Bios copy not working

[neazoi]

hello I have an IBM PC-XT 5160 motherboard and I have copied the bios from these 27256 eproms. When I reprogram the bios to a 27C256 eprom it does not work on the computer but the willem programmer verifies that the chip has been programmed ok.

The BAD thing is that I re-insert the original chip on the motherboard and now it does not boot at all! I have only read the original eprom.


Please tell me what should I do?
I thought just reading original eproms cannot harm the program inside the eprom....

 

[eltonjohn]

see with a scope if you have any bus activity .Something happened to you board with the new eproms. I never had any problems copying eproms . See if your pin voltages are there .

 

[neazoi]

see with a scope if you have any bus activity .Something happened to you board with the new eproms. I never had any problems copying eproms . See if your pin voltages are there .

I compared the copied bios with the eprom readed bios and two locations are different... maybe something went wrong when I mixed original and copied chips...

 

[betwixt]

When the system starts up, it does a quick count through the BIOS, adding all the bytes it finds. Normally, the count is 16-bit so it will roll over many times when so many bytes are added into the total. The result should be zero, if it isn't the BIOS is deemed faulty and the system will not start. Somewhere in the chip there s a location reserved to to hold the value that adjusts the total to be zero. It only takes one bit to be wrong for the total to indicate a failure.

Brian.