Schneider Electric Modicon BM85 - Manual (Page 44)

Configuring the Modbus Models 890 USE 103 00 34
3.4.7 Modem Booster
How the Parameter Applies to the BM85
The Modem Booster applies to BM85 Modbus ports that will be
configured as Network ports. You will need this information for any port
you will be using as a Modbus Network port.
The parameter does not apply to a port that is being configured as a
single Master or Slave device port. You can bypass the information
below for any port you will be using as a single Master or Slave port.
The Purpose of the Parameter
The Modem Booster parameter determines how the BM85 will handle
Modbus commands to networked slave devices at a Modbus port.
When the BM85 receives a command that is addressed to a slave device
at a Modbus network port, a response is expected from the slave device.
Two methods are available for handling the response, defined by the
function code in the command issued by the master. The two kinds of
commands are:
V `Suspend' Transactions (Function Code 126) With this type
of command, the slave device will send a response after it
completes the action requested in the command. The master
device waits for the response, and therefore other commands on
the Modbus network cannot be processed until the slave's action
has been completed and its response received. Other transactions
intended for the port must remain pending until that time.
SomeĂcommands, such as `start' and `search' commands from a
Modicon P230 programming panel to a slave controller on the
Modbus network, can take a relatively long time for processing in
the controller, in the range of 15 seconds to 2 minutes.
V `Polling' Transactions (Function Code 13) With this type of
command, the slave will send an acknowledgement of receipt of the
command, and must be polled later by the master device to
determine if the requested action has been completed. The
command acknowledgement frees the Modbus network for
processing messages to other devices on the port. The BM85 will
poll the slave device periodically to determine if the command
action has been completed. It will then send the final response to
the originating device.
The Modem Booster parameter allows you to specify which of the two
methods will be used at the BM85 Modbus port.