11
Rev. 1.0
8/26/2015
STM11 Hardware Manual
3.2 Connecting the STM11 Communications
The STM11 is available with RS-422/485 serial communication. Below are descriptions of how to
interface RS-422/485 to a PC.
3.2.1 Connecting to a host using RS-422/485
RS-422/485 communication allows connection of 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 300 meters or
1000 feet). The use of Category 5 cable is recommended as it is widely used for computer networks,
inexpensive, easily obtained and certified for quality and data integrity.
The STM11 can be used with either Two-Wire or Four-Wire RS-422/485 implementation. 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 Configuration
Four-Wire Systems utilize separate transmit and receive wires. One pair of wires must connect
the host’s transmit signals to each drive’s RX+ and RX- terminals. The other pair connects the
drive’s TX+ and TX- terminals to the host’s receive signals. The logic ground on each drive shares
with the same potential of power ground (V-). So if all the drives on the RS-422/485 network
are powered from the same supply it is not necessary to connect (V-) as the logic grounds. One
drive’s (V-) terminal should still be connected to the host computer ground.
Because the host in a four-wire system never needs to disable its transmitter, software is
simplified. Some converters make this process very difficult to implement and can delay
communications.
to PC TX-
to PC TX+
to PC RX-
to PC RX+
to PC GND
120
Drive#1
Drive#2
Drive#3
GND
RX-
RX+
TX-
TX+
GND
RX-
RX+
TX-
TX+
GND
RX-
RX+
TX-
TX+
RS-422/485: 4-wire system diagram
NOTE: If the PC does not have an RS-422/485 serial port, a converter will be required.
NOTE: Send SCL Command “PR261” to the drive will enable the full duplex communication.
