Electronics > Projects, Designs, and Technical Stuff
Cloning a Tandy TRS-80 Model 1
biff:
--- Quote from: davefiddes on January 04, 2019, 01:05:10 pm ---I
A recent(ish) find on the VCF board was two character generator EPROM replacements which can be used as an improved lower-case mod. They offer 1-line or 3-line decenders depending on how much you are willing to mod the video generation circuit from the Tandy original.
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Yup - this is the mod being sold by Ian which I mentioned earlier.
The 1 line descender mod (which I installed) has the underscore replaced with a funky graphic block, and the 3 line version - while it has a true underscore, looked too different from what I was used to on my Model 1.
Jim
MrAl:
Hi,
Not sure what you guys are going for and it does sound very interesting, but just to note the TRS80 was based around the Z80 CPU. I would think you would want to use that unless you plan to emulate that too. Z80's were cheap 20 years ago, not sure anymore though.
Planning to expand the memory too? That would be cool.
What kind of disk drive, if any?
USB stick capability?
GK:
--- Quote from: davefiddes on January 04, 2019, 01:05:10 pm ---If you are looking for official character generator ROM dumps the xtrs emulator has the best collection I'm aware off: https://github.com/TimothyPMann/xtrs/blob/master/trs_chars.c There is quite a storied history behind these roms.
A recent(ish) find on the VCF board was two character generator EPROM replacements which can be used as an improved lower-case mod. They offer 1-line or 3-line decenders depending on how much you are willing to mod the video generation circuit from the Tandy original. The thread with ROM dump links can be found here: http://www.vcfed.org/forum/showthread.php?58665-GENDON3-improved-character-generator-for-the-Model-I-Discussion
If I'm following your schematic it seems that you are preserving the delay-base mechanism for horizontal sync generation. A recent article in the TRS8BIT newsletter discusses the issues with the original circuit and offers some suggestions on how to fix the resulting character wobble which can develop with the original chips: Page 48 of http://www.fabsitesuk.com/tandy/trs8bit1202.pdf
Hope this helps. Looks to be a fascinating project. I always figured my TRS-80 will keep going long after any of the other computers I've used because it used such basic logic building blocks. This promises to keep things going for even longer!
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Thanks a lot for the links, I'll check them out. For the sync generation delays I am using proper monostable chips (74HC4538) which are rock stable, unlike the original design which looked a bit dodgy to me as it used some funky combination of 74C04 inverters and RC networks.
GK:
--- Quote from: biff on January 04, 2019, 12:44:43 pm ---I'm hoping to wire up the adapter board so I can read it using my TL866 - and then modify the graphic block to a proper underscore - and write it to a 27XX and swap out the masked ROM.
Assuming the TL866 supports reading it - I'd be happy to read my original chargen from my Model 1. Probably wouldn't happen till Sunday though.
Jim
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That would be awesome. I don't see a problem with wiring either your adapter unit or the original MCM6670 masked ROM to a 27XX footprint and reading it with your TL866. I use the same programmer.
Here is how you'd wire it. I'd use a machined-pin IC socket in place of the 2716, and plug that directly into the TL866. It's easy to solder wires to the pins from the top.
EDIT: oops, don't connect anything to A10.
davefiddes:
--- Quote from: biff on January 04, 2019, 01:10:19 pm ---Yup - this is the mod being sold by Ian which I mentioned earlier.
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Sorry. Our posts crossed mid-flight. :)
--- Quote from: GK on January 04, 2019, 01:33:17 pm ---For the sync generation delays I am using proper monostable chips (74HC4538) which are rock stable, unlike the original design which looked a bit dodgy to me as it used some funky combination of 74C04 inverters and RC networks.
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The original circuit is definitely a bit dodgy. After 38 years my TRS-80 needs fixing as the 74C04 has probably "gone off". The approach in the TRS8BIT article is to use a 74HC164 to derive the required timing from the main clock signal. Your approach seems simpler to be honest.
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