GE MM300 RS485 interface (Modbus RTU)

2MM300 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE

RS485 INTERFACE (MODBUS RTU) COMMUNICATIONS GUIDE

RS485 interface (Modbus RTU)

The RS485 interface is a serial two-wire port intended for use as a Modbus RTU slave. The

RS485 port has the following characteristics.

• Address: 1 to 254

• Baud rate: 9600 to 115200 bps

• Supported Modbus function codes: 3, 4, 5, 6, 7, 8, 16

The MM300 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 MM300 Modbus

implementation employs two-wire RS485 hardware. Using RS485, up to 32 MM300s can be

daisy-chained together on a single communication channel.

The MM300 is always a Modbus slave. It can not be programmed as a Modbus master.

Computers or PLCs are commonly programmed as masters.

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 MM300 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 W 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 MM300s 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.

NOT

NOTE: Polarity is important in RS485 communications. The '+' (positive) terminals of every device

must be connected together.

One data frame of an asynchronous transmission to or from a MM300 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).

Modbus protocol can be implemented at any standard communication speed. The

MM300supports operation at 9600, 19200, 38400, 57600, and 115200 baud.

A complete request/response sequence consists of the following bytes (transmitted as

separate data frames):