The solution is to use a null-modem connection as shown below. To summarize everything, here's a typical communications session . Both devices are powered up. The external device responds by turning on it's CTS which says it's ok to for the plc to send data. The plc sends the data on its TD terminal and the external device receives it on its RD terminal.
Some data is sent and received.
After a while, the external device can't process the data quick enough. So, it turns off its CTS terminal and the plc pauses sending data. The external device catches up and then turns its CTS terminal back on. The plc again starts sending data on its TD terminal and the external device receives it on its RD terminal. The plc runs out of data to send and turns off its RTS terminal.