74LS139 - Sanity Check....

Alan173

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I'm doing some checking of the chips involved in the RAM circuit on my Rally-X board and need help to kickstart my brain. Probing chip 5A - 74LS139.....getting some strange readings or confusing myself. For example - Ea input is High/Low-pulsing...A0 & A1 are High/Low pulsing as well. However.....outputs 0o, 01 & 03 are sitting high...rest are High/Low pulsing.

Bad chip?

Alan
 
- Ea input is High/Low-pulsing...A0 & A1 are High/Low pulsing as well. However.....outputs 0o, 01 & 03 are sitting high...rest are High/Low pulsing...
I am assuming the following:
Ea=Enable pin=pin 15
A0=Select A1=pin 14
A1=Select A2=pin 13
O0=1Y0=pin 12
O1=1Y1=pin 11
O2=1Y2=pin 10
O3=1Y3=pin 9

if you look at the truth table for the 74ls139 your symptoms as you describe suggest that :
if pin 15 is pulsing, pin 14 is stuck low, and pin 13 is stuck high then outputs are p12=1,p11=1,p10=pulsing,p9=1 and match your symptoms.

Possible scenarios:
If as you say A0/A1 (pins 14/13) are pulsing it could mean that both the xor gate feeding pin 13 AND address bus line AB10 buffered by chip 2H signals are below TTL spec voltage levels for high level input - a less likely scenario than a faulty 74ls139. so possibly a dud chip. check for shorted tracks and replace the 74ls139 to start with.

Assuming that the outputs of the XOR ic and the AB10 line are good, If Ea (enable, pin 15) is pulsing, and output 2 is pulsing , then 1H driving pin 15 is prob ok. once again points to likely stuck inputs for A0/A1 on pins 14,13 of the 74ls139. check for shorted tracks and replace the 74ls139 to start with.

this shouldnt exlude:
- faulty 2H
- 5A pin 14 shorted to ground, possible faulty ic 74xx157 buffer providing buffered ab10 address line to pin 14
- 5A pin 13 shorted to 5v, possible faulty XOR ic feeding pin 13 of 5A

just some thoughts!
 
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Can you give pin numbers?

1 H/L P
2 H/L P
3 H/L P
4 H
5 H
6 H/L P
7 H
8 L
9 H
10 H/L P
11 H/L P
12 H/L P
13 H/L P
14 H/L P
15 H/L P
16 H

The last 2 Defender boardsets I repaired the 74LS139 tripped me up...so I'm interested to hear what you guys think...

Alan

H/L P = High/Low Pulsing
 
Another question.....

The reason I am going through this circuit is I'm getting a RAM failure code 2H...which is position 6B on the board. ROM's test o.k...all new RAM's in place...Z80A was toast so that is new as well. The only unknown is the custom chip...but I flip-flopped it between the boards (6P and 9R) without a change...so at this point I'm assuming it is good and checking into anything feeding 6B...
 
The only true way to test that LS139 is with a logic analyzer... it sounds bad based on the inputs to the chip BUT... if the timing of the signals is right then it could be acting just fine. Only the logic analyzer will show you all the inputs and outputs at once to help you determine if it's really bad.

Of course, it's a lot quicker to just desolder it, put a socket in, and put in a replacement. ;)
 
Righty-O.....

Yeah....been meaning to pick up a logic analyzer for situations just like this. I of course have a pretty significant collection of 74XX chips but used my last 74LS139 last month on another repair! :( So I was hoping to hone in on this problem a little more before placing my next chip order.

That said....I was only going by the fact if inputs on pins 1, 2 & 3 are all pulsing shouldn't the outputs at pins 4 & 5 be pulsing as well? (not stuck high) That was leading me to believe this 74LS139 chip was bad.

At this point everything upstream and downstream from the 74LS139 seems fine....
 
you could achieve a simple test using a 7400 or 74ls00 quad 2 input nand gate and logic probe in lieu of a logic analyser, by effectively ANDing the relevant signals (ie if Enable is low AND A0 is LOW and A1 is LOW and O1 is LOW then output is active HIGH), that can be displayed as an ON pulse on the logic probe when 6B is selected.

or, if you have a dual QUAD input NAND then you only need two gates, one to NAND four inputs, one to invert the output to active HIGH that can be displayed as an ON pulse on the logic probe when 6B is selected.

replied to PM. hopefully my logic was sound ;)
 
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