GE 1601-0149-A2

Modbus Protocol 489

Communications Guide

http://www.GEindustrial.com/multilin 1

GE Multilin

489 Communications Guide

GE Publication Code: GEK-106495A

GE Multilin Part Number: 1601-0149-A2

Modbus Protocol

Electrical Interface The hardware or electrical interface is one of the following: one of two 2-wire RS485

ports from the rear terminal connector or the RS232 from the front panel connector.

In a 2-wire RS485 link, data flow is bidirectional. Data flow is half-duplex for both

the RS485 and the RS232 ports. That is, data is never transmitted and received at

the same time. RS485 lines should be connected in a daisy chain configuration

(avoid star connections) with a terminating network 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 1nF ceramic capacitor

when used with Belden 9841 RS485 wire. The value of the terminating resistors

should be equal to the characteristic impedance of the line. This is approximately

120 Ω for standard #22 AWG twisted pair wire. Shielded wire should always be used

to minimize noise. Polarity is important in RS485 communications. Each '+' terminal

of every 489 must be connected together for the system to operate. Refer to the

489 Instruction Manual for correct serial port wiring.

Modbus RTU

Description

The 489 implements a subset of the AEG Modicon Modbus RTU serial communication

standard. Many popular programmable controllers support this protocol directly with

a suitable interface card allowing direct connection of relays. Although the Modbus

protocol is hardware independent, the 489 interfaces include two 2-wire RS485

ports and one RS232 port. Modbus is a single master, multiple slave protocol

suitable for a multi-drop configuration as provided by RS485 hardware. In this

configuration up to 32 slaves can be daisy-chained together on a single

communication channel.

The 489 is always a slave; it cannot be programmed as a master. Computers or

PLCs are commonly programmed as masters. The Modbus protocol exists in two

versions: Remote Terminal Unit (RTU, binary) and ASCII. Only the RTU version is

supported by the 489. Monitoring, programming, and control functions are

performed with read/write register commands.

Data Frame Format and

Data Rate

One data frame of an asynchronous transmission to or from a 489 is default to 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 parity bit

is optional as odd or even. If it is programmed as odd or even, the data frame

consists of 1 start bit, 8 data bits, 1 parity bit, and 1 stop bit.

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

489 RS485 ports support operation at 1200, 2400, 4800, 9600, and 19200 baud.

The front panel RS232 baud rate is fixed at 9600 baud.