ProSoft Technology 5202-DFNT-MCM4 User Manual
Page 47
Modbus Port Expander (4102/5102-MBS3-MBM)
MCM ♦ ProLinx Gateway
Driver Manual
Modbus Master/Slave
ProSoft Technology, Inc.
Page 47 of 80
October 16, 2009
7
Modbus Port Expander (4102/5102-MBS3-
MBM)
In a typical Modbus serial network implementation, one Modbus Master device
would be configured to poll (send commands) to one or more Modbus slave
devices. The Modbus protocol specification does not provide a way for multiple
Modbus Masters to exist and actively poll slaves on the same physical network.
Therefore, more than one Master is usually not able to communicate with one,
particular slave.
The ProLinx Modbus Port Expander Communication Gateway is a specialized
implementation of the Modbus protocol. It allows multiple Modbus Masters to
communicate directly with one Modbus slave, something not normally possible
using the Modbus RTU/ASCII serial protocol. It provides this unique capability by
multiplexing the inputs received on up to three Modbus slave ports and
retransmitting them out one Modbus Master port to one Modbus slave device.
When the device responds, the response is routed back to the slave port on
which the command was received and returned to the originating Modbus
Master.
All internal multiplexing and routing is accomplished automatically by the
gateway with no special command configuration required. The only configuration
needed is for typical serial port communication, such as baud rate, parity, stop
bits, hardware handshaking, and such. The following two sections describe these
configuration settings in greater detail.
7.1
[Modbus Master Port]
There is one Master port on the Port Expander gateway. The purpose of this port
is to pass all commands received on any of the three slave ports to the single
Modbus slave which will be attached to this Master port. There is no way to
define a Command List for this Master port. Therefore, it cannot act as a typical
Modbus Master and poll one or more slaves on a serial network. It's sole purpose
is to pass-through commands received on the gateway's slave ports.