Data Packet Format; Timing - GE 735 Instruction Manual

4.1 OVERVIEW
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
a) SLAVE ADDRESS:
This is the first byte of every transmission. It is 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 using the front panel switches and only the
addressed slave will respond to a transmission that starts with its address.
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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. A master transmission with a SLAVE ADDRESS of 0 indicates a broadcast command. All slaves on the commu-
nication link will take action based on the transmission but no response will be made.
b) FUNCTION CODE
This is the second byte of every transmission. Modbus defines function codes of 1 to 127. The 735/737 implements some
of these functions. See section 4.3 for details of the supported function codes. In a master request transmission the FUNC-
TION 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 then the slave performed the function as requested. If the
high order bit of the FUNCTION CODE sent from the slave is a 1 (that is, if the FUNCTION CODE > 127) then the slave did
not perform the function as requested and is sending an error or exception response.
c) DATA
This will be a variable number of bytes depending on the FUNCTION CODE. Data may be actual values, setpoints, or
addresses sent by the master to the slave or by the slave to the master. See Section 4.2: SUPPORTED MODBUS FUNC-
TIONS on page 4–4 for a description of the supported functions and the data required for each.
d) CRC
This is a two byte error checking code (see the following section for additional details).
Data packet synchronization is maintained by timing constraints. The receiving device must measure the time between the
reception of characters. If three and one half character times elapse without receiving a new character or completion of the
message, then the communication link must be reset (that is, all slaves start listening for a new transmission from the mas-
ter). Thus at 9600 baud a delay of greater than 3.5 × 1 / 9600 × 10 = 3.6 ms will cause the communication link to be reset.
4-2 735/737 Feeder Protection Relay
4 MODBUS COMMUNICATIONS

4.1.4 DATA PACKET FORMAT

4.1.5 TIMING

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