ProSoft Technology MVI69-MCM User Manual

Page 137

MVI69-MCM ♦ CompactLogix or MicroLogix Platform

Reference

Modbus Communication Module

User Manual

ProSoft Technology, Inc.

Page 137 of 167

March 22, 2011

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 will reflect the register content at the end of the scan.

Some Slaves will limit the quantity of register content provided each scan; thus

for large register quantities, multiple transmissions will be 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

Hi Data

Lo Data

Error Check Field

CRC

5.6.5 Read Input Registers (Function Code 04)

Query
Function code 04 obtains the contents of the controller's input registers at

addresses 3xxxx. 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.

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

Data #of Regs Lo

Error Check Field

CRC