Joe R. wrote:
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.
Oh I see. But the high drive is surely limited in some way - there's no current behind it.
The databus has eprom,I went away. I have to keep work flowing through, with or without this new toy.
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.
> Exactly what do you mean "opens tests"? Under what conditions (clockpresent, RESET state, etc).
> When you say OPENS do you mean that it's high Z
/looks it up.and not logic 0 or 1?
An opens test is for input pins. If inputs won't switch, the truth table is modified accordingly,
and the bit of the chip that isn't being used isn't tested.
Guarding is the only name these guys gave the technique of protecting a chip test from
unwanted influence from other chips. I don't know why they used a name that was
already around meaning something else. Lack of Imagination?
-- Author: Declan Moriarty INET: [EMAIL PROTECTED]
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