At 09:09 AM 6/30/04 -0800, you wrote: >Hello All, > >Will someone with a little testing savvy explain the basics of guarding >to me. Let me >give you an example. This board is populated with 74HC, which is not >good stuff to >short high or low, as it is driven both ways.
Most TTL is driven both ways. The databus has eprom, >cpu, & 3 support >chips with /CE going to a 74HC138. That device I desoldered, and the >Eprom was pulled, >to test the support chips. >/RESET on the cpu is driven from a watchdog chip, via a 74HC4049 >/STDBY is wedded to +5V, and things fail. I grounded the /RESET anyhow >and what I'm getting >is a fail like "H(or L) expected - Z received" from every output pin. It may be normal for the device to go into a Tristate (high Z) mode when it is reset. I said WHEN it's reset not afterwards. As long as you're keeping RESET grounded then this is probably normal. You'd have to look at the data sheet to tell for sure. FWIW I've found that the same device made by different manufacturers doesn't always behave them same during reset so you need to look at the actual manufacturer's data sheet if possible. (I found this out the hard way with 555 timers made by Signetics). BTW did you disconnect the logic that drives RESET? It may not like it's output being forced to ground. >The chips are an 8255 and a 6840. >Additionally, the 6840 fails an opens test on random pins Exactly what do you mean "opens tests"? Under what conditions (clock present, RESET state, etc). When you say OPENS do you mean that it's high Z and not logic 0 or 1? > >The board is known to work. It has been tested and returned to me.Am I >safe pulling 74HC o/ps >low? High? It's only switched in fior the functional test, btw, and the >devices are not >continuously powered. I don't understand why you're asking about guarding? Guarding to me means shielding a sensative input from stray signals. Usually by surrounding it with a continous metal circuit trace or metal shielding that's gounded. This has nothing to do with guarding. Joe > >How does one go about solving this sort of thing? > > TIA from the learning curve > > Declan Moruarty > >-- >Author: Declan Moriarty > INET: [EMAIL PROTECTED] > >Fat City Hosting, San Diego, California -- http://www.fatcity.com >--------------------------------------------------------------------- >To REMOVE yourself from this mailing list, send an E-Mail message >to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in >the message BODY, include a line containing: UNSUB CHIPDIR-L >(or the name of mailing list you want to be removed from). You may >also send the HELP command for other information (like subscribing). > -- Author: Joe R. INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
