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GE Consumer & Industrial
Multilin
ACTUAL
STORE
SETPOINT
RESET
MESSAGE
RUNNING
RELAY A
RELAY A
RELAY B
RELAY B
STOPPED
VALUE
TRIPPED
AUX RELAY 1
AUX RELAY 1
AUX RELAY 2
AUX RELAY 2
ALARM
AUTO
MANUAL
STOP
START
START
START
START
A
A
B
B
8.1.1
Overview
8.1.2
Electrical
Interface
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL
MM3 Motor Manager 3
Chapter 8: Communications
Communications
8.1
MM3 Motor Manager 3 Modbus Protocol
The MM3 Motor Manager 3 implements a subset of the Modicon Modbus RTU serial
communication standard. The Modbus protocol is hardware-independent. That is, the
physical layer can be any of a variety of standard hardware configurations. This includes
RS232, RS422, RS485, fibre optics, etc. Modbus is a single master / multiple slave type of
protocol suitable for a multi-drop configuration as provided by RS485 hardware. The MM3
Motor Manager 3 Modbus implementation employs two-wire RS485 hardware. Using
RS485, up to 32 MM3 Motor Manager 3s can be daisy-chained together on a single
communication channel.
The MM3 Motor Manager 3 is always a Modbus slave. They can not be programmed as
Modbus masters. Computers or PLCs are commonly programmed as masters.
Modbus protocol exists in two versions: Remote Terminal Unit (RTU, binary) and ASCII. Only
the RTU version is supported by the MM3 Motor Manager 3.
Both monitoring and control are possible using read and write register commands. Other
commands are supported to provide additional functions.
The hardware or electrical interface in the MM3 Motor Manager 3 is two-wire RS485. In a
two-wire link, data is transmitted and received over the same two wires. Although RS485
two wire communication is bidirectional, the data is never transmitted and received at the
same time. This means that the data flow is half duplex.
RS485 lines should be connected in a daisy chain configuration with terminating networks
installed at each end of the link (i.e. at the master end and at the slave farthest from the
master). The terminating network should consist of a 120 Ω resistor in series with a 1 nF
ceramic capacitor when used with Belden 9841 RS485 wire. Shielded wire should always
8–1
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