The IO scanner makes it possible to periodically read or write to/from remote inputs/ouputs on the Ethernet network, without requiring any specific programming.
This service comprises the following essential elements:
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a read field containing all the values of the remote inputs,
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a write field containing all the values of the remote outputs,
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scanning periods independent of the PLC cycle and dedicated to checking each remote device.
The scan will only be performed if the PLC is in Run mode.
This service works with all devices supporting Modbus communication on the TCP/IP profile in server mode.
The exchange mechanism, which is transparent for users, involves:
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read requests,
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write requests,
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read and write requests.
The following diagram shows how scanning of remote inputs/outputs works.
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As soon as the PLC goes into Run mode, the module opens one connection per scanned device.
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The module then periodically reads the input words and periodically writes the output words of each device.
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If the PLC goes into Stop mode, the connections with each device are closed.
The functions of the IO scanning service are to:
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manage the connection with each remote device (one connection per scanned device),
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scan the inputs/outputs of the device by using the Modbus read/write requests on the TCP/IP profile,
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update the read/write fields in the application memory,
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refresh the status bits of each remote device.
NOTE: The status bits indicate whether the input/output words of the module have been refreshed.
