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8 COMMUNICATIONS
A complete request/response sequence consists of the following bytes transmitted as separate data frames:
MASTER QUERY MESSAGE:
SLAVE ADDRESS:
(1 byte)
FUNCTION CODE:
(1 byte)
DATA:
(variable number of bytes depending on FUNCTION CODE)
CRC:
(2 bytes)
SLAVE RESPONSE MESSAGE:
SLAVE ADDRESS:
(1 byte)
FUNCTION CODE:
(1 byte)
DATA:
(variable number of bytes depending on FUNCTION CODE)
CRC:
(2 bytes)
A message is terminated when no data is received for a period of 3½ character transmission times. Conse-
quently, the transmitting device must not allow gaps between bytes larger than this interval (about 3 ms at 9600
baud).
•
SLAVE ADDRESS: This is the first byte of every message. This byte represents the user-assigned
address of the slave device that is to receive the message sent by the master. Each slave device must be
assigned a unique address, and only the addressed slave will respond to a message that starts with its
address. In a master query message the SLAVE ADDRESS represents the address of the slave to which
the request is being sent. In a slave response message the SLAVE ADDRESS is a confirmation represent-
ing the address of the slave that is sending the response. A master query message with a SLAVE
ADDRESS of 0 indicates a broadcast command. All slaves on the communication link will take action
based on the message, 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 message with
broadcast mode slave address 0 will be ignored.
•
FUNCTION CODE: This is the second byte of every message. Modbus defines function codes of 1 to 127.
The GE Power Management 745 implements some of these functions. In a master query message the
FUNCTION CODE tells the slave what action to perform. In a slave response message, if the FUNCTION
CODE sent from the slave is the same as the FUNCTION CODE sent from the master then the slave per-
formed the function as requested. If the high order bit of the FUNCTION CODE sent from the slave is a 1
(i.e. if the FUNCTION CODE is > 7Fh) then the slave did not perform the function as requested and is
sending an error or exception response.
•
DATA: This will be a variable number of bytes depending on the FUNCTION CODE. This may include
actual values, setpoints, 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 two byte CRC-16 (16
bit cyclic redundancy check) with every message. The CRC-16 algorithm essentially treats the entire data
stream (data bits only; start, stop and parity ignored) as one continuous binary number. This number is first
shifted left 16 bits and then divided by a characteristic polynomial (11000000000000101B). The 16 bit
remainder of the division is appended to the end of the message, MSByte first. The resulting message
including CRC, when divided by the same polynomial at the receiver will give a zero remainder if no trans-
mission errors have occurred. If a GE Power Management Modbus slave device receives a message in
which an error is indicated by the CRC-16 calculation, the slave device will not respond to the message. A
CRC-16 error indicates that one or more bytes of the message were received incorrectly and thus the
entire message should be ignored in order to avoid the slave device performing any incorrect operation.
The CRC-16 calculation is an industry standard method used for error detection.
GE Power Management
745 Transformer Management Relay
8.2 MODBUS PROTOCOL
8.2.5 DATA PACKET FORMAT
8
8-3
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