Reference MVI69E-MBTCP ♦ CompactLogix Platform
User Manual Modbus TCP/IP Enhanced Communication Module
Page 114 of 150 ProSoft Technology, Inc.
June 28, 2017
Response
The addressed server 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 server 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 servers 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)
7.2.7 Read Input Registers (Function Code 04)
Query
This function retrieves the contents of the controller's input registers from the
Modbus 3x range. These locations receive their values from devices connected
to the I/O structure and can only be referenced, not altered from within the
controller, The addressing allows retrieving up to 125 registers at each request;
however, the specific server device may have restrictions that lower this
maximum quantity. The registers are numbered for zero (30001 = zero, 30002 =
one, and so on). Broadcast mode is not allowed.
The example below requests the contents of register 30009 in server 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 Number
of Points High
Data Number
of Points Low
Error Check
Field
(2 bytes)
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