EPROM Data Output

XianXi

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Yesterday I noticed an EPROM on a PCB I was fixing was flashing very faintly with the logic probe on the data output lines (27C1001). What causes the output to do that? Is it the component the output is maybe connected to? All the other roms on the board flash nice and bright.

I tried replacing the rom and got the same results on the data outputs with the new chip.

I thought it was kinda strange.
 
Thats the problem with logic probes, you are still partially blind, a scope would let you see what the signal actually looks like and you could tell if it was unwell or not.

You need to see what the /OE pin is doing, if it is high then the ROM is held in a mode where it doesnt drive the data pins at all, so what you are seeing is something else talking on the bus. Assuming you have found a fault (which is not certain at all) then the issue is upstream of the ROM as you have replaced that already.

I would focus on what the /OE pin is doing to see if the ROM should even be driving the bus.
 
If you put a scope on it you'll see very narrow pulses. This is a spot where a scope will show you more than a probe. It happens.

I'd take a look at the schematics or in Caesar to see what the ROM is for - GFX, CPU, etc...
 
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