Solution:

Sometimes certain RS-232 device needs to pull high DCD pin, then the communication can be going. But ADAM-4520 just provides RX, TX, GND and RTS signals, the others are useless except loopback connections. When this type of RS-232 device connects to ADAM-4520, the communication could be failed because DCD isn't pulled to high. There are two ways to improve this situation: 1. Send DTR signal from RS-232 device The detailed pin connections of RS-232 connector on ADAM-4520 is shown as follows: TX (connect to RX of RS-232 device) RX (connect to TX of RS-232 device) GND (connect to GND of RS-232 device) RTS--CTS (connected together, connect to RTS of RS-232 device) DTR--DSR--DCD (connected together) According the above pin connections of ADAM-4520, you can use pin-to-pin DB9 cable to link RS-232 device and ADAM-4520. After that, please send DTR signal from RS-232 device to ADAM-4520, then DCD of RS-232 device will be pulled to high via internal loopback of ADAM-4520 (DTR#1 => DSR#2 => DCD#2 => DCD#1, #1 means RS-232 device, #2 means ADAM-4520) 2. Connect proper voltage to DCD directly If your RS-232 device can't send DTR signal, you can adopt this way to overcome it. Please open the shell of ADAM-4520 and look for C3, C4 labels on PCB (close to RS-232 connector), then you can measure about -7.6V at the right point of C3 and +8.7V at the right point of C4. After that, you can connect the proper voltage to DCD pin of RS-232 connector on ADAM-4520.