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MOONS’
MSSTAC6 Step Motor Drive User Manual
MOONS’
www.moons.com.cn
RS-485/422 allows more than one drive to be connected to a single host PC, PLC or other computer
with an RS-485/422 port. It also allows the communication cable to be long (more than 1000 feet [300
meters]).
A Short tutorial on RS-485 & RS-422
RS-485, in the strictest definition, is a 2 wire interface that allows multi-node connections limited to
half-duplex serial communications. Up to 32 nodes that both transmit and receive can be connected to the
network. RS-422, also in the strictest definition, is a 4 wire point-to-point connection that allows full-duplex
serial communications when connected to a single node. RS-422 has one node that is the driver or
transmitter and up to 10 nodes that are receivers. RS-422 was not designed for a true multi-node network.
Two wire interfaces also require a network node, master or slave, capable of tri-stating its transmitter
to allow other node to use the network when required. For high speed baud rates this must be done very
quickly to avoid communications collisions
Four wire interfaces can go beyond simple point-to-point connections and communicate on multi-node
networks if the slave nodes are also capable of tri-stating their transmitters. Some RS-485 devices are able
to do this and can be used in a 4-wire configuration
On MOONS' servo drives, the RS-485 can be implemented with either a 2-wire or a 4-wire interface, but
communication is still limited to half-duplex because of the serial communications protocol being used. A
4-wire interface is often easier due to the larger number of host RS-232 to RS-485 adaptors which support
it. A 2-wire interface may require a special host adaptor that supports Auto Enable of the adaptor transmitter,
as PC software does not typically support tri-stating the output of serial adaptors.
In both 2-wire and 4-wire systems one extra wire is always required to connect the grounds of all the
nodes on the network. Even though, in both cases, the signals are sent differentially, a common ground
connection is absolutely required.
Proper cable shielding must not be neglected. The high voltage, high frequency, high current signals
present on the servo motor cables can emit a significant amount of electrical interference. Without proper
shielding these signals can disrupt even “noise tolerant” differential line drivers.
4 wire RS-485/RS-422 Network
On the MSSTAC6, RS-485 is implemented as a RS485 implementation is a 4-wire multi-drop network
with separate transmit and receive wires. One pair of wires connects from the host computer’s TX+ and TX-
signals to each drive’s RX+ and RX- terminals. Another pair connects the drive’s TX+ and TX- terminals to
the host computer’s RX+ and RX- signals. A common 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. One drive’s ground terminal should be connected to the host
computer ground. Before wiring the entire system, each drive must be connected individually to the host
computer and assigned unique address. Proceed as follows: (Also see figure #1 below)
1. Connect the drive’s TX+ to the PC’s RX+ terminal.
2. Connect the drive’s TX- to the PC’s RX-.
3. Connect the drive’s RX+ to the PC’s TX+.
4. Connect the drive’s RX- to the PC’s TX-.
5. Connect one drive’s GND to the PC’s GND.
Getting and Connecting an RS-485 4-wire adaptor to your PC
