ProSoft Technology MVI94-MCM-MHI - Manual (Page 73)

MVI94-MCM ♦ Flex I/O Platform Reference
Serial Communications Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 73 of 109
March 29, 2011
5.3.1 About the MODBUS Protocol
MODBUS is a widely-used protocol originally developed by Modicon in 1978.
Since that time, the protocol has been adopted as a standard throughout the
automation industry.
The original MODBUS specification uses a serial connection to communicate
commands and data between Master and Slave devices on a network. Later
enhancements to the protocol allow communication over other types of networks.
MODBUS is a Master/Slave protocol. The Master establishes a connection to the
remote Slave. When the connection is established, the Master sends the
MODBUS commands to the Slave. The MVI94-MCM module can work as a
Master and as a Slave.
The MVI94-MCM module also works as an input/output module between itself
and the Rockwell Automation backplane and processor. The module uses an
internal database to pass data and commands between the processor and
Master and Slave devices on MODBUS networks.
5.3.2 Virtual Modbus Database Concepts
Central to the functionality of the module is the virtual Modbus database. This
database is used as the interface between remote Modbus master and slave
devices and the Flex I/O bus. The size, content and structure of the database are
completely user defined.
The Flex I/O bus reads data from and write data to the database using the
backplane interface. The module interfaces data contained in remote Modbus
slave devices to the virtual Modbus database when using the Modbus master
port. User commands are issued out the master port from a command list. These
commands gather or control data in the Modbus slave devices. When configured
as a slave, control information from the Modbus master and data from the
processor are exchanged over the backplane. Pass-through mode allows the
passing of single bit or register data directly to the processor, bypassing the
Modbus database. The following diagrams illustrate the relationships discussed
above.