TRS80 Model 1 Level II

From: Witchy <>
Date: Wed Feb 19 02:36:12 2003

> -----Original Message-----
> From: []On
> Behalf Of Tony Duell
> Sent: 18 February 2003 00:12
> To:
> Subject: Re: TRS80 Model 1 Level II
> You say it's an L2 machine. Does it have the ROM in 2 off 24 pin chips on
> the mainboard (one is 8K, the other 4K, and there are some cut-n-jumper
> mods), or is it old enough to have the extra PCB with 3 24 pin chips
> (eack 4K) and the address decoder?

It's the former - 2 ROMs and 2 DIP shunts.

> > ROMs. I found a link to the Tech Ref for the Model 1 -
> excellent book, given
> Or ROM failure itself?

Yep, of course. I haven't got a ROM reader (yet) so I can't do any checksum
tests on the ROMs.

> > the writing style it could've been written by me, same sense of
> humour - and
> Indeed. The introduction is rather amusing... (As you may have guessed, I
> have this manual...)

The whole thing is amusing IMO, I loved the 'real world' section where he's
done a schematic to allow your TRS80 to control the power on your coffee
machine :)

> Be warned that much of the tech manual is written assuming you have a L1
> machine. I don't know what L2 does if it can't find any RAM, for
> example...

It does predictable things along the lines of no ROM; I can't remember the
resulting screen though. The couple of replies I got on comp.sys.tandy also
suggest ROM failure and/or address lines and/or data lines.

> With the ROMs pulled, all the address lines should be toggling (as the
> CPU tries to continually push 39 00 onto the stack). You can check this.

Yep, I'm being dense again and struggling to find a good link to ground I
can fasten my probe too; +5V can be got from the right side of C9. Just need
to study the schematics a bit more.

> > This I now know.....good job the tests indicate the RAM is
> happy! I guessed
> > when the RAM turned out to be 16 pin chips instead of the 2114's 18.....
> Yes, the M1 uses DRAMs for the main memory. Either 4K or 16K parts.

The schematic also mentions 8K parts, but since that's from the tech ref it
might only apply to L1 machines? I suppose I can always steal memory from
the model III that's still in bits? (mail in your inbox about that)


adrian/witchy - the online computer museum - monthly gothic shenanigans
Received on Wed Feb 19 2003 - 02:36:12 GMT

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