Schneider Electric Modbus Plus Network Bridge Multiplexer none - Manual (Page 45)

Configuring the Modbus Models 31007492 35
Setting the Parameter
You can select `No' or `Yes' for the Modem Booster parameter at any
serial port. The parameter will have the following effect:
If you select `No' for this parameter, incoming commands with function
code 126 or 13 are passed directly to the slave device at the port. If the
command is function code 126, the slave response will be returned to the
originator of the Modbus command after the slave has completed the
requested action, or when it returns an exception response indicating
that it cannot complete the action.
Selecting `Yes' causes the second type of response (function code 13) to
be used. If the command specifies function code 126, the BM85 will
perform a conversion to function code 13 before sending the command to
the slave device. The BM85 will then automatically handle the polling of
the slave device until the command action is completed. It will return
the final response to the originator of the command. The polling process
is handled by the BM85 transparently to the originator.
Modem Booster Example
A P230 programming panel connected to a Modbus Plus networked
controller can access a slave controller that is part of a Modbus network
at a BM85 port. The P230 panel can remotely interrogate, program, and
start the slave controller.
If the Modem Booster parameter is set, the P230 can issue a command
with function code 126. The BM85 converts the function code and sends
the converted command to the slave. The BM85 then handles polling of
the slave, returning a response to the P230 when the slave device's
action is completed.
To the originating device, the response appears as the one from the slave
for function code 126. During this time, however, normal network
transactions can continue between other devices in the user application
and the other slave devices on the BM85 port's Modbus network.
Note:ąFor Modicon 584 Controllers The Modem Booster will not
function with 584 controllers that are in SAFE84 mode. Function code
13 is not recognized by those controllers in that mode. You should
select the Modem Booster `No' option for a BM85 port that will have
584 controllers connected to that port's Modbus network.