Reference MVI56E-MCM ♦ ControlLogix Platform
User Manual Modbus Communication Module
Page 172 of 209 ProSoft Technology, Inc.
Because the Slave interface device is normally serviced at the end of the
controller's scan, the data reflects 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.
6.8.5 Read Input Registers (Function Code 04)
Query
Function code 04 obtains 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 up to 125 registers to be obtained 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 3009 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.
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 reflects 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 3009 contains the decimal value 0.
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