3 Sep
2009
3 Sep
'09
12:41 p.m.
But the thing that caught my eye was the HP 548A
"logic clip"--a
device built into a DIP test clip with an LED for each pin. It
featured self-seeking logic and would display the state of all pins
for RTL, TTL, DTL and CMOS logic families. It sounds very cool--and
I have never seen one in the flesh.
I have (both the HP one and clones), but I don't own one. To be honest,
it _looks_ cool, but it's fairly useless. The ones I used had no
pulse-stretching circuitry, the LEDs just displayed the state of each pin
(on for '1', off for '0' and on-but-dim for toggling -- the brightness
depended on the mark/space ratio). It was OK on slow circuity, but that's
easy to debug anyhow. On anything with a reasonable clock frequency, it
didn't tell you much.
HP made a couple of other devices that I don't own and could never see
the point of. One was the 'signature analyser' which will tell you if a
logic signal differes from the correct version (well, provided you have a
'correct version' to also test) but won't tell you _how_ it differs. So
actually finding the fault doesn't seem to be any easier.
The other is the 'logic comparator' which effectively lets you connect a
known-good example of a simple (SSI, MSI) TTL chip to the one under test.
The inputs get the same signals, the outputs of the 2 chips are compared.
Any differences are shown on LEDs. It seems to me to be a very slow way
of findign a fault by just testing each chip, rather than actually
looking for the symptoms and working out what's wrong.
-tony