Table of Contents
Advertisement
4 COMMUNICATIONS
269 PLUS INSTRUCTION MANUAL 4 COMMUNICATIONS
The 269Plus Motor Protection relays communicate with other computerized equipment such as programmable
controllers, personal computers or plant master computers with the Modicon Standard Data Communication
Network (Modbus RTU).
This document describes the Modbus protocol subset supported by the 269Plus Motor Protection relays. For
additional information about the Modbus protocol, visit the
•
GE Power Management 269Plus relays always act as Modbus slaves, meaning they never initiate a dia-
logue but listen and respond to the bus master computer.
•
Only the Remote Terminal Unit (RTU) format of the Modbus protocol is adopted for the 269Plus. The for-
mat is described later and features binary data format rather than ASCII character communications.
•
As the physical layer of the protocol, the two-wire (4-wire is also available as a modification for a moderate
cost) multi-drop RS485 standard is supported. This standard uses twisted pair cable. Up to 32 269Plus
relays using up to 4000 ft. of cable are supported.
•
Serial communication interface (SCI) boxes can be purchased to support the RS232 standard for the con-
nection of a group of relays on an RS485 bus to a PC or PLC RS232 port.
•
Since the operation of the 269Plus is register based, a master computer can monitor and control relay
operation by reading or writing to the relay registers. Therefore, only the Register Read and Write functions
of the MODBUS are supported. These are described in Section 4.6: SUPPORTED FUNCTIONS on page
4–5.
•
The 269Plus provides flexibility towards master computer corrective actions upon communication errors
and timeouts. This is left to the discretion of the master programmer.
The electrical interface is a two-wire bidirectional multi-drop RS485 on a shielded twisted-pair cable. A Belden
9841 insulated 24 gauge shielded twisted pair cable should be used. The connection to the 269Plus is by Ter-
minals 46 (–) and 47 (+), where + is the positive wire and – is the negative wire. The shield must be connected
at Terminal 88 on each 269Plus throughout the link and GROUNDED AT THE MASTER ONLY. If a GE Power
Management RS232 to RS485 converter is used, no grounding of the shield is necessary. The cable should be
terminated at the two extreme ends by resistors equal to its characteristic impedance (120 Ω for the Belden
9841 AWG twisted pair) in series with 1 nF / 50 V capacitors. See Figure 4–1: SERIAL COMMUNICATION
LINK for details. A terminator assembly with the resistor/capacitor network may be obtained from GE Power
Management.
Communication to the 269Plus is half-duplex. This means that the 269Plus listens to receive a command, ser-
vices it, then transmits the reply. The 269Plus will not service and process a new request until the current
request is processed.
The message bytes are communicated as asynchronous items with 1 start bit, 8 data bits, and one stop bit.
This produces a 10-bit data frame (some modems do not support 11-bit data frames at baud rates of greater
than 300 bps). The rates at which data can be communicated are 300, 1200, or 2400 bps. The unit defaults to
2400 bps. If 300 or 1200 bps is desired, contact the factory.
Each relay on the network is identified by a one byte address which is programmed into the 269Plus non-vola-
tile memory (NOVRAM). This address is located in Setpoints page 5.
GE Power Management
Courtesy of NationalSwitchgear.com
w w w . m o d b u s . o r g
269Plus Motor Management Relay
INTRODUCTION
4.1 INTRODUCTION
website.
4.2 MODES OF OPERATION
4.3 PHYSICAL LAYER
4
4-
1
Advertisement
Chapters
Table of Contents
