National Instruments NI PCI-8254R - Manual (Page 36)

Chapter 3 Signal Connections
NI PCI-8254R User Manual 3-10 ni.com
The isolated outputs power connector on the NI 8254R device
accommodates one power supply. The V terminal provides the isolated
output circuitry (5 to 30 VDC) for the NI 8254R device. The C terminal
provides the common-mode signal for the NI 8254R device.
Table 3-4. Power Connector Terminals
Terminal Description
V Isolated power (5 to 30 VDC)
C Isolated common-mode signal