GE DGP Series Instruction Manual page 198

9.2 MODBUS COMMUNICATIONS
b) FUNCTION CODE
This is the second byte of every transmission. Modbus defines function codes 1 to 127 but the DGP imple-
ments only a subset of these functions. In a master request, the function code represents the action to be per-
formed by the slave. The slave responds with function code identical to that sent by the master if there are no
errors. In case of an error or exception, the slave sets the MS bit of the function code to 1 to indicate an error.
c) DATA
This will be a variable number of bytes depending on the function code.
d) CRC HI & CRC LO
This is a two-byte cyclic redundancy check. The MS byte is sent first and the LS byte next in accordance with
the Modbus protocol reference guide.
In RTU mode, messages include an error checking field that is based on a cyclic redundancy check method.
The CRC field checks the contents of the entire message. It is applied regardless of any parity check method
used for the individual characters of the message.
The CRC field is two bytes, containing a 16-bit binary value. The CRC value is calculated by the transmitting
device, which appends the CRC to the message. The receiving device recalculates CRC and compares with
the value it received in the CRC field. If they are not equal an error message results.
The CRC is calculated by first pre-loading a 16-bit register to all 1's (in GE relays, all registers are pre-loaded
with zeros). Then using a CRC polynomial specified by the CCITT, the CRC is computed (0xA001).
Modbus RTU messages are separated by a silence period of at least 3.5 characters. The slave flushes the
COM port and reads for the first character. This marks the start of transmission. The slave keeps reading until
a silent interval of 3.5 characters which is about 3.65 ms at 9600 baud, which marks the end of transmission.
At this stage it builds the message and resets the port.
9
9-
6
DGP Digital Generator Protection System
9 COMMUNICATIONS
9.2.4 ERROR CHECKING
9.2.5 DATA FRAMING
GE Power Management