HP6034A TMS9981 CPU won't exit reset state.
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. Many thanks! |
HP6034A TMS9981 CPU won't exit reset state.
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? -- -- Graham W http://www.gcw.org.uk/ XP1800+ Page added, Graphics Tutorial WIMBORNE http://www.wessex-astro.org.uk/ Wessex Astro Society's Website Dorset UK Info, Meeting Dates, Sites & Maps |
HP6034A TMS9981 CPU won't exit reset state.
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. |
HP6034A TMS9981 CPU won't exit reset state.
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? -- -- Graham W http://www.gcw.org.uk/ XP1800+ Page added, Graphics Tutorial WIMBORNE http://www.wessex-astro.org.uk/ Wessex Astro Society's Website Dorset UK Info, Meeting Dates, Sites & Maps |
HP6034A TMS9981 CPU won't exit reset state.
On Wed, 12 Sep 2007 16:56:19 +0100 "Graham W"
wrote in Message id: : 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? Yes, but in my experience that would be expected, as TTL inputs would do that if nothing was driving them. |
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