I have a strange problem with interfacing a UART (16C550) with a 87C54X2. I'm sending data to and from a PC with it, and all appears to be well (If I send data from the 8051 I get the right data at the PC) except if I send data to the 8051, when I get an odd problem.

If I send 82H the '51 receives 12H >>edit I meant to say 02H

If I send CFH the '51 gets 4FH and so on.

I then tried sending FFH, and lo and behold the '51 receives 5FH. Seems obvious so far - my D7 and D5 lines between the UART and the '51 are bad..... except if I try it with my MetaIce emulator I get the correct data to the '51. If I try it with a prototype PCB (but the same layout) I get the correct data.

When I'm not using the emulator I know that the data is wrong because I'm echoing it back to the PC. I'm pretty sure it's arriving at the '51 incorrectly because I'm doing a comparison in the '51 which fails.

Any suggestions (for possible problems, or further problem analysis) warmly received. It's pretty hard to faulfind when it all works fine with the emulator!

Thanks,

Rob.