User Documentation

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Exception
The master sends a request to the slave. The slave may not support the command or an error is
detected. As result the slave sends an exception to the master.
Broadcast
The master sends a broadcast command, such as a reset command. Every slave on the network
accepts the command. No response is sent to the master.
Requests Need a Time Limit  Response timeout
The original Modbus protocol was not designed for simultaneous requests or simultaneous masters,
so only one request on the network can be handled at a time. When a master sends a request to a
slave, no other communication may be initiated until the slave responds. The Modbus protocol
specifies that masters use a response timeout function to identify when a slave is nonresponsive due
to device or communication failure. This function allows a master to discard a request if no response is
received timely.
To integrate a wide range of devices (with different baudrates or line conditions) into a Modbus
communication a limit for a response time is not set and can be determined by the manufacturers.
However, this also makes it difficult for system integrators to know what response timeout value to use
during configuration, especially with older or proprietary devices.
The Weidmüller Modbus Gateway provides a speciual function that tests all attached devices and
recommends a response timeout value. This function saves considerable time and effort for system
integrators, and results in more accurate timeout settings.