Data Packet Format; Error Checking - GE 369 Communications Manual

CGCOMMUNICATIONS GUIDE
5.2

Data Packet Format

5.3

Error Checking

369 MOTOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
A complete request/response sequence consists of the following bytes (transmitted as
separate data frames):
Master Request Transmission:
SLAVE ADDRESS
FUNCTION CODE
DATA
CRC
Slave Response Transmission:
SLAVE ADDRESS
FUNCTION CODE
DATA
CRC
SLAVE ADDRESS: This is the first byte of every transmission. It 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. The modbus protocol
defines function codes of 1 to 127. The 369 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 > 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 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 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
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.
- 1 byte
- 1 byte
- variable number of bytes depending on FUNCTION CODE
- 2 bytes
- 1 byte
- 1 byte
- variable number of bytes depending on FUNCTION CODE
- 2 bytes
MODBUS RTU PROTOCOL
CG39