TI 990/189 debugging

Josh Dersch derschjo at gmail.com
Thu Nov 17 01:57:20 CST 2016

Hi all --

Got myself a TI-990/189 single-board computer based around the TMS9980 
microprocessor (actually, a variant of it, the MP9529, which apparently 
differs only in that it has a lower maximum clock and only requires Vdd 
of 9.3V or so...)

It was advertised as "it looks like it's working, but who knows" and so 
of course it arrived and it's dead.  It powers up and nothing appears on 
the display,  and the CR1-CR4 and SHIFT LEDs are illuminated.  No 
response whatsoever.

I've spent some time yesterday and today probing the thing and I think 
the CPU is dead, but I wanted to run it past the braintrust here in case 
anyone has any experience with the 9980...

Here's what I see:

Voltages are all nominal on the +12, +5 and -5 supply; +5 and -5 are 
present at the CPU, as is 9.3V for the VDD.

At the CPU:

- CKIN is clocking at the right rate, the phi3 clock generated by the 
CPU is also correct.

- IAQ is not pulsing, so the CPU is not fetching instructions

- The Address and Data lines are all zeros with no activity whatsoever

- HOLDA is low, -HOLD is high (so the CPU is not being held)

- READY is high

- MEMEN is low (so no memory accesses are taking place)

- INT0 through INT2 is 010 (which indicates that a LOAD interrupt is 
active, more on this later)

I have verified that the POWERGOOD signal is going high after about a 
second after power-on, as expected (this causes things on the board to 
RESET appropriately).  This in turn causes the -LOAD signal from the 
Power Up/Reset circuit to go low, which causes INT 1 to go high.  (This 
is later supposed to be reset, once the CPU's IAQ line clocks after the 
first instruction is executed, but since that's dead, well, nothing 

Based on this, I believe the CPU to be faulty.  Anyone have any thoughts 
on this?

Given the VDD difference (12V vs 9.3V), I don't think a standard TMS9980 
will work; the MP9529 seems to be difficult to source, but it shouldn't 
be hard to get 12V to the CPU...



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