Electronics > Projects, Designs, and Technical Stuff

Cloning a Tandy TRS-80 Model 1

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MustardMan:
Bit late on the "hanging while booting Level II" (already solved), but something that may be interesting...

The book "The Custom TRS-80 and Other Mysteries" (1982 - Dennis Bathory Kitsz) on page 26 talks about the "hanging" and why (including disassembed code). Page 22 also mentions the "jumpers" that select level I or II. The book is a fairly good read, and I would have loved to have known of it back in the day. Ahhh, the world without internet!

I think it was the 'hackaday' project (??) that talked about the differences in how different chips (eg: LS, C, HCT etc) let their outputs idle. Some high, some low. Hence, using a modern replacement that has an almost identical part number can lead to some rather unexpected problems (eg: replacing a 2764 ROM with a 27C64 ROM).

MM.

nick_d:
I believe that TTL and LS-TTL inputs do not "float" as such but act as if pulled up, unless explicitly pulled down by a device on the bus. This is probably a point of incompatibility which would make the Z84C00 (CMOS Z80) not an exact drop in replacement for the Z8400 (NMOS Z80), despite that the input thresholds of the CMOS part have been set to approx 1.2V for NMOS compatibility (that is, the external interface is constructed with 74HCT-like gates).
cheers, Nick

GK:
Yes of course, in the original TRS-80 the data I/O pins of the Z80 were buffered to the data bus by a pair of 'LS244s, which will always read 0xFF when the bus is floating.

Good tip on "The Custom TRS-80 and Other Mysteries"! Complete PDF of the book available here: https://archive.org/details/Custom_TRS-80_and_Other_Mysteries_1982_Dennis_Bathory_Kitsz_a


BTW, can anyone out there elaborate upon what the lower-case modification incompatibility is supposed to be with Level II BASIC? :


--- Quote ---Unlike most other lower case modifications, the Electric Pencil modification required a toggle switch to enable and disable lower case. This was important because the modification was incompatible with Level II BASIC and needed to be disabled before using it. Those toggle switches were once a common site on Model I keyboards.
--- End quote ---

jeffheath:
Just found out you actually have the gerber files and schematics uploaded for the commodore pet... looks like I've got something to keep me busy for a looong time ;D Can't wait to see how this one turns out!

kizmit99:

--- Quote from: GK on February 03, 2019, 05:00:00 am ---BTW, can anyone out there elaborate upon what the lower-case modification incompatibility is supposed to be with Level II BASIC? :

--- End quote ---
Please note - the following is from very old memories, not from taking a look at the schema/ROMS...  I *could* be miss-remembering this, but I think it's basically correct:

I didn't do the Electric Pencil mod, but I *think* the issue was actually with the font ROM having "giberish" in the first 32 positions.  I believe BASIC actually stores values of 0 through 32 for the alphabet, and relies on the original "missing bit" circuitry to "push" those characters up into positions 64 through 96 in the font ROM.  When the font ROM has the alphabet in positions 0 through 32, this alleviates that issue (which is why you don't see it).

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