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4-4 735/737 Feeder Protection Relay

GE Power Management

4.2 SUPPORTED MODBUS FUNCTIONS 4 MODBUS COMMUNICATIONS

4.2 SUPPORTED MODBUS FUNCTIONS 4.2.1 DESCRIPTION

The following functions are supported by the 735/737:

• 03: Read Setpoints

• 04:Read Actual Values

• 05: Execute Operation

• 06: Store Single Setpoint (test/simulation)

• 07: Read Device Status

• 16: Store Multiple Setpoints (test/simulation)

4.2.2 FUNCTION CODE 03: READ SETPOINTS

Modbus implementation: Read Holding Registers

735/737 implementation: Read Setpoints

For the Modbus implementation, "holding registers" are equivalent to memory locations reflecting the user switch settings.

Holding registers are 16-bit (two byte) values transmitted high order byte first. Thus all setpoints are sent as two bytes. This

function code allows the master to read setpoints from a slave device.

The slave response 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 (setpoint) is sent as a two byte number with the high order byte sent first. Note that broadcast mode

is not allowed with this function. The master transmission will be ignored by all slaves if broadcast mode is used with this

function code.

Message Format and Example:

Request slave 11 to respond with 3 setpoints starting at address 0040. For this example the setpoint data is:

ADDRESS DATA

0040 0003

0041 0000

0042 0009

MASTER TRANSMISSION: BYTES EXAMPLE / DESCRIPTION

SLAVE ADDRESS 1 11 message for slave 11

FUNCTION CODE 1 03 read setpoints

START ADDRESS 2 00 40 data starts at 0040h

NUMBER OF SETPOINTS 2 00 06 3 setpoints = 6 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 03 read setpoints

BYTE COUNT 1 06 3 setpoints = 6 bytes

DATA #1 2 00 03 setpoints data at 0040h

DATA #2 2 00 00 setpoints data at 0041h

DATA #3 2 00 09 setpoints data at 0042h

CRC 2 ?? ?? CRC calculated by the slave

GE 735 User Manual

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