TRS-80 Model I

From: Tony Duell <>
Date: Sun Nov 23 11:13:53 1997

On Sun, 23 Nov 1997, Charles E. Fox wrote:

> On a TRS-80 Model I, no expansion unit, I am getting the left half of the
> screen filled with graphics and the right half with scrambled text. Does
> anyone have a suggestion of where I should start looking for the solution
> to this problem?

Well, it's been 5 years since I read the Model 1 technical reference
manual, but I can still remember some bits of it (I hope)..

Start by opening the case, laying the 2 boards out component-side up, and
connecting power and video. If you want step-by-step instructions on how
to do that, please ask. Turn on the machine. Is it fixed? If so, suspect a
solder-ball short (these were quite common one some batches, I believe) or
a problem in the keyboard cable, which carries the Z80 bus. In fact,
checking that keyboard cable for continuity (with the machine off, of
course) wouldn't be a bad idea.

OK, still not fixed? Check the PSU. The 0V line is the -ve side of the
largest capacitor on the board, and all the supply rails go to the DRAM
chips (-5V on pin 1, +12V on pin 8, +5v on pin 9).

Now switch off and pull the shunt block (it looks like an IC, but has
metal shorting bars on it only). That disables all the DRAMs. Turn on
again. On a Level1 machine (and I believe a level 2 machine, but my manual
isn't that clear) you'll get a 32*16 display of colons. If that occurs,
then you've probably got RAM or RAM addressing problems.

Switch off again, and pull the ROMs (or on a level 2 machine, pull the 24
pin ribbon cable from the ROM socket). Turn on again. The Z80 data bus is
pulled high, so the machine executes continual RST38 instructions and
fills all of the memory with 39 00 (the return address, of course). The
display will fill with alternate '_at_' and '9' symbols, in 64*16 mode. If
that works, then you have ROM trouble, I think.

If you still don't get the right display, reseat the Z80, and then use a
scope/logic probe/LogicDart to find out (a) what the Z80 is doing, (b)
what the buses are doing and (c) what the video controller is doing.

> Thanks
> Charlie Fox

Received on Sun Nov 23 1997 - 11:13:53 GMT

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