2-wire RS-485 connects the transmitter and receiver to the same data channel. Many applications are not designed to receive the same data they are transmitting and expect the serial port to be set to No Echo. Therefore, when the serial port is transmitting data, the receiver is disabled. For this reason an external loopback adapter will not work with 2-wire RS-485.
To successfully perform a loop back test with 2-wire RS-485, the easiest method is to connect two adjacent serial ports together and use the BERT function in the Sealevel WinSSD utility. Open two instances of WinSSD by running it twice. Set the first instance of WinSSD to the first port to transmit and the second instance of WinSSD to the second port to receive. Visually verify that the data transmitted is the same as the data received. To complete the loopback test, set the first instance of WinSSD to the first port to receive and the second instance of WinSSD to the second port to transmit. Visually verify that the data received is the same as the data transmitted.
On single port serial adapters configured for 2-wire RS-485, enable Echo to allow the receiver to listen during transmission. This allows you to perform an internal loopback test. No external loopback adapter is required.
Note: Some single-port 2-wire RS-485 adapters do not support Echo. These adapters must be configured for RS-422 or 4-wire RS-485 in order to perform a loopback test.