PROTOCOL 6. COMMUNICATIONS
6-2
6.2.3
DATA PACKET FORMAT
A complete request/response sequence consists of the following bytes (transmitted as separate data frames):
Master Request Transmission:
SLAVE ADDRESS - 1 byte
FUNCTION CODE - 1 byte
DATA - variable number of bytes depending on FUNCTION CODE
CRC - 2 bytes
Slave Response Transmission:
SLAVE ADDRESS - 1 byte
FUNCTION CODE - 1 byte
DATA - variable number of bytes depending on FUNCTION CODE
CRC - 2 bytes
SLAVE ADDRESS - This is the first byte of every transmission. 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 transmission that starts with its address. In a master request transmission the SLAVE ADDRESS represents the address of
the slave to which the request is being sent. In a slave response transmission the SLAVE ADDRESS represents the address of the
slave that is sending the response. 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 SR469 implements
some of these functions. In a master request transmission the FUNCTION CODE tells the slave what action to perform. In a slave re-
sponse 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 > 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, Setpoints, or ad-
dresses 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.
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