TRS80 Model 1 Level II

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
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).

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