Author Topic: Eurorack modular 8085 computer  (Read 16883 times)

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Offline ChristofferB

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Eurorack modular 8085 computer
« on: March 21, 2016, 12:37:44 am »
Hey! I'm currently setting out to build the 8085 system, and thought I'd share.

The computer is going to be modular, with each card being as universal as possible.
I'm sticking strictly to the 19'' 3U subrack Euro Card Bus standard, A) because it's not very widely seen in DIY computers and B) because it's just a convenient form factor, I think.

 Mechanically that means:

    - 3U subrack  with DIN 4.1612 AC connectors to backplane
    - PCB sliding rails
    - Either proper eurorack front panels with handles (a bit pricey) or pcb's without handles
    - 16cm x 10 cm std. vero-boards for creating the circuits on (probably with vero-wire)

Computer wise, I plan on distributing the boards as such:

    - CPU card: 8085, clk, bus drivers, reset
    - RAM/ROM card(s): Plan on going for 8k*8 RAM/ROM - will have decoding and type patchable                              for universality.
    - Serial card: (8250/8251), baudrate gen, etc
    - Display/keypad card: Hex keypad and 7seg displays, either with an 8279 or some 8255's
Display and keypad won't be on the module plate. I think I'll add a separate 3U front plate above for that. (and other controls, too) - Just neater that way.
    - Cassette interface card (for when I get a 19''rack reel to reel for it)
    - 8255 parallel centronics printer interface

Aand in the further future:

    - CRT card (I have not looked into driving a CRT
    - ASCII keyboard card (not difficult, but not that useful without a CRT as display)
    - Punched tape interface (Yeah, like I ever find a rack paper tape reader)
    - 8088 CPU card (maybe)

I'll try and stick to 74LS and HCT logic (I have so much LS, but HCT would be easier on the PSU)
And 82XX intel peripheral IC's.

I'm aiming for using the standard ECB pinout for the bus, but I have to make some modifications. The standard has specified z80 interrupt lines, but no /INTA for intel CPU's.

Lastly, I need some bus termination, I think. I plan on just adding that to the bus side of the CPU card, as you'd usually always have that installed. I think passive resistive termination is enough for now.

Currently working on the power supply, which will be 5V, at first. Later, a +5 +-12V will probably be needed.

I'm not sure if I should have more features on my CPU card than I've planned. Right now, it's literally only the CPU module.


Let me know what you think! Has anyone ever worked with the 3U subrack /ECB standard?
Is there any cards / functionalities you'd add?

--Christoffer


--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 

Online MK14

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Re: Eurorack modular 8085 computer
« Reply #1 on: March 21, 2016, 01:13:03 am »
Surely with specs like that (8085 etc), you may want to try/use CP/M. Hence want at least a (8085 era) floppy, to more easily allow that.

I never make jokes, on forums. With your 8088 cpu, and paper tape unit, by creating 8088 compatibility versions, of some of the newer (needed) instructions. You can download/transfer win10, onto 1,472,446,284 miles of paper tape. Then after about 345,347,399,585 hours, to read the paper tape, you can play with win10.
« Last Edit: March 21, 2016, 01:20:12 am by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #2 on: March 21, 2016, 01:56:17 am »
It would at one point be nice to be able to run some "real" software, and not just small homemade routines.
5 1/4 inch floppy drive (dual?) would be convenient, if not necessary for that. I'll add that to the list!

Running win10 from paper tape would be "a bit" ...ambitious... but If I do upgrade to 8088, it'd be nice to be able to run IBM DOS.

--Christoffer
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #3 on: March 21, 2016, 02:03:18 am »
It would at one point be nice to be able to run some "real" software, and not just small homemade routines.
5 1/4 inch floppy drive (dual?) would be convenient, if not necessary for that. I'll add that to the list!

Running win10 from paper tape would be "a bit" ...ambitious... but If I do upgrade to 8088, it'd be nice to be able to run IBM DOS.

--Christoffer

I think you would find, trying to run IBM DOS (or similar), a MASSIVE undertaking. Because (presumably), you would have to turn the 8088 system, into a COMPLETE (compatible) IBM PC, of that era.
E.g. CGA graphics (or whatever standard applies with it), BIOS (compatible with both IT, AND your system), all sorts of other bits and pieces, adding to the major headache.

BUT it may be possible to get hold of a FULLY open sourced "DOS", by the time you are ready. Which lets you configure/compile/tweek it to your particular hardware.
 

Online MK14

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Re: Eurorack modular 8085 computer
« Reply #4 on: March 21, 2016, 02:05:07 am »
It would at one point be nice to be able to run some "real" software, and not just small homemade routines.
5 1/4 inch floppy drive (dual?) would be convenient, if not necessary for that. I'll add that to the list!

Running win10 from paper tape would be "a bit" ...ambitious... but If I do upgrade to 8088, it'd be nice to be able to run IBM DOS.

--Christoffer

I think you would find, trying to run IBM DOS (or similar), a MASSIVE undertaking. Because (presumably), you would have to turn the 8088 system, into a COMPLETE (compatible) IBM PC, of that era.
E.g. CGA graphics (or whatever standard applies with it), BIOS (compatible with both IT, AND your system), all sorts of other bits and pieces, adding to the major headache.
E.g. Good luck with building your "Compatible Chip Set", out of 1,000? raw TTL 74LS/HCT devices.

BUT it may be possible to get hold of a FULLY open sourced "DOS", by the time you are ready. Which lets you configure/compile/tweek it to your particular hardware.
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #5 on: March 21, 2016, 12:15:51 pm »
It would be a pretty big task, for sure. At this point that's also only some far-fetched dream.
But the original IBM PC had only a handful of specialty IC's as chipset: CPU, DMA, Interrupt controller, timer, and some peripheral interface (8255). But I do think CP/M would be the better alternative.

--Christoffer
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #6 on: March 21, 2016, 12:34:25 pm »
It would be a pretty big task, for sure. At this point that's also only some far-fetched dream.
But the original IBM PC had only a handful of specialty IC's as chipset: CPU, DMA, Interrupt controller, timer, and some peripheral interface (8255). But I do think CP/M would be the better alternative.

--Christoffer

I completely agree.

CP/M opens up a world of opportunity, with a huge range of software. Its programming language interpreters/compilers, for example, give tremendous ability to play with it for hundreds of hours or more.
The CP/M software base, will really bring your computer to life, and maybe bring back memories of fun computing times, from a long time ago.

I agree the original IBM PC, is NOT that bad, to recreate in hardware. Complicated chipsets, were later.

The 8085 (8080), is a good choice, because (in real terms), it (8080), was the first, seriously successful microprocessor. There were some before it, but they never really hit the market big time, especially as regards computer systems.
I.e. Apart from calculators and display terminals, 4004/4040/8008 were NOT in many computer systems, from a quantity point of view.

The Z80 (a sort of semi-illegal, improved clone of the 8080/8085), has been one of the most successful microprocessors, of all time. With the Arm probably being the MOST successful, or the X86, depending on ones opinion.

I hope your vero wire, DOES NOT give off toxic fumes. Someone warned me against it, because it can/does give off highly toxic/poisonous fumes. Assuming you are talking about the stuff, which is covered in insulation, which "boils" away, when soldered at appropriately high enough temperatures.
I had bad/poor experiences with my early (long time ago), vero wire constructions, whereby I had TOO MANY non-connecting joints, to viably build stuff.
Looking back, it was probably because I had a relatively POOR quality, mains type (SCR/TRIAC in the handle), of temperature controlled soldering irons. I should have got something of better  quality (at that time).
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #7 on: March 21, 2016, 12:53:41 pm »
Quote
The Z80 (a sort of semi-illegal, improved clone of the 8080/8085), has been one of the most successful microprocessors, of all time.

That's one of the reasons I'm going for the 8085. 90% of homebrew vintage-ish computer are z80 based. I also really like the intel 82xxx series support chips over the z80 based ones, and I have a lot of them.

My vero-wire DOES have the fuming issue (it all does, as far as I'm concerned), so I make sure to have adequate ventilation and hold my breath when soldering it.

Vero-wire seems very picky about soldering iron. I've found that a high thermal capacity iron at medium temp. is pretty good. The tips I use are flat-hammered and chisel-ground desoldering tips with my PACE vacuum desoldering station. Works pretty o.k.

I finished the PSU enclosure, see attached, it's built from an old 24V euro-rack module.

--Christoffer
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Offline Fungus

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Re: Eurorack modular 8085 computer
« Reply #8 on: March 21, 2016, 01:50:41 pm »
I think you would find, trying to run IBM DOS (or similar), a MASSIVE undertaking. Because (presumably), you would have to turn the 8088 system, into a COMPLETE (compatible) IBM PC, of that era.
Quote

In theory the BIOS does everything, it's the hardware abstraction layer.

In practice, many programs went direct to the hardware.

 

Online MK14

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Re: Eurorack modular 8085 computer
« Reply #9 on: March 21, 2016, 02:32:18 pm »
Quote
The Z80 (a sort of semi-illegal, improved clone of the 8080/8085), has been one of the most successful microprocessors, of all time.

That's one of the reasons I'm going for the 8085. 90% of homebrew vintage-ish computer are z80 based. I also really like the intel 82xxx series support chips over the z80 based ones, and I have a lot of them.

My vero-wire DOES have the fuming issue (it all does, as far as I'm concerned), so I make sure to have adequate ventilation and hold my breath when soldering it.

Vero-wire seems very picky about soldering iron. I've found that a high thermal capacity iron at medium temp. is pretty good. The tips I use are flat-hammered and chisel-ground desoldering tips with my PACE vacuum desoldering station. Works pretty o.k.

I finished the PSU enclosure, see attached, it's built from an old 24V euro-rack module.

--Christoffer

Thanks for the information, on how to use the Vero system, more reliably and safely. The powerful/meaty desoldering station, makes a lot of sense. As it (I would imagine) has the raw heating power to reliably melt the insulation. Getting PCBs made (externally), is quite expensive, and time consuming (to design each one).
So just quickly vero wiring the board(s), is a quick and convenient solution to that issue.

The relative lack of surface mount parts and relatively low frequencies, mean that, that method, is perfectly suitable for such projects.

Your power supply, looks VERY authentic. It looks like a genuine 1970s, computer/electronic item, and built VERY robustly. Lots of cooling power, visible.

In theory the BIOS does everything, it's the hardware abstraction layer.

In practice, many programs went direct to the hardware.

That's a good point. So sensible creating of the BIOS in the new computer (assuming the software exclusively uses the BIOS etc, only), can allow partly non-PC hardware, to be coped with. Via adapted software (BIOS).
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #10 on: March 22, 2016, 05:39:55 pm »
Quote
The relative lack of surface mount parts and relatively low frequencies, mean that, that method, is perfectly suitable for such projects.

I think if you lay out your boards properly, and use vero-board with ground plane, you can use vero-wire (and wire-wrap) at pretty high frequencies (for the 80's) - There's a guy actually doing surface mount vero-wiring of boards: http://elm-chan.org/docs/wire/wiring_e.html
Incredible stuff..

I've thought about diagnostics/development cards in addition to the afforementionned cards.

 - A bus monitor card might be nice to have, in the style and function of the altair front panel.
 - Memory access card: A German Euro Card Bus // eurorack enthusiast made a card to decode the regions of memory accessed, by means of 74138's and latches. Looked very useful.
 - Single-step and instruction step card: These existed for the S-100 (ALTAIR/IMSAI) systems, allowing single-stepping either one cycle or one instruction (8, 16 cycles i think).

Any other debug module one could add?

--Christoffer
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Offline woodchips

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Re: Eurorack modular 8085 computer
« Reply #11 on: March 22, 2016, 06:26:02 pm »
Might be worth considering using 6U cards otherwise half the card will be buffer chips.
 

Online MK14

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Re: Eurorack modular 8085 computer
« Reply #12 on: March 22, 2016, 06:27:08 pm »
I think if you lay out your boards properly, and use vero-board with ground plane, you can use vero-wire (and wire-wrap) at pretty high frequencies (for the 80's) - There's a guy actually doing surface mount vero-wiring of boards: http://elm-chan.org/docs/wire/wiring_e.html
Incredible stuff..

That is quite amazing. Offhand, I would have simply dismissed vero wiring, surface mount IC's (excluding 0.1 pitch conversion boards) as being somewhat impossible. Because they don't do 0.1" pitched IC sockets.
Ground planes, and improved frequencies, is interesting as well.

Some of the parts (IC's), that are required for projects like these. Are becoming increasingly difficult to get hold of, in through hole packages, such as DIPs. So that method (you linked to), could still be useful, even on something like a mostly/all DIP IC packaged computer project. Because the odd chip(s) here and there, may be unavailable to you.
E.g. DIP SRAM chips.
So having that solution as a backup plan, in case you can't get hold of all the bits you need, is very useful.


- A bus monitor card might be nice to have, in the style and function of the altair front panel.

If you are trying to make me REALLY jealous and want to play with your computer, when it is finished. You are doing a VERY GOOD JOB!
Yes, the huge panels, with tons of flashing LEDs and nice quality toggle switches (or whatever), still looks really COOL!


I've thought about diagnostics/development cards in addition to the afforementionned cards.

 - A bus monitor card might be nice to have, in the style and function of the altair front panel.
 - Memory access card: A German Euro Card Bus // eurorack enthusiast made a card to decode the regions of memory accessed, by means of 74138's and latches. Looked very useful.
 - Single-step and instruction step card: These existed for the S-100 (ALTAIR/IMSAI) systems, allowing single-stepping either one cycle or one instruction (8, 16 cycles i think).

Any other debug module one could add?

--Christoffer

If it was me...
There are a (somewhat) large number of modern "logic analyser", capability devices available, at reasonable prices, these days that I have heard about.
So unless you are trying to build it, only using equipment/tools that were available in the 1980s. Then I would seriously consider giving them a look into.

But building dedicated debug stuff, makes lots of sense as well.

In those days (and a decade later), there were VERY powerful (but sadly very expensive at the time, such as >$10,000), "proper", stand alone logic analyzers. They were amazingly powerful, and very, very useful.
You could capture the code using various triggers, while it was running at full speed, in real time.
When it triggers, you could see a fully disassembled listing (scroll-able) on screen, which was very useful for seeing what was going on.

Presumably such logic analysers (or their modern equivalents), are available used (or new in the case of the modern ones), on ebay (or test equipment sellers for new/modern stuff). At reasonable prices.

« Last Edit: March 22, 2016, 06:30:57 pm by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #13 on: March 22, 2016, 06:35:07 pm »
I actually have a nice late 80's 40 ch stand alone logic analyzer. But being able to debug via a couple of LED's would be a lot quicker (and NO vintage computer is complete without TONS of flashing LED's)

Another issue is which Europe Card Bus pins to let deviate from the standard to make this useful with the 8085 and other 8-bit Intel's. D8-15 can of course go, if you never intend to go 16 bit, but the standard states it was developed for the z80 and 8080/8085, so there must be a standardized way of doing that.

--Christoffer
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #14 on: March 22, 2016, 07:08:47 pm »
That makes perfect sense.
Taking the big/massive/heavy logic analyser, is quite a bit of work. Many/most things DON'T needs its capabilities.
So a quantity of permanent front panel LEDs and maybe seven segment displays (depending on what you want, I think you prefer the LEDs by the sound of it), should work out just fine.

I find hanging a number of LEDs (or at least one), onto I/O pins (or PIOs or whatever in your case), useful. Because you can set/clear/flash them in various places in the code. So if it crashes (totally), and you have no idea why/where. You can move their on/off settings, throughout your code, until you find out where it is crashing,

Also such outputs, make useful scope/logic analyser trigger signals as well.

Some people delight in using serial ports. They then send diagnostic information, maybe continually to it. So if it goes wrong, suddenly. They can trace through (what might be a big file), and see where/how it failed. Using a PC or something to record the data. E.g. "Starting xfer operation". "Finished Ram init.". Etc. Via printf's or whatever.

But yes, LEDs and switches, seems to be the direction you are going, which sounds really good.

Two birds with one stone. You can use it to debug your system, and then later have some really cool flashing lights, which will look really impressive and authentic (for the 1980s).
« Last Edit: March 22, 2016, 07:13:16 pm by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #15 on: March 22, 2016, 07:22:26 pm »
Right, that's the plan. I like serial ports too, but software-wise the LED's are faster and easier to deal with. I'd love some numeric displays, but I hate the fact that there are no common available 4 bit to hex display drivers. And the single hex digit displays with built-in 4bit decoder are VERY expensive.

So LED's it is.

Maybe I should add a jumper to disconnect the CPU clock from the board and have a slow clock module. Though that might cause issues with other boards..

--Christoffer //IG:Chromatogiraffery
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #16 on: March 22, 2016, 07:35:54 pm »
It is indeed a big pain, there are no commonly, reasonably priced hex displays and/or driver chips.

A long while ago, I managed to bag, a modest pile of these:



at reasonable cost. As I think they look REALLY cool, and are very quick and convenient, for such projects.

Additionally, I got some similar displays, but they are actually alphanumeric (I also managed to bag a modest quantity of those, as well, if I remember correctly, if I'm mixed up, then they are seven segment and/or hex, rather than alphanumeric).

I think your idea of slowing or even stopping the clock (as long as compatibility issues and any (if applicable, you could be all sram) Dram refresh issues, are resolvable), is a good one.
« Last Edit: March 22, 2016, 07:42:57 pm by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #17 on: March 22, 2016, 09:16:04 pm »
Those are the ones I meant!

If one were really smart, one would use a MC14411 baud rate generator for the slow clock card, having a switch to select between clock rates, all the way from xtal freq to 300 hz.

-This board would be an obvious place to put the single step/instruction step circuit too.

Farnell sells the card front panels in kits with all hardware, that seems to be the cheapest option at
about 6£ for the 4 hp one:

http://uk.farnell.com/schroff/20818-018/card-frame-plug-in-3u-4hp/dp/1455894

I couldn't buy 1,5mm aluminium plate and loose hardware that cheap (unless I planned a LOT of modules)
 
--Christoffer
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Offline rfeecs

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Re: Eurorack modular 8085 computer
« Reply #18 on: March 22, 2016, 10:13:56 pm »
You are probably familiar with the N8VEM project.  It sounds similar:
https://en.wikipedia.org/wiki/N8VEM

They have done a lot of EuroCard Bus boards, though it doesn't look like any use an 8085:
https://www.retrobrewcomputers.org/doku.php?id=boards:ecb:start

Here's a project using an 8085 combined with PICs that runs the old Altair software and CP/M,
wire wrapped construction:
http://mini-altair.tripod.com/
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #19 on: March 22, 2016, 10:46:49 pm »
Quote
They have done a lot of EuroCard Bus boards, though it doesn't look like any use an 8085:

Wow, their floppy board and video/keyboard board looks really interesting!

Neat looking Altair compatible too! Might just grab some ideas from that.

--Christoffer

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Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #20 on: March 23, 2016, 02:49:01 pm »
I've completed design of the 8085 CPU module.
It's pretty minimalist, and only uses one of the 8085's interrupts (it has 5) -and then int acknowledge, thus I can just use the Intel 8259 interrupt controller to get more interrupt options.
I've included the HOLD line, that also controls all (most) tri-state gates to the bus, allowing either DMA or punching code in via front panel, Altair-style.

-See attached.

--Christoffer
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #21 on: March 23, 2016, 03:00:54 pm »
I hope you don't mind me criticizing the circuit.

You seem to have one or more (such as the SID pin), unconnected input pins on the MCU (8085). I believe that is NOT recommended on the 8085 (or most ICs). (obvious solution, to tie them, with resistor or whatever is  recommended in datasheets, rather than leave them floating).

The reset pins, resistor/capacitor, does not have a diode across the resistor. The possible problem with that way of doing it, is a very fast power on/off/on cycle, may be able to NOT actually properly reset the 8085.
The diode forces the capacitor to rapidly discharge when Vcc is switched off, potentially giving very reliable reset operation.

Disclaimer:
I'm NOT a 8085 guru. So I could be wrong.
« Last Edit: March 23, 2016, 03:03:34 pm by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #22 on: March 23, 2016, 03:11:57 pm »
Thanks for the feedback!
I never quite got the diode seen in power on reset circuits, thanks for clarifying that!
And you're right, there shouldn't be floating inputs, I'll work that out too.

I'm still undecided whether I should make dedicated RAM and ROM boards, or a combined.
Dedicated allows upgrade of the one without messing with the other (could have multiple ROM cards with different software)
Combined is obviously more space/board load optimized - but with room for 28 boards that's not very critical.

-Christoffer
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Online MK14

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Re: Eurorack modular 8085 computer
« Reply #23 on: March 23, 2016, 03:44:42 pm »
Having combined RAM/ROM board(s), or separate ones, both have their own, pros and cons.

Looking at this page (I know it is different to what you are doing):

http://www.s100computers.com/My%20System%20Pages/RAM+ROM%20Board/RAM+PROM%20Board.htm

The apparent high complexity, high packing density and high complexity with configuration switching. Gives me a slight inclination to say, go for separate RAM/ROM boards.

There have been times, when ROMs (including Flash(if available/invented in that era?)/EPROM etc) were too slow, to run at full MCU speed, without waitstates. Nowadays, you can easily get very fast ones. Depending on how authentic, you want to go.

Off-hand, I'm NOT sure if when the 8085 was introduced, current ROM/EPROM/FLASH technologies, were fast enough for full speed operation.

Switching ROM (only) boards around, to change OSs or boot code etc. Is a useful advantage.

Expanding memory is also an advantage.

Interestingly, the 8085 may actually be qualifiable/used as a spaceworthy cpu. That is a good sign.

Apparently there is some talk of "secret" 8085 instructions.

https://en.wikipedia.org/wiki/Talk%3AIntel_8085

Quote
extended instructions[edit]
Long ago, in the 1970's, when I was building some new industrial products that incorporated the 8085, I came across an article in, as I remember, Electronics magazine, which described a small set, a half dozen or so, of undocumented op codes that the authors had discovered. I do not have a reference for the article at present. After I studied these new op codes, I became quite excited, because they were very useful extensions of the instruction set, with for instance, really useful indexing capabilities. These new instructions were far more useful that the horde of new op codes introduced with the Z80, most of which made code simultaneously longer and slower if used; we avoided all but a couple of the Zilog opcodes, even when the target machine was a Z80 and not an 8080. I asked management if I could use these 8085 op codes, the decision was not to. Our inquiry, through the grapevine, was that Intel, for marketing reasons, disowned them, because they were not compatible with the upcoming 8086/88, for which they had bright hopes, correctly as we see now. Anyway, anyone interested in the 8085 today, especially in an emulator, could benefit from these extensions. --AJim (talk) 18:43, 25 March 2013 (UTC)
« Last Edit: March 23, 2016, 03:48:05 pm by MK14 »
 

Offline ChristofferB

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Re: Eurorack modular 8085 computer
« Reply #24 on: March 23, 2016, 04:24:08 pm »
That S-100 board does seem rather complex! But it seems it's also intended to be able to run 16bit.
I don't think the speed is problematic in my case, if it turns out to be, I can always switch to a bit faster memory - I'll probably stay in the 2-4 MHz clock range.

I'm drawing up a 4K RAM board at the moment, featuring little more than a 74138 decoding for 2 6116 2K sram's. I've added a 16 pin socket as jumper between the '138 outputs and the /CS lines you you can map the 4K where it wouldn't interfere with booting.
This leaves plenty of room for expansion (even on the same card) if needed.
An EPROM card would be almost identical, of course. If one were to create a pcb for these, you'd make them identical and allow ram/rom/mapping with jumpers of DIP switches.


I've also added a jumper on the /WR line, to allow for a "memory protect" switch to be placed on the front panel, disabling writing to memory - This might be nice if you've punched in manually and don't want the machine plowing through your code.

 
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