ProSoft Technology MVI71-MNET - Manual (Page 76)

MVI71-MNET ♦ PLC Platform Reference
Modbus TCP/IP Interface Module User Manual
Page 76 of 109
ProSoft Technology, Inc.
June 23, 2009
An exception to this normal mode is when the pass-through mode is
implemented. In this mode, all write requests will be passed directly to the
processor and will not be placed in the database. This permits direct, remote
control of the processor without the intermediate database. This mode is
especially useful for Master devices that do not send both states of control. For
example, a SCADA system may only send an on command to a digital control
point and never send the clear state. The SCADA system expects the local logic
to reset the control bit. Pass-through must be used to simulate this mode. The
following illustration shows the data flow for a slave port with pass-through
enabled:
Client Driver
In the client driver, the MVI71-MNET module is responsible for issuing read or
write commands to servers on the Modbus TCP/IP network. These commands
are user configured in the module via the Client Command List received from the
module's configuration file (MNET.CFG) or issued directly from the PLC
processor (event command control). Command status is returned to the
processor for each individual command in the command list status block. The
location of this status block in the module's internal database is user defined. The
following flow chart and associated table describe the flow of data into and out of
the module.