🚀 go-pugleaf

     I'm currently working on a custom board that uses 8 KBytes of the
IBM PC/AT's memory space.  We're running under DOS 3.1 and so are constricted
to the 1 MByte limit of the 80286 in Real mode.  Our original intention was
to map the 8K block into the E000 segment since that's where the unused
ROM sockets on the AT motherboard are mapped.  We tried this and found that
there was some kind of bus contention which prevented the custom board from
working correctly.  We could write to it, but when we tried to read, the data
bus (on a logic analyzer) had the appearance of undergoing a tug of war between
states.  The board, when mapped to the 8000 segment (with our extra memory card
removed) worked fine.

     The other cards in the system include two parallel/serial cards, an
IBM EGA card, an Intel Above board memory card, and the disk controller card.
We suspected that the memory card may have been causing the problem, but this
was ruled out when we tried mapping to the E000 segment with the Intel card
removed.  The only thing left that we suspect is the EGA card.  Without our
custom card in the unit, the E000 segment appears to be floating; i.e., the
data read is usually all 1's, although there are occassional 0's (not always
consistant).  With our card mapped to E000, our card, when trying to place
0's on the bus, fights with whatever is putting out those 1's.

     Does anybody have an answer for this one?  I don't have the EGA technical
documentation immediately available, but I'm not convinced that that's what it
is anyways.  Any suggestions would be appreciated.

                                    Jim Curl
                                    Watkins-Johnson, San Jose