Overview; Introduction; Physical Layer; Data Link Layer - GE UR Series Instruction Manual

APPENDIX B
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APPENDIX B MODBUS RTU PROTOCOLB.1 OVERVIEW
The UR series relays support a number of communications protocols to allow connection to equipment such as personal
computers, RTUs, SCADA masters, and programmable logic controllers. The Modicon Modbus RTU protocol is the most
basic protocol supported by the UR. Modbus is available via RS232 or RS485 serial links or via ethernet (using the Mod-
bus/TCP specification). The following description is intended primarily for users who wish to develop their own master com-
munication drivers and applies to the serial Modbus RTU protocol. Note that:
• The UR always acts as a slave device, meaning that it never initiates communications; it only listens and responds to
requests issued by a master computer.
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• For Modbus , a subset of the Remote Terminal Unit (RTU) protocol format is supported that allows extensive monitor-
ing, programming, and control functions using read and write register commands.
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The Modbus RTU protocol is hardware-independent so that the physical layer can be any of a variety of standard hard-
ware configurations including RS232 and RS485. The relay includes a faceplate (front panel) RS232 port and two rear ter-
minal communications ports that may be configured as RS485, fiber optic, 10BaseT, or 10BaseF. Data flow is half-duplex in
all configurations. See Chapter 3: HARDWARE 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 can be 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, 14400, 19200, 28800, 33600, 38400, 57600, or 115200 bps are available. Even, odd, and no parity are avail-
able. Refer to the COMMUNICATIONS section of the SETTINGS chapter for further details.
The master device in any system must know the address of the slave device with which it is to communicate. The relay 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 setting selects the slave address used for all ports, with the exception that for the faceplate port, the relay will
accept any address when the Modbus
Communications takes place in packets which are groups of asynchronously framed byte data. The master transmits a
packet to the slave and the slave responds with a packet. The end of a packet is marked by 'dead-time' on the communica-
tions line. The following describes general format for both transmit and receive packets. For exact details on packet format-
ting, refer to subsequent sections describing each function code.
Table B–1: MODBUS PACKET FORMAT
DESCRIPTION
SLAVE ADDRESS
FUNCTION CODE
DATA
CRC
DEAD TIME
SLAVE ADDRESS
This is the address of the slave device that is intended to receive the packet sent by the master and to perform the desired
action. Each slave device on a communications bus must have a unique address to prevent bus contention. All of the
relay's ports have the same address which is programmable from 1 to 254; see Chapter 5 for details. Only the addressed
slave will respond to a packet that starts with its address. Note that the faceplate port is an exception to this rule; it will act
on a message containing any slave address.
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RTU protocol is used.
SIZE
1 byte
1 byte
N bytes
2 bytes
3.5 bytes transmission time
L90 Line Differential Relay
B.1 OVERVIEW

B.1.1 INTRODUCTION

B.1.2 PHYSICAL LAYER
B.1.3 DATA LINK LAYER
B-1
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