Sterlco M2B+ Controller - Manual (Page 51)

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MODBUS Function 03 Request. The format for a function 03 request is shown below.
Device
Address Function Code
03 Register Address 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 Register Address 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