74S287 Help?

Scucci

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Got a PROM here where everything seems to be working right... all inputs are pulsing like crazy, but outputs at 10 and 11 (Q1 and Q2) are just pegged high.

This is for the (not)SBCLK1 and (not)SBCLK2... brain fart, and I don't remember how to type NOTs in text. lol You get the picture.

Is there ANY combination of inputs that would peg JUST those two outputs high like that? ... Seems like a reeeaaaaalllly long shot, but I just have to ask.

I'm okay with most logic ICs, but when you get to the PROMs and stuff like that... if there's not a logic diagram in my big yellow book, then I'm more or less lost.

I've compared those outputs to the same chip, same location, on a working board and they're pulsing like crazy.
 
A PROM is a memory chip like an EPROM...

You have inputs (address lines)
You have outputs (data lines)
and
You have control signals (CE* and/or OE*)

If the control signals are active low (* or a bar over the top of the name) then they must be low for the chip to be read. Sometimes you'll see some active high and some chips will have active low and active high control pins.

The difference is that a PROM sometimes is 4 bits and it's much faster than a typical EPROM. It also is programmed differently - it's a one shot deal by blowing fuses on the chip to do it.
 
A PROM is a memory chip like an EPROM...

You have inputs (address lines)
You have outputs (data lines)
and
You have control signals (CE* and/or OE*)

If the control signals are active low (* or a bar over the top of the name) then they must be low for the chip to be read. Sometimes you'll see some active high and some chips will have active low and active high control pins.

The difference is that a PROM sometimes is 4 bits and it's much faster than a typical EPROM. It also is programmed differently - it's a one shot deal by blowing fuses on the chip to do it.

I was outside cleaning up around the house, thinking about this stuff off and on... I GUESS if the next 2 chips in the line are stuck high, it would pull the lines high, but each of the lines out (Q1, Q2) go to different chips, and they don't seem to have anything in common. There's a resistor from +5 to SBCLK2* that could account for pulling that line high if the resistor has gone bad like that... But that still wouldn't effect SBCLK1 (gets inverted at the next chip).

I might just have to see about getting a couple of the PROMs burnt and swapping them in and out to test as needed. I'll keep digging to see if there's anything crazy going on down the line.
 
Resistors go open... they don't go shorted.

PROM is probably bad.

Put the bad PROM in the good board and try it there. I wouldn't put the good PROM in the bad board right off the bat without checking for shorts.
 
Resistors go open... they don't go shorted.

PROM is probably bad.

Maybe not shorted, but off tolerance enough to pull the line, is what I was thinking... but that's REALLY a long shot.

I thought about swapping PROMs... was try'ng to put that off... while I'm decent with desoldering... I never trust myself 100%... But, gotta do what I gotta do I guess. I'll give that a shot... if all else fails, hey... two more socketed chips.

Thank you.
 
Yeah, bad PROM.

Put the suspicious one (117) in my good board, no sounds. Put the good one back in... everything's good to go. Well, close... Minus the voices, engine sound, explosing, and tire squeal the other sounds are static... BUT it's given me something to go off of.

So, I guess I'll have to order a couple more PROMs.

BTW... socketing these things SUCKS... the legs are just a liiiiiiitle too short and they refuse to stay in dual-swipe sockets... just a heads up.
 
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