GE Industrial Solutions Entellisys Integrator’s Guide - Manual (Page 7)

Modbus ® protocol implementation 7
1 Integrator’s Guide
1.1 Modbus
®
protocol implementation
1.1.1 Introduction
The CPU supports a number of communications protocols to allow connection to the HMI
computer, as well as other equipment which includes personal computers, RTUs, SCADA
masters, and programmable logic controllers. The Modicon Modbus
®
RTU protocol is the most
basic protocol supported. Modbus is available via ethernet as specified by the Modbus/TCP
specification. Note that:
• The CPU always acts as a slave device, meaning that it never initiates communications; it
only listens and responds to requests issued by a master computer.
• For Modbus, a subset of the Remote Terminal Unit (RTU) protocol format is supported that
allows extensive monitoring, programming, and control functions using read and write
register commands.
• The CPU will support a maximum of 8 concurrent Modbus sessions. Four sessions are
reserved for use by HMI computers. A remote device that attempts to connect when all
sessions are in use will receive a response message indicating the number of maximum
connections has been exceeded. If a remote device does not make a request within
30 seconds, the session will be timed out and made available to the next device that
establishes a session.
1.1.2 Physical layer
The Modbus RTU protocol is hardware-independent so that the physical layer can be any of a
variety of standard hardware configurations. The CPU includes a faceplate (front panel)
100BaseT Ethernet port. Data flow is auto-configuring full or half-duplex. Each data byte is
transmitted in an asynchronous format consisting of 1 start bit, 8 data bits, 1 stop bit, and
possibly 1 parity bit. This produces a 10 or 11 bit data frame. The master device in any system
must know the address of the slave device with which it is to communicate. In the case of
ModbusTCP communications, the CPU will not act on a request from a master if the address in
the request does not match the CPU’s slave address. A single setting selects the slave address
used for ModbusTCP. The default slave address for a CPU is 1.