B&B Electronics 3PCIU4 - Manual (Page 66)

RS-422/485 Connections/Operation
60 Chapter 8 Manual Documentation Number 3PCIoUx-1008
RS-422/485 Signal Designations
Typically RS-422 and RS-485 interfaces use five lines including two signal
pairs and ground. One signal pair is the transmit pair, labeled
TD(A)- and
TD(B)+ . The other signal pair is the receive pair, labeled RD(A)- and RD(B)+ .
Signal ground is labeled GND .
RS-422/485 Differential Signals
In RS-422 and RS-485 interfaces signals are sent on differential pairs. In the idle, or
Mark, state, the TD(A)- line will be 0V with respect to (wrt) ground and the TD(B)+
line will be about 4.4V wrt ground. Sensing differentially across the two lines, the
voltage will be –4.4V (representing a Mark state). When the transmitter changes to the
Space state TD(A)- goes to +4.4V and TD(B)+ goes to 0V. In that state the differential
voltage will be +4.4V
Figure 44. Differential transmitter/receiver
The receiver input is a differential circuit which senses the difference in
voltage across the transmission line as described above. In addition, any
common mode noise picked up equally on both wires of the twisted pair is
cancelled. Receiver sensitivity is rated at 200mV. A separate signal
ground/common connection provides a common mode reference for the
transmitter and receiver (and is often used to ground a shield in the cable).
These factors provide reliable communications at much greater distances than
RS-232.