750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
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750/760 Feeder Management Relay
Chapter 2: Modbus Protocol
GE
Grid Solutions
Modbus Protocol
Description
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 sufficient information for a user
to develop an interface should DNP wish to be used for communications with the relay.
Data Link Layer
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 communications line. The following
describes general format of both transmit and receive packets. For exact details on packet
formatting refer to subsequent sections describing each function code.
The Modbus packet format is a set of five sequential information blocks:
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
Block
Size
Slave Address
1 byte
Function Code
1 byte
Data
n
bytes,
n
≥
1
Cyclic Redundancy Check (CRC)
2 bytes
Dead Time
3.5 bytes transmission time