User manual

Modbus

Modbus-Function 03, Read Holding Registers

Reads the binary contents of holding registers (4X references) in the slave.

Broadcast is not supported.

Appendix B lists the maximum parameters supported by various controller

models.

Query

The query message specifies the starting register and quantity of registers to be

read. Registers are addressed starting at zero: registers 1&endash;16 are

addressed as 0&endash;15.

Here is an example of a request to read registers 40108&endash;40110 from

slave device 17:

Field Name (Hex)

Slave Address 11

Function 03

Starting Address Hi 00

Starting Address Lo 6B

No. of Points Hi 00

No. of Points Lo 03

Error Check (LRC or CRC) &endash;&endash;

Response

The register data in the response message are packed as two bytes per register,

with the binary contents right justified within each byte. For each register, the

first byte contains the high order bits and the second contains the low order bits.

Data is scanned in the slave at the rate of 125 registers per scan for

984&endash;X8X controllers (984&endash;685, etc), and at the rate of 32

registers per scan for all other controllers. The response is returned when the

data is completely assembled.

Here is an example of a response to the query on the opposite page:

Field Name (Hex)

Slave Address 11

Function 03

Byte Count 06

Data Hi (Register 40108) 02

Data Lo (Register 40108) 2B

Data Hi (Register 40109) 00