Read Input Registers (Function Code 04) - ProSoft Technology MVI56E-MNETC User Manual

MVI56E-MNETC/MNETCXT ♦ ControlLogix Platform
Modbus TCP/IP Client Enhanced Communication Module - Client/Server
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.
Node Function
Address Code
0B 03

5.4.5 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.
Node Function
Address Code
0B 04
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 2 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.
ProSoft Technology, Inc.
September 20, 2018
Byte High Low
Count Data Data
06 02 2B
Data Start Data Start
Point High Point Low
00 08
High Low High
Data Data Data
00 00 00
Data Number Data Number
of Points High of Points Low
00 01
User Manual
Low Error Check
Data Field
(2 bytes)
64 CRC
Error Check
Field
(2 bytes)
CRC
Page 139 of 172
Reference