We're getting a bit off into the weeds but I've got a reminder about serial ports in general that also applies to the PLC-5 Channel 0 port.
Your PC and the PLC are both "DTE" type devices, so generally you need a "null modem" cable to connect one to the other directly. Tx on one connector is connected to Rx on the other, and vice-versa. The Data Common pin is connected straight across.
For ordinary PLC upload/download applications, only those three pins (Rx, Tx, Signal Ground) are necessary. No RTS/CTS or DSR/DTR or loopback jumpers are needed.
On a DB9 <-> DB9 cable, most of us have the pins memorized. A null modem cable is:
Rx(2) - Tx(3)
Tx(3) - Rx(2)
Sg(5) - Sg(5)
But the PLC-5 Channel 0 port is a DB25 port, and the standard DTE pinout for a DB25 port is the reverse of a DB9 port on Pins 2 and 3. And Signal Ground on a DB25 port is Pin 7, not Pin 5.
So a "null modem" cable like the 1784-CP10 does not "swap pins" on the relevant conductors.
A PLCTalk thread with the appropriate pinout diagrams, from back-in-the-day:
http://www.plctalk.net/qanda/showthread.php?t=26918