Multitek MultiLed M812-LD Series Operating Manual page 50

5.0 RS485 COMMUNICATION
The RS 485 allows remote reading and programming of the
MultiLed via a host computer. The communication protocol used is
a subset of Modicons Modbus, enabling the use of standard host
computer, PLC and scada programs. Up to 32 MultiLeds can be
connected in parallel to a single communication bus
5.1 MultiLed Modbus Protocol
5.1.1 Physical Connections
The interface to the Modbus network is via RS485 connection. This
takes the form of a twisted pair cable, preferably screened, onto which
up to 32 devices may be connected, one of which will be the host PC or
controller node. The devices on the network are connected in parallel
+ to +, - to - and shield to shield ( screen to screen). The shield should
be connected to earth at one point of the network only. Other
manufacturers of devices intended for use on the Modbus network
may have different + and - designations : some have Hi, Lo or A ,B. In
any case reversing the + and - connections may have the desired results
if the device fails to communicate with the master node.
The maximum length of the twisted pair cable specified by the RS
standard is 1.2km. Any length over 200m should be terminated at each
end with a resistor equal to the cable's characteristic impedance in
series with a 0.1µF capacitor.
A typical network cable is Beldon 8761.
5.1.2 Communication Format
Baud rate : 2400, 4800, 9600, 19200
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