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7.2 MODBUS PROTOCOL
7.2MODBUS PROTOCOL
This section is dedicated to discussion of details of the Modbus protocol. As noted above, specifics of DNP are best
obtained directly from the DNP Users Group. Along with the Device Profile Document, the DNP specification provides suffi-
cient information for a user to develop an interface should DNP wish to be used for communications with the relay.
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 of both transmit and receive packets. For exact details on packet format-
ting refer to subsequent sections describing each function code.
The Modbus packet format is a set of five sequential information blocks as shown below:
BLOCK
Slave Address
Function Code
Data
Cyclic Redundancy Check (CRC)
Dead Time
Each block of the Modbus packet is described below:
•
Slave Address: This is the address of the slave device intended to receive the packet sent by the master and perform
the desired action. Each slave device on a communication bus must have a unique address to prevent bus contention.
All the relay ports have the same address which is programmable from 1 to 254 (see Port Setup on page 5–8). Only
the addressed slave will respond to a packet that starts with its address. Note that the front port is an exception to this
rule; it will act on a message containing any slave address.
A master transmit packet with a slave address of 0 indicates a broadcast command. All slaves on the communication
link will take action based on the packet, but none will respond to the master. Broadcast mode is only recognized when
associated with Function Codes 05h, 06h, and 10h. For any other function code, a packet with broadcast mode slave
address 0 will be ignored. See Section 7.3.3: Clock Synchronization of Multiple Relays on page 7–8 for an example of
broadcast mode.
•
Function Code: This is one of the supported functions codes of the 750/760 which tells the slave what action to per-
form. See page 7–4 for complete details. An exception response from the slave is indicated by setting the high order bit
7
of the function code in the response packet. See page 7–7 for further details.
•
Data: This will be a variable number of bytes depending on the function code. This may include actual values, set-
points, or addresses sent by the master to the slave or by the slave to the master.
•
Cyclic Redundancy Check (CRC): This is a two byte error checking code. The RTU version of Modbus includes a 16-
bit cyclic redundancy check (CRC-16) with every packet which is an industry standard method used for error detection.
If a GE Multilin Modbus slave device receives a packet in which an error is indicated by the CRC, the slave device will
not act upon or respond to the packet thus preventing any erroneous operations. See the following section for a
description of how to calculate the CRC.
•
Dead Time: A packet is terminated when no data is received for a period of 3.5 byte transmission times (about 15 ms
at 2400 baud, 2 ms at 19200 baud). Consequently, the transmitting device must not allow gaps between bytes longer
than this interval. Once the dead time has expired without a new byte transmission, all slaves start listening for a new
packet from the master except for the addressed slave.
7-2
Courtesy of NationalSwitchgear.com
SIZE
1 byte
1 byte
n bytes, n ≥ 1
2 bytes
3.5 bytes transmission time
750/760 Feeder Management Relay
7 COMMUNICATIONS
7.2.1 DESCRIPTION
7.2.2 DATA LINK LAYER
GE Multilin
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