Hello Glen,

This is mostly taken care of automatically (even if You want it or not)
by the read/modify/write function of I/O pins of all PIC processors.
That is, when you modify a pin in a port by clearing or seting a bit, the
whole port byte is first read, the bit is updated and the whole byte is
written back to the port. When the port value is first read it is the
logical level on the pin that is read and not the output latch in the PIC.

If You have unused output pins in a port and want them to follow what ever they
are externaly forced to just make sure You update another bit in the
port often enough without explicitly seting the unused output bit. If no bit in
port is modified often enough this can be done by MOV PORT_X,f which reads
the portvalue and writes it back again. Any forced output pins will then
change state. Be ware, though, that this may have unwanted effects on used
IO pins which shouldn't be allowed to change it's state. To overcome this,
use a shadow register in ram for the port and whenever the portvalue is to
be updated, modify it first in the shadow register, read the portvalue,
mask off the used pins, OR the result with the shadow register (which
should have the unused bits set to 0) and write it back to the port.
This way only the unused pins will change their state to whatever they are
forced to externaly.

Good luck

> ruben ,
> where you said
> "What I also do, and
> have not seen mentioned here before, is to frequently read the value of the
> outport and write it back. This way the pin won't be destroyed if it is
> forced low when written high (short circuit to ground or something), instead
> the pin state is switched to whatever it is forced to externaly."
>
> thats a good idea.
> how do you do it .for 16f84 & 12c508
> glen
> *******
==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +4640142078
FAX INT +4640947388
ruben@2.sbbs.se
==============================