GE EPM 9650/9800 manual Introduction, Communication Packets, Slave Address and Broadcast Request

Chapter 1

Modbus Protocol Overview

1.1: Introduction

EPM Monitors can communicate with other devices using the RTU transmission mode of the AEG Modicon Modbus protocol. Communication is available through RS232 or RS485 standards.

•RS232 communication supports a single connection between one EPM Meter and one other device, available on the EPM Meter’s Port 1 ONLY.

•RS485 communication supports multiple EPM Meters connected on a network. It is a two-wire connection operating up to 115200 baud, available on Ports 1 through 4.

•See the EPM 9450/9650 and EPM 9800 Operation and Installation manuals for wiring details.

1.2: Communication Packets

Communication takes place between a Modbus Master and one or more EPM Slaves. The Master initiates all communication by transmitting an information packet called the “request” to a specific Slave. The Slave replies with its own packet, called the “response”. A packet is a serial string of

8-bit bytes consisting of the following:

•	Slave Address	1 byte
•	Function Code	1 byte
•	Data	N bytes: high-ordered byte first, low-order byte second
• CRC (RTU Error Checksum)		2 bytes
•	Dead Time	3.5 bytes transmission time

A single packet can transmit a maximum of 127 registers.

1.3: Slave Address and Broadcast Request

Each Slave device on a communication bus has its own unique address. Only the Slave addressed by a Master will respond. The response packet returned to the Master will have the same value in the Slave Address Field as the request packet. Addresses are programmable and range from 1 to 247.

A Slave Address of 0 is a broadcast command that allows the Master to send the same packet to all devices at once. All Slaves will obey the packet’s instructions, but none will respond. The broadcast request feature is available only with function codes 6 and 10, Preset Single Registers and Preset Multiple Registers, respectively. See Tables 1.3 and 1.4.

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