things to check
is the RS232 light flashing when your connected?
If not null modem your connection or swap 2 and 3 on your cable.
Go to RSLinx, select a Rs232 DF1 driver, Select the correct comm port on your pc, select the device=slc, push the autoconfig, if it does not find it, If not null modem your connection or swap 2 and 3 on your cable.
If the rs232 is flashing then its a baud rate issue
Under RsLogix500 this is generally what it is set to below
general tab = mode=system,
channel 0 - system = DF1 full duplex, Baud = 19200 <<change here, parity = none, stop bits=1, protocal control = no handshaking,
Go to device manager and see if you have problems with your comm port.