ProSoft Technology MVI69-MCM - Manual (Page 134)

Reference MVI69-MCM ♦ CompactLogix or MicroLogix Platform
User Manual Modbus Communication Module
Page 134 of 167 ProSoft Technology, Inc.
March 22, 2011
5.6 Modbus Protocol Specification
The following pages give additional reference information regarding the Modbus
protocol commands supported by the MVI69-MCM.
5.6.1 Commands Supported by the Module
The format of each command in the list depends on the MODBUS Function Code
being executed.
The following table lists the functions supported by the module.
Function
Code Definition Supported in
Master Supported in
Slave
1 Read Coil Status X X
2 Read Input Status X X
3 Read Holding Registers X X
4 Read Input Registers X X
5 Set Single Coil X X
6 Single Register Write X X
8 Diagnostics X
15 Multiple Coil Write X X
16 Multiple Register Write X X
17 Report Slave ID X
22 Mask Write 4X X
23 Read/Write X
Each command list record has the same general format. The first part of the
record contains the information relating to the communication module and the
second part contains information required to interface to the MODBUS slave
device.
5.6.2 Read Coil Status (Function Code 01)
Query
This function allows the user to obtain the ON/OFF status of logic coils used to
control discrete outputs from the addressed Slave only. Broadcast mode is not
supported with this function code. In addition to the Slave address and function
fields, the message requires that the information field contain the initial coil
address to be read (Starting Address) and the number of locations that will be
interrogated to obtain status data.
The addressing allows up to 2000 coils to be obtained at each request; however,
the specific Slave device may have restrictions that lower the maximum quantity.
The coils are numbered from zero; (coil number 1 = zero, coil number 2 = one,
coil number 3 = two, and so on).
The following table is a sample read output status request to read coils 0020 to
0056 from Slave device number 11.
Adr Func Data Start Pt Hi Data Start Pt Lo Data # Of Pts Ho Data # Of Pts Lo Error Check Field
11 01 00 13 00 25 CRC