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8.3 MultiGen Modbus Protocol
8.3.1 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 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 dif 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.
8.3.2 Communication Format
Baud rate : 2400, 4800, 9600, 19200
Low level character format :
With Odd or Even parity = 1 start bit, 8 data bits, 1 parity
bit, 1 stop bit
With NO parity = 1 start bit , 8 data bits, 2 stops bits.
8.3.4 Modbus Format
Data format :
RTU
32 bit, floating point format to IEEE-754 standard,
exponent bias 127, most significant byte transmitted first.
There is no word reversal.
GENMOM.PM5
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