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6 COMMUNICATIONS
6 COMMUNICATIONS 6.1 MM3 MODBUS PROTOCOL
The MM3 implements a subset of the Modicon Modbus RTU serial communication standard. The Modbus pro-
tocol is hardware-independent. That is, the physical layer can be any of a variety of standard hardware config-
urations. 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 Modbus imple-
mentation employs two-wire RS485 hardware. Using RS485, up to 32 MM3s can be daisy-chained together on
a single communication channel.
The MM3 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.
Both monitoring and control are possible using read and write register commands. Other commands are sup-
ported to provide additional functions.
The hardware or electrical interface in the MM3 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 bi-directional, 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 be used to minimize noise. The shield should be connected to all of the
MM3s 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 2–4: RS485 TERMINATION on page 2–5 and Chapter 2: INSTALLATION for more information.
One data frame of an asynchronous transmission to or from a MM3 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 MM3
has the capability of adding an odd or even parity bit if necessary.
Modbus protocol can be implemented at any standard communication speed. The MM3 supports operation at
1200, 2400, 4800, 9600, 19200, and 57600 baud.
GE Power Management
6.1.3 DATA FRAME FORMAT AND DATA RATE
MM3 Motor Manager 3
6.1 MM3 MODBUS PROTOCOL
6.1.1 OVERVIEW
6.1.2 ELECTRICAL INTERFACE
6
6-1
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