TRS80 Model 1 Level II

From: Tony Duell <>
Date: Wed Feb 19 17:42:01 2003

> > 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.

So it's a fairly late one. The older version still has the 2 DIP sockets
(filled with 2K chips for the L1 BASIC!) but the L2 upgrade was a PCB
that was stuck to the solder side of the main PCB. It had a ribbon cable
that plugged into one of the ROM sockets (now empty) to pick up power,
data lines, and most of the address lines, and a 4-wire (I think) cable
that was soldered topins of the chips on the mainboard for the extra
address lines.

> > 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 :)

Oh yes :-)...

> > 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

L1 machines give a 32*16 mode display of colons if there's no RAM (or if
the RAM DIP shunt is pulled). I don't know if L2 machines do the same.

Any machine will give the _at_9 display with no ROMs. Fairly obviously it
doesn't matter if those ROMs were L1 or L2...

> 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

The -ve side of the largest capacitor in the PSU section is ground IIRC.

> > 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

Whill this is probably true, I've never seen an 8K machine, or indeed, an
8K DRAM. I would think that L2 would work with 8K RAM (it will work with
4K RAM, although it's not very useable).

> 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)

Yes, both machines use the same 16K DRAM chips (4116s or equivalent).

Received on Wed Feb 19 2003 - 17:42:01 GMT

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