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I have a 2-wire serial device (RS-232) that I want to connect to an RS-485 serial port. Since RS-485 communication is usually 4-wire, how do I connect it and how do I configure Windows to communicate with the device?
RS-485 is, by default, set up for four-wire communication. Two pins send data (TXD+ and TXD-) and two pins receive data (RXD+ and RXD-) as differential signals as shown below.
To connect a 2-wire device, you must connect the 2-wire device's send pin (TXD) to both TXD- and RXD- and the device's receive pin (RXD) to both TXD+ and RXD+. In the image above, jumper wires would be placed between pins 4 and 8, and between pins 5 and 9.Because the send and receive pins are shorted together on the RS-485 COM port, the COM port must be configured to communicate in 2-wire half-duplex mode. To do this, follow the steps below:
Go to Windows Control Panel
Click on System
In the system menu choose the Hardware tab.
Click on Device Manager
Expand the Ports menu and right-click on the RS-485 serial port in use and choose Properties.
Choose the Port Settings tab and choose Advanced.
Choose the 2-Wire Auto setting from the Tranceiver Mode drop-down menu.
Serial Quick Reference Guide
What Are the Different Transceiver Modes on the NI RS-485 Serial Cards?
How Can I Self-Test My PCI-485i-2 to Check That All Ports Work?
Pins Used for 2- and 4- Wire Transmission with RS-485
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