Communications; Mm2 Modbus Protocol; Overview; Electrical Interface - GE MOTOR MANAGER 2 Instruction Manual

6 COMMUNICATIONS

6.1 MM2 MODBUS PROTOCOL

6 COMMUNICATIONS
The MM2 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 stan-
dard 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 MM2 Modbus implementation employs two-wire RS485 hardware. Using
RS485, up to 32 MM2s can be daisy-chained together on a single communication channel.
The MM2 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 MM2. 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 MM2 is two-wire RS485. In a two-wire link, data is trans-
mitted and received over the same two wires. Although RS485 two wire communication is bi-direc-
tional, 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 termi-
nating 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 be used to minimize noise. The shield
should be connected to all of the MM2s as well as the master, then grounded at one location only.
This keeps the ground potential at the same level for all of the devices on the serial link.
Polarity is important in RS485 communications. The '+' (positive) terminals of every
device must be connected together.
NOTE
See Figure 2–6: RS485 TERMINATION on page 2–7 and Chapter 2: INSTALLATION for more infor-
mation.
One data frame of an asynchronous transmission to or from a MM2 typically consists of 1 start bit, 8
data bits, and 1 stop bit. This produces a 10 bit data frame. This is important for transmission through
modems at high bit rates (11 bit data frames are not supported by Hayes modems at bit rates of
greater than 300 bps). The MM2 has the capability of adding an odd or even parity bit if necessary.
Modbus protocol can be implemented at any standard communication speed. The MM2 supports
operation at 1200, 2400, 4800, 9600, 19200, and 57600 baud.
GE Power Management

6.1.3 DATA FRAME FORMAT AND DATA RATE

MM2 Motor Manager 2
6.1 MM2 MODBUS PROTOCOL

6.1.1 OVERVIEW

6.1.2 ELECTRICAL INTERFACE

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