Mark Crosbie wrote:

> I realise that the serial backpack needs inverted serial data - which
> requires swapping any bcf and bsf opcodes.  However, does this include
> inverting the start and stop bits?

  In this context, start and stop bits are in fact "data", and you must
indeed swap *any* bcf and bsf opcodes.  It would be much more useful to
summarise the situation as "the serial backpack needs mark-negative
serial communications".  It is expected that you would feed it with one
of the inverting (almost all are) RS-232 interfaces such as 14C89 or
MAX-232.

> There data is generated, but then it suddenly goes dead (after about 2
> seconds) - no more output is generated.

> loop:
>         movlw 170               ; alternating 1 and 0 pattern
>         call txbyte
>         goto loop
> temp    dt 0

  "temp" appears to be a label to a program instruction, whereas I think
you wanted it to be a file register.  Just maybe if you allocate it to
a file register somewhere, things might be different!
--
  Cheers,
        Paul B.