16 Nov
2016
16 Nov
'16
11:57 p.m.
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
happens.)
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...
Thanks,
Josh
17 Nov
17 Nov
3:05 a.m.
9.3V might actually work fine for a TMS9980, even though it's below spec.
It's not going to damage the part, so it may be worth a try before
modifying the board for 12V to the CPU socket.
In NMOS digital parts that predate depletion loads, Vdd needs to be
significantly higher than the most positive logic level in order to bias
the enhancement nFET used for the loads (pull-ups). The Vdd voltage
doesn't have to have a precise value, but it needs to be somewhat more than
the FET gate threshold above the most positive logic level, and below the
breakdown voltage. The higher the Vdd voltage (below breakdown), the
faster the pullup will operate, so running below spec will reduce the
maximum speed at which the part will operate. This is also dependent on
temperature. The part is spec'd for operation over a fairly wide
temperature range (even if only "commercial" rated). Since the logic high
level is no more than 5.0V, and generally somewhat less, a Vdd of 9.3V is
probably more than adequate at room temperature, but may fail at
temperature extremes.
The MP9529 is a "selected" TMS9980. In most contexts, a "selected" IC
is
one that has been tested and found to meet specifications more stringent
than the normal specifications. However, in this case I think the MP9529
might actually be "selected" in the sense of being tested to *lower*
specifications than a standard TMS9980. It's unclear why they would want to
use the lower Vdd, except possibly to reduce power consumption.
With the introduction of depletion loads in later NMOS ICs, generally
starting around 1976, and becoming ubiquitous by 1980, the requirement for
a supply above +5V was eliminated. Similarly, by adding an on-chip
substrate bias generator, the need for an externally supplied substrate
bias voltage (Vbb, typically -5V) was removed.
30 Dec
30 Dec
6:53 p.m.
Just to bring some closure to this old thread, I finally picked up a
working TMS9980 cpu chip (after getting one faulty one off of eBay -- it
was even more dead than the one it was replacing). And it appears to work
properly in the 990/189 board, with the 9.3V supplied on the CPU socket.
Thanks,
Josh
On Thu, Nov 17, 2016 at 3:05 AM, Eric Smith <spacewar at gmail.com> wrote:
9.3V might actually work fine for a TMS9980, even
though it's below spec.
It's not going to damage the part, so it may be worth a try before
modifying the board for 12V to the CPU socket.
In NMOS digital parts that predate depletion loads, Vdd needs to be
significantly higher than the most positive logic level in order to bias
the enhancement nFET used for the loads (pull-ups). The Vdd voltage
doesn't have to have a precise value, but it needs to be somewhat more than
the FET gate threshold above the most positive logic level, and below the
breakdown voltage. The higher the Vdd voltage (below breakdown), the
faster the pullup will operate, so running below spec will reduce the
maximum speed at which the part will operate. This is also dependent on
temperature. The part is spec'd for operation over a fairly wide
temperature range (even if only "commercial" rated). Since the logic high
level is no more than 5.0V, and generally somewhat less, a Vdd of 9.3V is
probably more than adequate at room temperature, but may fail at
temperature extremes.
The MP9529 is a "selected" TMS9980. In most contexts, a "selected"
IC is
one that has been tested and found to meet specifications more stringent
than the normal specifications. However, in this case I think the MP9529
might actually be "selected" in the sense of being tested to *lower*
specifications than a standard TMS9980. It's unclear why they would want to
use the lower Vdd, except possibly to reduce power consumption.
With the introduction of depletion loads in later NMOS ICs, generally
starting around 1976, and becoming ubiquitous by 1980, the requirement for
a supply above +5V was eliminated. Similarly, by adding an on-chip
substrate bias generator, the need for an externally supplied substrate
bias voltage (Vbb, typically -5V) was removed.
31 Dec
31 Dec
12:22 a.m.
On Fri, Dec 30, 2016 at 7:53 PM, Josh Dersch <derschjo at gmail.com> wrote:
Just to bring some closure to this old thread, I
finally picked up a
working TMS9980 cpu chip (after getting one faulty one off of eBay -- it
was even more dead than the one it was replacing). And it appears to work
properly in the 990/189 board, with the 9.3V supplied on the CPU socket.
Excellent!
11:09 a.m.
On Fri, Dec 30, 2016 at 6:53 PM, Josh Dersch <derschjo at gmail.com> wrote:
Just to bring some closure to this old thread, I
finally picked up a
working TMS9980 cpu chip (after getting one faulty one off of eBay -- it
was even more dead than the one it was replacing). And it appears to work
properly in the 990/189 board, with the 9.3V supplied on the CPU socket.
Thanks,
Josh
Now that it is working what are you going to do with it? I have
collected at least 3 of the TI-990/189 boards but have never really
gotten around to doing anything with them.
At least one has the optional serial port hardware already installed
and I have picked up a couple more TMS 9902 chips (eBay specials from
China, no guarantee they are legit and functional) to install in the
others. At some point I'll have to try getting BASIC EPROMs installed
and running on them. The cheap calculator keypads are a bit noisy and
bouncy to try to use reliably.
-Glen
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