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How to make a RS232 to RS485 Port Check

To check if your RS232 to RS485 converter (or USB to RS485 converter) is working properly you can make a loop-back test using a terminal program such as AccessPort. This way you can determine if the converter can send and receive characters properly.

If you have a RS485 converter without an 'Echo On' option, you will need a second RS485 port or converter to receive the signal transmitted by the first converter OR to transmit a signal to be received by the first converter. This is because a 2-wire RS485 port is half duplex; it either transmits or receives, but not both at the same time. While transmitting, receive is disabled, while receiving, transmit is disabled. The Data A(-) line from one converter is connected to Data A(-) on the other, and the Data B(+) from the first is connected to Data B(+) on the second.

First connect the converters. Following is a schematic of how a RS232 to RS485 converter test can be setup. If your computer does not have 2 serial ports you can use a USB to RS485 converter for COM2 instead; or even two USB to RS485 converters instead.

rs485 test

Following is the same loop-back test using USB to RS485 converters

Please refer to the datasheet for your particular converter in regards to correct pin configuration.

After you have made the loop-back connections on the converter you can now open two instances of AccessPort, one for each converter. AccessPort is a terminal software which allows you to send and receive characters to and from the converter. The software can be downloaded here.

Within AccessPort you can now open the COM ports to where you have connected the converters (check the COM port number in Device Manger if you use a USB to RS485 converter).

accessport window1

You can now enter a string of characters (in the lower window) and click the Send button to send the data to the COM port of the first converter. If the converters are working properly, and the connections are made properly, the characters will be sent out of the first converter and into the other converter and will then show up in the second AccessPort window (in the upper window). This confirms that the converters are sending and receiving data.


If data is received and transmitted in both directions, the ports/converters are most likely working properly.