Read Input Registers (Function Code 04) - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module

8.2.7 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 slave 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 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
04
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 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.
Because the slave interface is normally serviced at the end of the controller's scan, the
data reflect the register content at the end of the scan. Each PC limits the quantity of
register contents provided each scan; thus for large register quantities, multiple PC
scans are required, and the data provided is from sequential scans.
In the example below the register 30009 contains the decimal value 0.
Function
Node
Code
Address
04
0B
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Data Start Data Start
Point High Point Low
00 08
Byte Count Data Input
Register High
02 00
Data Number Data Number
of Points High of Points Low
00 01
Data Input Error Check Field
Register Low (2 bytes)
00 CRC
Reference
User Manual
Error Check
Field
(2 bytes)
CRC
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