MVI69E-MBS ♦ CompactLogix Platform Reference
Modbus Serial Enhanced Communication Module User Manual
ProSoft Technology, Inc. Page 109 of 154
June 28, 2017
7.2.6 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.
Data Start
Registers
High
Data Number
of Registers
High
Data Number
of Registers
Low
Error Check
Field
(2 bytes)
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.
Error Check
Field
(2 bytes)
To Next Page
Loading...
