Refresh Request Bit (Bit 'Ra'); Liveness Flag (Bit 'Lm'); Data Set Download Active (Bit 'Dda') - Omron NT4S Operation Manual

Standard Mode

4.20.3 Refresh Request Bit (Bit 'RA')

4.20.4 Liveness Flag (Bit 'LM')

4.20.5 Data Set Download Active (Bit 'DDA')

The refresh request function is set up by using an input variable with the PLC-
Handshake attribute. With this function, a data release for the next input variable in
the same mask will not be set until a refresh acknowledgement has been received
from the controller.
Upon request, the controller can then refresh the next variable (bit 'RA' = 1) before
sending a refresh acknowledgement (bit 'RQ' in the Write coordination byte = 1),
thereby ensuring that the value the operator works with is current.
With some communication protocols it is not possible to check, from the controller
end, whether or not the interface is in operating condition. This is why the liveness
flag function has been introduced - a simple functionality that has proven to be very
effective in practice.
Whenever the PLC wants to know whether a connection is still established, it writes
a logical 1, and later a logical 0, to bit 3 of the Write Coordination Byte.
The operating terminal constantly monitors the liveness flag in the Write Coor-
dination Byte and compares it with the status of the liveness flag in the Read
Coordination Byte.
As soon as a discrepancy occurs, the operating terminal copies the Liveness Flag
Bit from the Write Coordination Byte to the Read Coordination Byte.
The controller is now responsible for checking within a timeout whether both states
are in agreement. The transfer times and polling times must be taken into account
when defining the timeout.
The Data Set Download Active bit remains set to logical 1 for as long as a data set
is being transferred. The bit is reset after all of the data have been transferred. The
controller can then work with the new values in the recipe.
Section 4-20
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