GE 735

GE Power Management
735/737 Feeder Protection Relay 4-5
4 MODBUS COMMUNICATIONS 4.2 SUPPORTED MODBUS FUNCTIONS
4
4.2.3 FUNCTION CODE 04: READ ACTUAL VALUES
Modbus Implementation: Read Input Registers
735/737 Implementation: Read Actual Values
For the Modbus implementation, "input registers" are equivalent to 735/737 actual values. Input registers are 16 bit (two
byte) values transmitted high order byte first. Thus all 735/737 actual values are sent as two bytes. This command allows
the master to read a group of actual values from a slave device. The maximum number of actual values that can be read in
one transmission is 60 in the 735/737. The slave response to this function code is the slave address, function code, a count
of the number of data bytes to follow, the data itself, and the CRC. Each data item (actual value) is sent as a two byte num-
ber with the high order byte sent first.
The broadcast mode is not allowed with this function code. The master transmission will be ignored by all slaves if broad-
cast mode is used with this function code.
Message Format and Example:
Request slave 11 to respond with 1 actual value starting at address 0008. For this example the actual value in this address
(0008) is 01AE.
MASTER TRANSMISSION: BYTES EXAMPLE / DESCRIPTION
SLAVE ADDRESS 1 11 message for slave 11
FUNCTION CODE 1 04 read actual values
DATA STARTING ADDRESS 2 00 08 data starts at 0008h
NUMBER OF ACTUAL VALUES 2 00 01 1 actual value = 2 bytes
CRC 2 ?? ?? CRC calculated by the master
SLAVE RESPONSE: BYTES EXAMPLE / DESCRIPTION
SLAVE ADDRESS 1 11 response message from slave 11
FUNCTION CODE 1 04 read actual values
BYTE COUNT 1 06 1 actual value = 2 bytes
DATA 2 01 A3 data at 0008h
CRC 2 ?? ?? CRC calculated by the slave