17
ST5/10-Si,-Q,-C, IP Hardware manual
920-0004 Rev. C
4/21/11
RS-485 Four Wire System
+RX- +TX- GND
+RX- +TX- GND
+RX- +TX- GND
to PC TX+
to PC TX-
to PC RX+
to PC RX-
to PC GND
Drive #1
Drive #2
Drive #3
Connecting to a host using RS-485 option card
RS-485 allows you to connect more than one drive to a single host PC, PLC, HMI or other
computer. It also allows the communication cable to be long (more than 1000 feet). But
the device to which you connect must have an RS-485 port.
Pin diagram is shown to the right. Wiring diagrams can be found on the next page. We
recommend the use of Category 5 cable. It is widely used for computer networks, it is
inexpensive, easy to get and certified for quality and data integrity.
The ST drives can be used with either two wire or four wire RS-485 implementations. The connection can
be point to point (i.e. one drive and one host) or a multi-drop network (one host and up to 32 drives).
Four Wire Systems utilize separate transmit and receive wires. One pair of wires must connect the host
computer’s transmit signals to each drive’s RX+ and RX- terminals. Another pair connects the TX+ and TX-
drive terminals to the host computer’s receive signals. A logic ground terminal is provided on each drive
and can be used to keep all drives at the same ground potential. This terminal connects internally to the DC
power supply return (V-), so if all the drives on the RS-485 network are powered from the same supply it is
not necessary to connect the logic grounds.
You should still connect one drive’s GND terminal to the host
computer ground.
Four wire systems are better than two wire types because the host can send and receive data at the same
time, increasing system throughput. Furthermore, the host never needs to disable its transmitter, which
simplifies your software.
RX+ RX
–
TX
–
TX+
GND
RS-485/422
