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8 COMMUNICATIONS
8 COMMUNICATIONS 8.1 OVERVIEW
The GE Power Management 745 Transformer Management relay communicates with other computerized
equipment such as programmable logic controllers, personal computers, or plant master computers using
either the AEG Modicon Modbus protocol or the Harris Distributed Network Protocol (DNP), Version 3.0. Fol-
lowing are some general notes:
•
The 745 relay always act as slave devices meaning that they never initiate communications; they only lis-
ten and respond to requests issued by a master computer.
•
For Modbus, a subset of the Remote Terminal Unit (RTU) format of the protocol is supported which allows
extensive monitoring, programming and control functions using read and write register commands.
•
For DNP, the functionality is restricted to monitoring of essential relay data and control of important relay
functions. A complete description of the services available via DNP may be found in the Device Profile
Document which is included in this chapter.
DNP is a complex protocol. As a consequence, it is not possible within the scope of this manual to provide a
description of the protocol's operation in anything approaching the detail required to understand how to use it
to communicate with the relay. It is strongly recommended that interested users contact the DNP Users Group
at www.dnp.org to obtain further information:
Members of the DNP Users Group are eligible to receive complete descriptions of all aspects of the protocol.
The Users Group also operates a website (www.dnp.org) where technical information and support is avail-
able.
Both the MODBUS and DNP protocols are hardware-independent so that the physical layer can be any of a
variety of standard hardware configurations including RS232, RS422, RS485, fiber optics, etc. The 745
includes a front panel RS232 port and two rear terminal RS485 ports, one of which can also be configured as
RS422. Data flow is half duplex in all configurations. See Section 3.2.16: RS485 / RS422 COMMUNICATION
PORTS on page 3–12 for details on wiring.
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. This is important for transmission through
modems at high bit rates (11 bit data frames are not supported by many modems at baud rates greater than
300).
The baud rate and parity are independently programmable for each communications port. Baud rates of 300,
1200, 2400, 4800, 9600, and 19200 are available. Even, odd, and no parity are available. See Section 5.3.4:
COMMUNICATIONS on page 5–26 for further details.
The master device in any system must know the address of the slave device with which it is to communicate.
The 745 will not act on a request from a master if the address in the request does not match the relay's slave
address (unless the address is the broadcast address -- see below).
A single setpoint selects the slave address used for all ports with the exception that for the front panel port the
relay will accept any address when the Modbus protocol is used. The slave address is otherwise the same
regardless of the protocol in use, but note that the broadcast address is 0 for Modbus and 65535 for DNP. The
relay recognizes and processes a master request (under conditions that are protocol-specific) if the broadcast
address is used but never returns a response.
DNP may be used on, at most, one of the communications ports. Any port(s) not selected to use DNP will com-
municate using Modbus. Setpoint S1 RELAY SETUP / COMMUNICATIONS / DNP / DNP PORT is used to select which
port will communicate using DNP.
The maximum time for a 745 relay to return a response to any (non-broadcast) master request never exceeds
1 second.
GE Power Management
745 Transformer Management Relay
8.1 OVERVIEW
8.1.1 PROTOCOLS
8.1.2 PHYSICAL LAYER
8-1
8
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