GE Power Management 750/760 Feeder Management Relay 16-
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16 COMMUNICATIONS 16.3 MODBUS PROTOCOL
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16.3 MODBUS PROTOCOL 16.3.1 DESCRIPTION
The remainder of the text in this chapter 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.
16.3.2 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.
MODBUS Packet Format:
SLAVE ADDRESS 1 byte
FUNCTION CODE 1 byte
DATA N bytes
CRC 2 bytes
DEAD TIME 3.5 bytes transmission time
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 conten-
tion. All the relay ports have the same address which is programmable from 1 to 254; see Section 8.2: COM-
MUNICATIONS on page 8–2. 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 commu-
nication 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 16.3.11: CLOCK SYNCHRONIZA-
TION OF MULTIPLE RELAYS on page 16–12 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 perform. See
Section 16.3.4: SUPPORTED FUNCTION CODES on page 16–5 for complete details. An exception response
from the slave is indicated by setting the high order bit of the function code in the response packet. See Sec-
tion 16.3.10: EXCEPTION RESPONSES on page 16–11 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.
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 Power Man-
agement 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 Section 16.3.3: CRC-
16 ALGORITHM 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 lis-
tening for a new packet from the master except for the addressed slave.
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