a) Reset (emulator isolated from reset pin and works).
b) Pullups (emulator "pull better" than chip).
c) clock/crystal (the emulator uses internal).
d) timing right on the edge.
e) configuration errors (emulator set to internal memory when MMIO used)

My original post was going to be "What are the differences between an emulator and a real chip" but it seemed a bit vague.

a) Using Dallas 1812-10 on the PCB.
b) Would that affect the transmit side as well? How could I confirm / check that. Also the same chip works in a prototype PCB (for what it's worth)
c) Using external clock on emulator.
d) Yes, that may make sense.
e) Sorry, not familiar with the abbreviation MMIO, but I believe the emulator is set up as identically to the chip as possible - I'm only using on-chip memory. >>edit Ah. Memory Mapped I/O

Thanks for the time Erik.