National Instruments RS485 - Manual (Page 8)

NI 9871 Operating Instructions 8 ni.com
You must connect an external power supply to the NI 9871.
This power supply provides the power for the RS485/RS422
transceivers on the module. You can use the included female
four-position pigtail to connect to an external voltage source.
Figure 1 lists the connections between an external voltage source
(of +8 V to +28 V) and the NI 9871.
Figure 1. Four-Position External Power Connector
1 V
SUP
2 V
SUP
3 COM 4 COM
1
3
4
2