TRS80 Model 1 Level II

From: Tony Duell <>
Date: Mon Feb 17 20:15:01 2003

> Tony wrote:
> > Good! If you have the disk controller in the system (read : If you have
> > the Expansion Interface connected), then the machine tries to boot the
> > disk _before clearing the screen_. If you don't have a disk in the drive,
> > then you get a screen of garbage.
> <snip good advice>
> I should've said that because the previous owner showed me the machine gave
> a garbage screen I didn't bother getting anything other than the machine and
> monitor out of their boxes - no point testing the rest when the machine
> won't even power up correctly :)

Ah, OK....

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?

> So far I've got the problem down to probably bad address/data lines at the
> ROMs. I found a link to the Tech Ref for the Model 1 - excellent book, given

Or ROM failure itself?

> 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 'remove and test' method leads me down the bad ROM path - without the
> ROMs in the machine I get a screenful of _at_9, which indicates the CPU and
> video RAM are happy.

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...

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.

> 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.

Received on Mon Feb 17 2003 - 20:15:01 GMT

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