Electronics > Projects, Designs, and Technical Stuff
Cloning a Tandy TRS-80 Model 1
GK:
--- Quote from: nick_d on January 03, 2019, 01:08:07 am ---I love it. :)
I bought some Atmel 5V CPLDs for this kind of purpose. Might be easier?
cheers, Nick
--- End quote ---
Kizmit99 mentioned that he/she might start a thread on his/her FPGA TRS-80 Model 1 clone here: https://www.eevblog.com/forum/projects/otp-eprom-programmer/msg2083648/#msg2083648
As per my PET clone, I am doing a traditional traditional hardware construction again with this one. Like the PET, the TRS-80 Model 1 keyboard unit didn't use any propriety/custom CRT controller chips and a plug-in equivalent for its original microprocessor is still being produced. I think this would not apply to and rule out cloning a model 100 similarly. I am seriously contemplating an Apple II next though.
GK:
--- Quote from: PA0PBZ on January 02, 2019, 07:38:51 pm ---Tandy sold a lower case upgrade kit (26-1104) for the model 1 which had the missing ram and an updated character generator. You probably want to implement that in your design.
--- End quote ---
Yes, I mentioned the Electric Pencil modification in my opening post, which was similar. My clone will include the lower case function - it is just a matter of providing a switch to toggle the MS address line of the character ROM between D6 of the video SRAM and the faux bit-6.
One thing about the Electric Pencil lower case mod is that it didn't come with a ROM upgrade, as the ROM originally had a lower case set programmed into it, even so it wasn't accessible with the standard hardware. The Tandy lower case upgrade kit, as you mentioned, actually updated the ROM as well. I am not sure at the moment in what way the upgraded lower case set differed from the original, but I intend to find out eventually as I am adding a manual ROM page select to my design so that you can choose to run either the original lower case set or the upgraded one.
Anyway, I have already programmed my ROM with a lower case set which I found on the net. It is an additional 32 characters. I just took a break from my PCB layout and changed a few jumper wires on my breadboard. Here is the lower case set dumped to the screen in addition to the upper case set.
Yes, I made a data-entry boo-boo on another ROM character - "g".
kizmit99:
I've noticed quite a few differences between the font your using and the one I have... I'm pretty sure mine is an accurate recreation of the font rom used in the "official" lowercase mod (with one line descenders). Although I have to say that I've found conflicting info on the net for a few of the character positions (0x60 and 0x7E specifically).
I've seen 0x60 with the pound-symbol, and 0x7e with a yen-symbol on this site: https://www.kreativekorp.com/software/fonts/trs80.shtml
but those symbols seem far too modern for the TRS-80, so I went with the original ROM glyphs (open-tic and tilda) for those two positions.
I've attached a hex file of the ROM I'm using in case you're interested...
(Note -- the 'graphics' characters are not in this rom-image, I'm generating them dynamically as in the original design)
German_EE:
My first computer was a Tandy TRS-80 Model 1. I thought I could save money by buying a 4K machine then getting a friend to upgrade it to 16K and install the lower-case mod. He did this but a couple of months later the machine died and he couldn't fix it. Tandy computers used to come with a warranty sticker on the bottom and if that was broken the repair cost was a minimum of four hours labor. Tandy fixed my machine and I got another three years of life from it before the Apple //e came along.
Other happy memories include building my own high-speed cassette interface that loaded and saved about eight times faster and playing my first adventure game through the night. Good times! :clap:
GK:
--- Quote ---Quote from: kizmit99 on Today at 03:21:00 am
I've noticed quite a few differences between the font your using and the one I have... I'm pretty sure mine is an accurate recreation of the font rom used in the "official" lowercase mod (with one line descenders). Although I have to say that I've found conflicting info on the net for a few of the character positions (0x60 and 0x7E specifically).
I've seen 0x60 with the pound-symbol, and 0x7e with a yen-symbol on this site: https://www.kreativekorp.com/software/fonts/trs80.shtml
but those symbols seem far too modern for the TRS-80, so I went with the original ROM glyphs (open-tic and tilda) for those two positions.
I've attached a hex file of the ROM I'm using in case you're interested...
(Note -- the 'graphics' characters are not in this rom-image, I'm generating them dynamically as in the original design)
--- End quote ---
Thanks for the ROM file and yeah, the issue I am also having is with conflicting info. Last night I watched Dave's TRS-80 tear down blog video from 2014 and noticed that the ROM was originally socketed. It's amazing that no one seems to have just wired one to a 27XX footprint and read the bloody thing. When I have time I might go over to the Tandy/RS section of the VCF forum and see if anyone with an original machine might be kind enough to dump the character set to the screen and make a high-res. photo.
I am also generating the 64 graphics block characters with discrete logic rather than with the ROM, but now that you bring this up I have to smack myself on the head. Doh! WTF am I doing that? By shifting this function to the ROM (which has way more space than I actually need) I can wipe four chips off the schematic. Going back to the breadboard now........
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version