Hmm... I would probably be at the point where I'd carefully snip just the pin (for whatever data line) until I found the culprit. How many 374s are you talking about? 4 or 5? That's not too bad and not too much of a hack if you're careful and have good snips. I've never found the bus check on the Fluke to be helpful... ever :-) I take it when you read a given address, you always have this bit set to 1. (I always like to confirm with the Fluke probe sync'ed to data ... makes it very easy to catch a problem when the bit is stuck on the output of a buffer chip, but that doesn't seem to be your problem.) Before snipping, I'd just double-double check. I hate cutting things without knowing for sure that I'm on the right track. But if you're always seeing this bit set at 1, then you're probably right... I've never found a great technique for isolating these types of problems. When you throw your scope on that node, is it always 1 as well? JB --James Bright www.QuarterArcade.com Restored Arcade Games for your Home
-----Original Message----- From: owner-techtoolslist@flippers.com [mailto:owner- techtoolslist@flippers.com] On Behalf Of Phillip Eaton Sent: Saturday, June 05, 2004 4:16 PM To: techtoolslist@flippers.com Subject: 68000 Pod Query
I have a broken pinball motherboard here that I'm trying to fix. I'm using a Fluke to spy on the data bus because it has a line stuck high.
Connected to the data bus there appears to be: 1 EPROM 1 RAM 1 Resistor pack A couple of 74HC244's bringing in the switches matrix data A load of 74AC374's driving the transistor-controlled outputs
I've isolated the EPROM, RAM and 244's from the circuit and it makes no difference.
I don't really fancy removing all the 374's so I thought I'd ask if anyone knows whether the 374 IC would ever fail in a state where it would drive a bus line high from an input to that IC? I could understand an output from a 244 doing it but they have all been isolated.
Do resistor packs die very often?
Is there a better way of testing for a stuck bit other than snipping the pins of the ICs :-(
The processor on the PCB is a 68000 and when I use the Fluke and 68000 pod to check the bus, it says it's OK, but every manual read from the databus gives a stuck bit. Has anyone ever seen the 68000 pod do this?
TIA! Phillip Eaton
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