Communication with the Modbus TCP slaves in the configuration is performed in parallel so in
comparison with Modbus RTU where the polling is made in a sequence with Modbus TCP will
establish a point-to-point connection with each device and poll with the time set of the BusTask
or the channel(Modbus request) itself.
With this mechanism the total update time will not be affected if a slave disconnects, the polling
of the others will not be affected.
Limitations and recommended settings
The maximum amount of slaves for the Modbus driver is 64.
If a slave disconnects and the desired function is that the Modbus TCP master should connect to
it automatic once available the “Auto-reconnect” option should be enabled.
Communication with Modbus TCP to Modbus RTU gateways
When using a gateway to communicate to Modbus RTU RS-485 devices one must pay attention
to how the connection establishment works in CODESYS. Normally the function of the gateway
is to work as a transparent device which converts the Modbus TCP requests into Modbus RTU
frames. When configuring a slave located on the Modbus RTU the slave id must be set in the
configuration for the slave.
As an example if we have 10 Modbus RTU slaves the driver in CODESYS will establish 10 TCP
sessions to the gateway. Normally this will cause a problem since Ethernet-to-RS485 gateways
only handles a few concurrent connections. Refer to the documentation of the gateway in use to
find out about the limitations.
To resolve this behaviour it is recommended to use a Modbus TCP function block to be able to
handle the connection of each device from the IEC code.
The open source library OSCAT contains a function block, MB_CLIENT, like this and has been
used with success in applications with gateways.
Please contact the support team for examples.
https://store.codesys.com/oscat-network.html
Dynamic configuration and application triggers
When inserting a Modbus slave in the project tree the configuration is static. If dynamic
configuration is required this must be handled in IEC code. Use the following link for details of
how to create a dynamic configuration.
https://faq.codesys.com/display/CDSFAQ/Modbus+TCP+Slave%3A+Dynamic+Setting+of+
the+IP+Address
To use an event based approach to enable communication of certain request (channels) the
application trigger approach is one way to achieve this. Use the following link for details of how
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