ProSoft Technology MVI71-MCM - Manual (Page 55)

Reference MVI71-MCM ♦ PLC Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 55 of 111
December 28, 2007
5.1.4 Functional Specifications
ƒ Support for the storage and transfer of up to 5000 registers to/from the PLC
processor's data files
ƒ User-definable module memory usage
ƒ Two ports to emulate any combination of Modbus master or slave device
ƒ Supports Enron version of Modbus protocol for floating point data
transactions
Slave Specifications
The MVI71-MCM module accepts Modbus function code commands of 1, 2, 3, 4,
5, 6, 8, 15, 16, 17, 22 and 23 from an attached Modbus master unit. A port
configured as a Modbus slave permits a remote master to interact with all data
contained in the module. This data can be derived from other Modbus slave
devices on the network, through a master port, or from the PLC processor.
Master Specifications
A port configured as a virtual Modbus master device on the MVI71-MCM module
actively issues Modbus commands to other nodes on the Modbus network. One
hundred commands are supported on each port. Additionally, the master ports
have an optimized polling characteristic that polls slaves with communication
problems less frequently. The PLC processor can be programmed to control the
activity on the port by actively selecting commands from the command list to
execute or issuing commands directly from the ladder logic.
5.2 Functional Overview
This section provides an overview of how the MVI71-MCM module transfers data
using the MCM protocol. You should understand the important concepts in this
chapter before you begin installing and configuring the module.
5.2.1 General Concepts
The MVI71-MCM is a module that allows the communication between the PLC
and a Modbus network. The module has 2 ports that can be individually
configured as a Modbus Master or as a Modbus Slave.
The module uses the rack backplane in order to transfer data to the PLC. Ladder
logic is used for different tasks including:
ƒ
Downloading configuration data
ƒ Updating the module's internal database (read or write)
ƒ Executing control blocks
The module uses block transfer instructions in order to read and write data to the
PLC. The MVI71-MCM also supports side connect transfer (requires a side
connect adapter) which allows a faster data transfer and does not require ladder
logic to transfer data from/to the PLC.