On Sun, 12 Jul 2009, Eric Smith wrote:
  Which bus signals are different? 
I'm not really certain, but I thought that the differences are the
"+External Horizontal Drive" and "+Printer Clock" signals.
  There are other 5103 models that are intended for
other IBM computers not in
 the 51xx line, and those won't work on the 5100, 5110, or 5120. 
I don't think that these printers are called 5103. They may look similar
but should have different model numbers.
Christian