ProSoft Technology MVI56E-MCM/MCMXT - Manual (Page 57)

MVI56E-MCM ♦ ControlLogix Platform Configuration as a Modbus Slave
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 57 of 199
June 18, 2014
3 Configuration as a Modbus Slave
In This Chapter
 Overview ............................................................................................... 57
 ModDef Settings .................................................................................... 58
 Slave Configuration ............................................................................... 62
 Floating-Point Data Handling (Modbus Slave) ....................................... 63
 Read and Write Same Modbus Address (Pass Through) ...................... 65
3.1 Overview
When configuring the module as a Slave, you will be providing whoever is
programming the Master side of the communications with a Modbus Memory
Map.
Note: If you are using the Sample Ladder Logic, the transfer of data is already done.
Information that is to be read by the Modbus Master device will be placed in the
MCM.DATA.W
RITE D ATA array as this will be pushed out to the module so that
values from the ControlLogix processor can be read by the Modbus Master.
Information that must be written to the ControlLogix processor from the Modbus
Master device will be placed into the MCM.DATA.R
EAD D ATA array.
To configure module as a Modbus Slave you must determine how much data you
must transfer to and from the module, to the Modbus Master.
The sample ladder file is configured to transfer 600 16-bit registers in each
direction. If more than that is required, please see Adjust the Input and Output
Array Sizes (Optional) (page 27).
Find out if the Master can read from one Modbus address and write to another
Modbus address, or, if the Master must use the same address to read and write
data points.
If a Modbus command must bypass the read and write areas of the slave's
memory area and send Modbus commands directly to another device on the
Modbus network (for example, to a PLC), you must use Pass-Through mode.
This allows the MCM.DATA.W
RITE D ATA array to be used for all data transfer to
the Master. Because the data transfer of the MVI56E-MCM module cannot be
bidirectional, when the Master issues a Modbus Write command in Pass-Through
mode, the MVI56E-MCM module builds a special block of information. This block
is then parsed by the ladder logic, and the value written from the Modbus Master
is then updated in the MCM.DATA.W
RITE D ATA array.
Note: You should only use Pass-Through mode when there is no other option, as there is a
drawback to this mode that is not present in the standard mode.