Hardware Manual for the PCD3 Series│Document 26/789; Version E 5│31.01.2005
Saia-Burgess
Controls Ltd.
Defi nitions of serial interfaces
Appendix
9-3
9
9.2.2 RS 485/422
Signals to RS 485 (RS 422)
VOZ =
0.9 V min ... 1.7 V
VOH =
2 V min (with load) ... 5 V max (without load)
VOL =
-2 V ... -5 V
In the idle state, RS 422 is in the “mark” position
RS 422:
Signal type
Logical state
Polarity
Data signal
0 (space)
1(mark)
TX positive to /TX
/TX positive to TX
Control/
message signal
0 (off)
1 (on)
/RTS positive to RTS
RTS positive to /RTS
RS 485:
Signal type
Logical state
Polarity
Data signal
0 (space)
1 (mark)
RX-TX positive to /RX-/TX
/RX-TX positive to RX-/TX
Not all manufacturers use the same connection configuration, so the data lines may
need to be crossed
To guarantee error-free operation of an RS 485 network, the network should be
terminated at both ends. Cable and line termination resistors should be selected in
accordance with manual 26/740 “Installation components for RS 485 networks”.
5V
2.5V
0V
4V
3V
2V
1V
VO
H
VO
L
VO
Z
not active
e.g. Startbit
/TX
TX
mark
space
= mark
