ProSoft Technology 5202-DFNT-MCM4 - Manual (Page 11)

Functional Overview MCM ♦ ProLinx Gateway
Driver Manual Modbus Master/Slave
ProSoft Technology, Inc.
Page 11 of 80
October 16, 2009
2 Functional Overview
In This Chapter
™ Modbus Master/Slave Port(s) ............................................................. 11
™ Serial Port Specifications ................................................................... 12
™ Modbus Master/Slave Port Specifications........................................... 13
™ Module Internal Database .................................................................. 14
The Modbus Master/Slave protocol driver may exist in a single port (MCM) or a
multiple port (MCM4) implementation. In either case, each port can be
independently configured as a Master or Slave port to interface, as required, with
other Modbus devices.
2.1 Modbus Master/Slave Port(s)
The Modbus Master ports can be used to continuously interface with Modbus
slave devices over a serial communication interface (RS-232, RS-422 or RS-
485). Each Master port may have up to 100 user defined commands which
determine the Modbus read/write messages issued to each slave attached to the
port.
In addition, the gateway can be configured to poll slave devices that are not
responding to commands at a lower priority (slow-poll mode). If a slave device
has failed to respond to a command after a user-defined number of retries, the
driver will recognize the slave is experiencing a communication failure. In this
case, if you have set a non-zero value in the Error Delay Count parameter in the
Master port configuration, then, as the driver comes to a command on the Master
Command List that should be sent to the failing slave, instead of sending the
command, the driver will skip the command and decrement the error delay
counter value by one.
When the number of commands have been skipped is equal to the value set in
the Error Delay Count parameter, the error delay counter will have been
decremented go zero and the driver will send next command for that node in the
command list. If the slave responds properly to this command, the driver will
resume normal polling of this slave. If the new command fails, the driver will
continue to recognize the slave is experiencing communication problems and
repeat the process of skipping commands and decrementing the error delay
counter before trying to send another command to that slave. This has the effect
of putting the slave in a low-priority, slow-poll mode until it begins to
communicate normally again.