Hi Dennis,

Yeah, I know about the bsf/bcf read-modify-write issue, but it should
only be a problem if something else is driving the pins, right? All
PORTC pins are outputs at this point, and should be the only things
driving the bus. Furthermore, the first listing works, and that uses
bsf/bcf, just in a different place!

Actually, the version I *want* to use is:

        movf    PC_SEND,0         ; get byte to send to PC
        andlw   b'00001111'       ; want to send low nibble first
        iorlw   b'11100000'       ; keep spare bits high, keep bit 4 low

lp1     btfsc   PORTB,7           ; wait for low on PC AUTOFD line
        goto    lp1

        movwf   PORTC             ; write data

        movf    PORTD,0           ; retrieve first byte from the PC
        movwf   PC_RECV_0         ; store first byte
        bsf     PORTC,4           ; *now* raise the handshake line

This should be a bit faster, since the outgoing portc value (minus the
handshake bit) is prepared in W while the PC is processing the previous
nibble. However, this code doesn't work either, despite (as far as I can
see) being functionally equivalent, as far as the protocol goes!

Cheers,
Ben

>         Read write modify instruction BSF PORTC.4
>         Reads the state written to PORTC, then writes it back
>         Try
>
>         andlw   b'00001111'       ; want to send low nibble first
>         iorlw   b'11100000'       ; keep spare bits high, keep bit 4 low
>         movwf   PORTC             ; write data
>          iorlw   b '00010000;
>          movwf    PORTC