Communication is by Request and Response
All Modbus communication is by request and response. A master sends a request and a slave sends a
response. The master will wait for the slave’s response before sending the next request. For
broadcast commands, no response is expected. This is illustrated by three scenarios as follows:
Normal
The master sends a request to the slave. The slave sends a response with the requested information.
Exception
The master sends a request to the slave. The slave may not support the command or an error is
detected, so it sends an exception to the master.
Broadcast
The master sends a broadcast command, such as a reset command. Every slave on the network
complies with the command, and no response is sent to the master.
Requests Need a Time Limit
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 after 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 line failure. This function allows a master to give up on a request if
no response is received within a certain amount of time. This is illustrated as follows:
Response Timeout
The master sends a request. The slave is unresponsive for the amount of time specified by the
response timeout function. The master gives up on the request and resumes operation, allowing
another request to be initiated.
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