74903 on the 4FDC

[Christopher Mallery]

Has anyone mapped inputs to outputs on the 4FDC's 74903?  I have two 4FDC boards having issues and their 74903s seem to report different outputs given the same inputs so I have no idea what is the correct mapping.  Any help would be greatly appreciated.

Thanks,

Chris

[Jay Cotton]

IC23 is listed as a resistor pack.  

BTW so far I can't find a schematic for the board.

jc

[Christopher Mallery]

Here is the schematic.  It appears to be some form of decoder.

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[rufreaknnuts]

https://groups.google.com/g/cromemco/c/LGwLhsDny3I

Just read my PROM.. doesn't match either of the tables from the above discussion. Bummer.

[William Sudbrink]

Looking at my 16FDC manual (my 4FDC manual is buried somewhere) it’s pretty clear that (in cromemco’s terms) a 74903 is a 32 byte high speed programmed ROM.  The four low bits of the S100 address and the write pin are used to select a byte. The bits programmed into that byte then (along with the card enable from the high address lines) are then used to activate various operations that the card provides.  The ROM monitor, the console serial port, the floppy controller, etc.  It is certainly possible that different revs of the 4FDC had different bytes programmed into that chip.  Or one of your chips could be bad.

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[Jay Cotton]

Hi Bill:

You beat me to it.  It's a TI part.  Can't make out the part number.  It's been scrubbed off.

If we can noodle out the part number we may be able to download the data from the rom

and decode what its up to.

From the schematic it looks like its decoding what function block to address and do I/O with.

Its a 4 line to 8 line decoder.

Inputs are address bits and sWO and a CE called 550I (no idea).   Since the driver is a 74138 and

the output is state 0 we can assume that the input address for that part is 0 so, a4 a5 a6 and a7 are low

when sWO is active.  Also a large portion of the 74903 output is directed to the ic12.

[Jay Cotton]

The last page in the other group text has this spread sheet.

[Christopher Mallery]

One is a 7749 and the other is a 7736

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[Christopher Mallery]

I used the truth table in the excel sheet to program a GAL, now I need to make a carrier socket to make it fit.