxgate.sourceforge.net/
The zxgate project has already been used to create a clone of the TRS80.
 

Online Macbeth

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #56 on: July 30, 2014, 07:14:38 pm »
My first ever computer was not far off the Trash-80 (or Commodore PET) , I happened to like the Japanese MZ range back in the early 80's. The MZ-80K is the original with it's built in blue/white screen & cassette deck. I got the later and much nicer MZ-80A with it's proper keyboard and 2MHz Z80 and 48K RAM and, wow, a 2K screenbuffer that let me scroll through 50 lines of code! (I actually would have preferred an 80 column option than the 40x25 display though - aftermarket options coul enable it).

I learned how to code pure hexadecimal Z80 machine code from the ref manuals and POKE'ing them from BASIC DATA statements. I have to say, the Z80 was a dream to code compared to the 6502.

An interesting much larger scale and working reproduction of the Sharp Z80 on glass is here www.sharpmz.org/z80glass.htm

Oh yes, I still have it and indeed my first BBC Micro in the loft. They both still work, but a lot of the ceramic caps on the MZ-80A are coated in a white powder and the tape drive needs some attention. The Beeb, it was dead, but I recently fixed it by properly soldering some socket mounted IC's and the major work of fixing the dead keys (desoldering each one, dismantling, cleaning out with isopropyl alcohol, re-soldering!)
« Last Edit: July 30, 2014, 07:19:30 pm by Macbeth »
 

Offline nsayer

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #57 on: July 30, 2014, 07:14:52 pm »
Slightly off-topic perhaps, but when I get nostalgic I always say I'm going to create a new CP/M machine. That is, a modern TQFP z-80 with a banked memory system - 64K of user RAM and 64K combined boot and BIOS ROM and display memory. For storage, it would have SD card slot(s).

I think about that... and then realize that a Raspberry Pi is $35 and that it could just run a Z-80 emulator and the next thought is, "Meh. Fuck it."
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Offline firehopper

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #58 on: July 30, 2014, 07:42:24 pm »
I have a model 100 lyin around.. only 24K ram. room for another module though to max it out at 32k
 

Offline Dave Turner

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #59 on: July 30, 2014, 11:01:06 pm »
I recall having a lot of fun writing a 'crude' cross assembler on the PET for my Z80 based homebrew,
 

Offline jlmoon

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #60 on: July 31, 2014, 04:18:26 pm »
My first ever computer was not far off the Trash-80 (or Commodore PET) , I happened to like the Japanese MZ range back in the early 80's. The MZ-80K is the original with it's built in blue/white screen & cassette deck. I got the later and much nicer MZ-80A with it's proper keyboard and 2MHz Z80 and 48K RAM and, wow, a 2K screenbuffer that let me scroll through 50 lines of code! (I actually would have preferred an 80 column option than the 40x25 display though - aftermarket options coul enable it).

I learned how to code pure hexadecimal Z80 machine code from the ref manuals and POKE'ing them from BASIC DATA statements. I have to say, the Z80 was a dream to code compared to the 6502.

An interesting much larger scale and working reproduction of the Sharp Z80 on glass is here www.sharpmz.org/z80glass.htm

Oh yes, I still have it and indeed my first BBC Micro in the loft. They both still work, but a lot of the ceramic caps on the MZ-80A are coated in a white powder and the tape drive needs some attention. The Beeb, it was dead, but I recently fixed it by properly soldering some socket mounted IC's and the major work of fixing the dead keys (desoldering each one, dismantling, cleaning out with isopropyl alcohol, re-soldering!)

I look at my old but really immaculate condition IMSAI 8080 and the box of S100 cards above it everyday.  Sure does stir up the old late night memories using that stuff.
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Offline HighVoltage

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #61 on: July 31, 2014, 08:22:39 pm »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?

May be there was a big difference between the Computer industry and the TV Broadcasting industry.
I was surprised at one time, when I read that it was a Washington legislation that prevented the US to switch to PAL for TV's
Who know, may be this limitation did not apply to computer screens.

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Offline miguelvp

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #62 on: July 31, 2014, 11:59:17 pm »
You loose lines if you use a B&W NTSC TV if you want to display  B&W  PAL. I know this because my dad kept his B&W TV that he brought from the States when he was stationed in Spain. Frequency didn't matter and you could get a bit more than 480 lines by adjusting the horizontal trim but it will loose lock if you pushed it too much and never was able to display the full 525 visible PAL lines.

I forget if it did clip the horizontal as well. But it was able to show the picture although cropped.
He had a transformer that will bring the 220V down to 115V or whatever it was.

But in the late 70s there where a lot of discarded B&W TVs because everyone was upgrading to color (Spain was a bit behind the times) So I had a bunch of old TVs to use with my ZX 81. Eventually I got an Spectrum but of course I wasn't allowed to use the color TV, so I used my beaten up B&W TVs.

When I came to the States in the mid 80s my paternal grandmother gave me her portable B&W TV, and I was able to use it with the spectrum, I don't recall if there was any problems with cropping I think it displayed it all but a bit smaller (and it was a tiny TV to begin with, but I was a college student so I didn't mind, it was better than nothing).
 

Offline mahjongg

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #63 on: August 02, 2014, 12:34:09 am »
Yeah, that brings back many memories, at the time I was working for a small Dutch company called MCP, and one of the things we did was that we designed an alternative for the (very) expensive expansion interface. It was a small 15x15cm PCB that contained the floppy disk and printer interface logic, it was sold combined with a pair of BASF floppy disk drives, and had double density and the drives were  double sided. To get all the benefits from an expansion interface we also offered a service to expand the 16K in the base unit to 48K, replacing the 16Kx1bit 4116 DRAMS by 64x1bit DRAMS. It was my job to do the conversion, so I have seen and worked on dozens of model 1 innards. At home I had built an LNW-80, including the highres color extension. The things we sold for the TRS-80 eventually lead us to design out own TRS-80 clone, that technically was much closer to what later became the model III.
« Last Edit: August 02, 2014, 12:47:29 am by mahjongg »
 

Offline Clocky

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #64 on: August 02, 2014, 12:43:36 am »
 Thanks for doing this teardown, really enjoyed it. I was gifted one of these with the original expansion interface, floppy drive and monitor (original but different to the one you had) and Tandy speech synthesizer expansion available for it at the time - just last year. It also came with original manuals and a heap of micro-80 magazines.
I played with it a bit and added a 3.5" floppy disk drive for easier file transfer and larger storage capacity. Given that it has virtually no graphics mode, the arcade conversions of popular games of the time where pretty good and certainly playable.
Anyway, I cobbled together a homebrew Z80 system dubbed the "Junkbox VZ" which runs a Dick Smith VZ200/300 ROM BASIC which is based on the TRS-80 Level II ROM. The VZ had a lot of the Level II commands disabled and the particular ROM I use is one that has most of the commands re-enabled.
The webpage is crappy and needs updating, but it's here: http://users.on.net/~clockmeister/Junkbox_VZ/index.htm if anyone is interested.
None of this FPGA rubbish ;-)

« Last Edit: August 02, 2014, 01:23:38 am by Clocky »
 

Online Macbeth

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #65 on: August 03, 2014, 05:31:31 pm »
My first ever computer was not far off the Trash-80 (or Commodore PET) , I happened to like the Japanese MZ range back in the early 80's. The MZ-80K is the original with it's built in blue/white screen & cassette deck. I got the later and much nicer MZ-80A with it's proper keyboard and 2MHz Z80 and 48K RAM and, wow, a 2K screenbuffer that let me scroll through 50 lines of code! (I actually would have preferred an 80 column option than the 40x25 display though - aftermarket options coul enable it).

I learned how to code pure hexadecimal Z80 machine code from the ref manuals and POKE'ing them from BASIC DATA statements. I have to say, the Z80 was a dream to code compared to the 6502.

An interesting much larger scale and working reproduction of the Sharp Z80 on glass is here www.sharpmz.org/z80glass.htm

Oh yes, I still have it and indeed my first BBC Micro in the loft. They both still work, but a lot of the ceramic caps on the MZ-80A are coated in a white powder and the tape drive needs some attention. The Beeb, it was dead, but I recently fixed it by properly soldering some socket mounted IC's and the major work of fixing the dead keys (desoldering each one, dismantling, cleaning out with isopropyl alcohol, re-soldering!)

I look at my old but really immaculate condition IMSAI 8080 and the box of S100 cards above it everyday.  Sure does stir up the old late night memories using that stuff.

Oh wow! Do you have a WOPR too?:-DD I love that old Wargames movie! As Hollywood films go, there was a lot of the stuff like hacking, wardialling, phone phreaking, etc. that was actually real and quite well presented. I know I was doing it all as a teen back in the '80s.
 


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