JW wrote:
On Wed, 12 Sep 2007 14:04:59 +0100 "Graham W"
wrote in Message id:
:

JW wrote:
Here's one that has me puzzled: I have a HP 6034A power supply that
does not come out of self-test when powered up. Internally I traced
the problem to the TI TMS9981 CPU itself
(Datasheet http://www.datasheetarchive.com/pdf/3434384.pdf)

On power up, the Interrupt code present on it's IC0 - IC2 pins is
000 (reset) it never exits reset. Tracing back through a few TTL
glue chips I found that a quad latch has no clock with which to
pass D to Q. The clock for the latch is derived from the TMS9981's
pin 21 "03" clock output - which appears to be floating at 2 volts.
If I isolate pin 21 of the CPU, it goes to a low state. It's not an
open collector output AFAICT, so I check the CLKIN and OSCOUT pins
(33 and 34) where the crystal is located. Looks fine at 9.2MHz,
levels look good. Check +5 and +12 and GND - looks good. So, must
be a bad CPU, right? Nope. I place a "known good" CPU in - same
result, no "03" clock output on pin 21. I'm stumped or missing
something here... Anyone have any ideas? Even a "That makes no
sense to me either" would be appreciated.



Is something holding up the "o3" o/p?

By "holding up" do you mean another device on the trace that is
defective? If so, I don't think so - as I said, I lifted and isolated
the pin 21 clock output on the CPU chip (it's in a socket) and it's
output went to zero volts, but that's it. Near as I can tell from the
datasheet, all that could be required for the clock output would be
GND, +5, +12, and an oscillator which appears to be functioning. It
makes no sense whatsoever.

Thanks Graham.

So with the IC pin lifted, does the socket trace still show +2V?


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