> Can anyone tell me the requirements for programming the C54 in circuit,
> what do you need to do with the MCLR pin. I can't find any data in the
> microchip data book. I take it that the C54 can be programmed serially
> as it can be programmed with the D. Tait type of programmer.

The 16C54 requires that you control--literally--all of its pins during
programming (well, Vss remains grounded, but...)  One person has posted
on the list that they in fact designed a circuit which would allow ISP'ing
the 54 even under those conditions, but I doubt such a thing would usually
be practical.

To program the '54, you need to power the thing up and take /MClr very
quickly from zero to VPP (about 13 volts I think).  After that each pulse
of the RTCC input will either output the contents of address zero onto
pins RA3-RA0:RB7-RB0 or else try to "burn" the contents of those pins into
address zero.  You should alternate between the read and write until add-
ress zero reads correctly.  After that, you should hit the /OSCin pin to
advance to the next memory location; RTCC will toggle between reading and
writing that.  Continue in this fashion until you have done the entire
chip.