Sterlco M2B+ Controller User Manual

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MODBUS Function 03 Request. The format for a function 03 request is shown below.

Device
Address

Function Code
03

Number Of Words
To Read

CRC

1 byte

1 byte
containing $03

MSB

LSB

MSB

LSB

MSB

LSB

MODBUS Function 03 Request Example. Suppose you want to Read Delivery
Temperature in Float 32 Bit IEEE Format, which has registers address of "4002" and if the
controller ID is "01".

Device
Address

Function Code
03

Number Of
Words To Read

CRC

$01

$03

$0F

$A2

$00

$02

$66

$FD

Function 03 Normal Reply. The controller reply will consist of the following data:

01= MODBUS unit ID

03= MODBUS command: read holding registers

0F = high byte of first register to read

A2 = low byte of first register to read

0FA2 = 4002 decimal

00 = high byte of number registers to read

02 = low byte of number of registers to read

2 = 16-bit registers are read in a single command, to get all of a 32-bit floating

point register

66 = low byte of CRC of command packet

FD = high byte of CRC of command packet

Following is an example of normal reply response showing Process Temperature = 37.6° C

Device
Address

Function
Code 03

Number Bytes
of Data

IEEE32 BitFloat Data Field See
Modbus Spec.

CRC

$01

$03

$04

$C0

$93

$42

$16

$87

$70