Polling receive method – Comtrol UP DeviceMaster User Guide User Manual
Page 64
64 - Chapter 2. Programming Interface
DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I
Polling Receive Method
2.4.7.3. Polling Receive Method
This method provides the polling method that allows the PLC to request data on a periodic basis.
In this method, the serial/socket data is
returned in the response to the data request
message.
The following restrictions apply to this
method:
•
The file on the PLC must be of integer
type and must be of sufficient size to
contain the sequence number, length,
and data field associated with the
maximum sized received data structure.
See
59 for more information.
•
New data is indicated with an incremented sequence number.
•
The same data may be returned more than once. However, the same data packet also returns the same
sequence number.
•
No data is indicated with a length of zero.
•
While the DeviceMaster UP queues received serial port data, the PLC program must poll for new data
faster than the data can be received on the serial port to ensure the receive queues on the DeviceMaster
UP do not overflow. (Example: If two packets can be received per second on the serial port, then the
polling rate would need to be at least once every 500 msec.)
Polling Receive Data Flow