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.