GE 489 Instruction Manual page 146

6.1 ELECTRICAL INTERFACE
ignores the slave address, so it will respond regardless of the value in the message. Note: A master transmission with
a SLAVE ADDRESS of 0 indicates a broadcast command. Broadcast commands can be used for specific functions.
• FUNCTION CODE: This is the second byte of every transmission. Modbus defines function codes of 1 to 127. The 489
implements some of these functions. In a master request transmission the FUNCTION CODE tells the slave what
action to perform. In a slave response transmission if the FUNCTION CODE sent from the slave is the same as the
FUNCTION CODE sent from the master indicating the slave performed 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 greater than 127) 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 be Actual Values, Set-
points, or addresses sent by the master to the slave or by the slave to the master. Data is sent MSByte first followed by
the LSByte.
• CRC: This is a two byte error checking code. CRC is sent LSByte first followed by the MSByte. The RTU version of
Modbus includes a two byte CRC-16 (16-bit cyclic redundancy check) with every transmission. The CRC-16 algorithm
essentially treats the entire data stream (data bits only; start, stop and parity ignored) as one continuous binary num-
ber. 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 transmission, LSByte first. The resulting message
including CRC, when divided by the same polynomial at the receiver will give a zero remainder if no transmission
errors have occurred.
If a 489 Modbus slave device receives a transmission in which an error is indicated by the CRC-16 calculation, the slave
device will not respond to the transmission. A CRC-16 error indicates than one or more bytes of the transmission were
received incorrectly and thus the entire transmission should be ignored in order to avoid the 489 performing any incorrect
operation.
The CRC-16 calculation is an industry standard method used for error detection. An algorithm is included here to assist
programmers in situations where no standard CRC-16 calculation routines are available.
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6-2 489 Generator Management Relay
6 COMMUNICATIONS
GE Power Management