Read Holding Registers (Function Code 03) - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module
Due to the quantity of input statuses requested, the last data field which is shown as 35
HEX = 0011 0101 (binary) contains the status of only 6 inputs (10213 to 102180) instead
of 8 inputs. The two left-most bits are provided as zeros to fill the 8-bit format.

8.2.6 Read Holding Registers (Function Code 03)

Query
This function allows you to retrieve the contents of holding registers 4xxxx (Modbus 4x
range) in the addressed slave. The registers can store the numerical values of
associated timers and counters which can be driven to external devices. The addressing
allows retrieving up to 125 registers at each request; however, the specific slave device
may have restrictions that lower this maximum quantity. The registers are numbered
form zero (40001 = zero, 40002 = one, and so on). The broadcast mode is not allowed.
The example below reads registers 40108 through 40110 (three registers) from slave
number 11.
Note: This is the structure of the message being sent out to the Modbus network. The byte values below are
in hexadecimal display.
Function
Node
Code
Address
03
0B
Response
The addressed slave responds with its address and the function code, followed by the
information field. The information field contains 1 byte describing the quantity of data
bytes to be returned. The contents of the registers requested (DATA) are two bytes
each, with the binary content right justified within each pair of characters. The first byte
includes the high order bits and the second, the low order bits.
Because the slave interface device is normally serviced at the end of the controller's
scan, the data reflect the register content at the end of the scan. Some slaves limit the
quantity of register content provided each scan; thus for large register quantities,
multiple transmissions are made using register content from sequential scans.
In the example below, the registers 40108 to 40110 have the decimal contents 555, 0,
and 100 respectively.
Function
Node
Code
Address
03
0B
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Data Start Data Start
Registers Registers Low
High
00 6B
Byte High Low
Count Data Data
06 02 2B
Data Number Data Number
of Registers of Registers
High Low
00 03
High Low High
Data Data Data
00 00 00
Error Check
Field
(2 bytes)
CRC
Low Error Check
Data Field
(2 bytes)
64 CRC
Page 118 of 159
Reference
User Manual