7 Sep
2006
7 Sep
'06
2:28 p.m.
Allison wrote:
That's good.
If I had only
tested a few devices, maybe I would agree. But in total I
tested
12 TI 74175 and 15 SIG 74S175. Each device behaved consistently. I could
never
get a TI '175 to pass (meaning yanking a high Q~ to gnd forced Q high);
and I
could only get the SIG 'S175 to pass with a pullup to +5V on Q. The
'best' S175
I found would work with a 2.2K pullup. To get them all to work reliably
I had
to go down to 470ohm pullup (~10mA current).
I also find it somewhat suspicious in that the BOM for the PDP-8f/m LED
console
panel calls out a plain 74175 device for these parts (rev F board etch).
However
in the three panels which I have, they are all BUILT with SIG 74S175 devices
of various date codes in 75 and 76. None of these parts looks to have been
previously reworked (or else someone did a real nice job; hard to tell
sometimes).
I find it somewhat fishy the BOM and the board have different parts; DEC was
usually a little more reliable than this.
But like I said earlier all the devices pass OK on the IC tester.
To
be clearer, the numbers I quoted just above are not databook numbers but
ACTUAL numbers I measured in circuit on the 7404 in my board (I have access
to all the inputs since I socketed both 74175 parts). I used a small diode
(1N4148) in series with my multimeter to measure input high/low currents.
I plan
to do that, just replacing the two S175 positions with sockets,
and I'll
dead bug the resistors over the top of the S175s. That way, someday when
I come
across some of these 'magic' 74x175s :-) that work without pullups I can
replace
them easily.
Don North wrote:
I had a 470ohm pullup to +5V on MR~ for all the tests, so there was no issue
WRT inadvertent or floating MR~ forcing the Q output low.
Prior to using all the '175 parts I ran them thru my PROM programmer /
IC tester
to validate they are good. All the 74x175s I used tested 'good' at both
4.5V and
5.5V VCC. I even added additional tests (repeated pattern sequences) to the
tune of about 25 sequential vectors total. All the functions (clocking,
clearing, holding data) tested out OK on all the parts I used. I only found
one bad part (an old TI 74175 dc 87; one Q output seemed to be floating).
Well, I am seeing something very different here.
I built a breadboard
with the circuit under question: 74x175 device, 470ohm to +5V on MR~,
CLK and Dx inputs forced to ground. I connected voltmeters to the
Q and Q~ outputs, and then tried forcing Q and Q~ alternately to ground
to see if I could change the state of the device.
I had a number of devices to test, here are the results:
DATE
MFG DEVICE CODE RESULT
--- ------ ---- ---------
TI 74175 87 FAIL, shorting Q~ to gnd never changes Q to high
SGS 74LS175 82 ditto
TI 74AS175 87 ditto
SIG 74S175 84 PASS, with 470ohm pullup to +5V on Q req'd for Q0-Q3
SIG 74S175 76 PASS, Q1-Q3 work w/ no resistor, Q0 requires 470ohm
For the failed devices, I tried with no pullup, and 100, 470, 1K, 4.7K
pullups to +5V on Qx. No value of pullup made any difference. Shorting
a Q~ signal (at ~4V) to ground never changed Q (at ~0.4V) to a HIGH.
It the MR/ is not in the correct state (may need pullup) I'd conclude
you have some bad parts. Especially the TI[I have the most data on those]!
I'd
give the 7404 the hairy eyeball! A quick test is socket a '175
with the Q and /Q output pins floating and using a jumper to ground
make it flip [It WILL NOT IF MR/ is asserted, you can bend out the
MR/ pin to avoid that.]. Then test the '04 for input changes output.
Allison
The 7404 on the output seems OK, as is the rest of the downstream logic
(the priority encoder). I measured the input currents required on a
suspect '04 input to set the output high and low and they are well within
spec (about +20uA for input high, -0.7mA for input low). With the 74S175
out of its socket I could set all the 7404 inputs H/L and observe the
downstream priority encoder outputs were just as expected. I have about 15 of the 1984 Signetics 74S175s, I
tried all of them in
the console board socket; none of them worked, even a little bit, with
no pullups added.
I'd pulse them slow with Q/ connected to D and see if they toggle.
I suspect you have a bad run of old chips. I just tossed a few tubes
of mid 80s NOS parts as they apprently died of silicon rust [moisture gets
into the plastc and they die].
So I added 470ohm pullups to +5V on the 74S175 Q
outputs to 7404 inputs.
Everything started working as would be expected. The switch decode logic
is now 100% functional.
You have something really messed up with those 175s your testing. My thought is that the output pullups on the
74S175 Q pins are trying to
pull those outputs high fairly strongly (10mA load) but the Q output can
still drive a valid low (it is a 20mA schottky driver). This pullup
'preloads' the output, so that a kick on the QB~ (by shorting to gnd)
gets the Q output moving high, the resistor keeps it moving high, able to
override some smaller internal driver trying to keep the output low.
At least that is the only rational explanation I can think of right now.
Sounds like you have some '175s with the upper device fried in the totem
pole outputs.
Just the 7404
by itself is a -1mA low, +40uA high load; not very much.
Unless it has an input with high leakage to Vcc or ground, i've seen both. In any event, the fix is simple (three resistors
really, but I'll add one
on each of the six used outputs). Turns out all the original logic chips
appear to be good (I had removed the original 74S175 intact for testing).
Thanks to everyone for all the suggestions and helpful hints.
Put all the parts on a header and plug it in rather than mess the board up.
Allison