Re: 68000 Pod Query

At 09:16 PM 6/5/04 +0100, you wrote:
>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 :-(

  I would remove the Fluke and manually check the line to verify the short.
Check the line both with power off and power on. (One of the gates with an
output connected to the line may have something driving it's input such
that it's output is pulling the line down. A hard short will be there even
with power off.) Then use a HP Current Probe to see where the short is.

   Joe
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Received on Sat Jun 5 17:07:33 2004