ProSoft Technology MVI56E-MCM/MCMXT User Manual

Page 164

Reference

MVI56E-MCM ♦ ControlLogix Platform

User Manual

Modbus Communication Module

Page 164 of 199

ProSoft Technology, Inc.

June 18, 2014

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 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.

Adr Func ByteCnt Hi Data

Lo Data Hi Data

Lo Data Error Check Field

CRC

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.

Adr Func Data Start Reg Hi Data Start Reg Lo Data #of Regs Hi

Data #of Regs Lo Error Check Field

0B 04

CRC