first question:
was anything already plugged into the port on the front of the SLC before you tried to connect to it yourself? ...
for example: sometimes a barcode reader or an operator's input terminal is already connected to the port ... if that is the case, then the port has probably been reconfigured to act as a "user" type port – instead of a "system" type port ... a "user" type port cannot be used for connection to a programming type device ...
next question:
with NOTHING plugged into the port on the front of the SLC, watch the LED marked "RS232" on the front of the processor ...
does the LED stay OFF – or does it blink once every three or four seconds? ...
if the LED blinks (with NOTHING plugged into it) then the port has probably been reconfigured to be a DH485 type port – and not an RS-232 type port ...
next question:
do you already have an existing copy of the program file? ... if so, can you post it here? ... (you'll have to zip it first – forum rule) ... if we can look at the file, we can often help you understand how your system is set up ...
finally be very careful with the terminology you're using ... in an Allen-Bradley system:
(1) you UP-load a program FROM the PLC ...
(2) you DOWN-load a program TO the PLC ...
which way are you trying to go? ... TO the PLC? – or FROM the PLC? ...
another tip: do not call the PLC a PC ... many people use the abbreviation PC for "Personal Computer" ... that can lead to all sorts of confusion when you're discussing communication issues ...
and welcome to the forum ...