When you tell Logicmaster to go online with the PLC, it will send a message out the serial port to try to establish a connection. If your serial cable has LEDs to indicate traffic on the port, you will see the LED for the transmit line blink. In the absence of LEDs on your cable, you could try connecting the cable to another computer running a terminal emulator like Hyperterminal and see if you can see any characters received there.