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Modbus RTU Function Codes

02/13 Modbus® RTU Serial Communications User Manual 9

3. Modbus RTU Function Codes

The Honeywell Universal Modbus RTU protocol uses a subset of the standard Modbus RTU function codes

to provide access to process-related information. Several MODICON function codes are employed. It is

appropriate to define instrument-specific "user-defined" function codes. Several standard Modbus RTU

function codes are supported. These standard function codes provide basic support for IEEE 32-bit floating

point numbers and 16-bit integer register representation of instrument’s process data.

Repolling of data is not supported by these instruments.

Table 3-1 Modbus RTU Function Codes Definitions

Function Code Name Usage

01 Read Coil Status Read the State of a Digital Output

02 Read Input Status Read the State of a Digital Input

Read Holding Registers /

Read Input Registers

Read Data in 16 bit Register Format (high/low). Used to

read integer or floating point process data. Registers

are consecutive and are imaged from the instrument to

the host.

05 Force Single Coil Write data to force Digital Output ON/OFF

Values of FF 00 forces DO ON

Values of 00 00 forces DO OFF

Values of FF FF releases the force of the DO

All other values are illegal and will not affect the DO.

RSX, VPR, VRX, UDC5300 ONLY

06 Preset Single Register

Write Data in 16-bit Integer Format (high/low) ONLY.

08 Loopback Test Used for diagnostic testing of the communications port.

16 (10h) Preset Multiple Registers Write Data in 16-bit Format (high/low). Used to write

integer and floating point override data. Registers

are consecutive and are imaged from the host to the

instrument.

Note: UDC will write floating point data using this

Function Code

17 (11h) Report Device ID Read instrument ID and connection information, ROM

version, etc.

20 (14h) Read General Reference Used to Read or upload the instrument’s configuration

into the host device. See Section 1.2 - Modbus RTU

Configuration Interface for a reference to the User

Manual for the specific instrument.

21 (15h) Write General Reference Used to Write or download an instrument’s

configuration into the instrument from a host device.

See Section 1.2 - Modbus RTU Configuration Interface

for a reference to the User Manual for the specific

instrument.

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