In reply to Youssef.El-Bahtimy:

The DeltaV Serial Output Data set can be configured with Read Back enabled.  This, I believe, has the Output data set added to the Input Data set reads.  If DeltaV writes to this data set, the value is written.  On the read scan, the same data set is read from the PLC.  If the PLC changes the value, DeltaV will be able to see it.

The DeltaV Module can read the current value of the register and depending on the logic execute a write as needed.  We used this a long while back on a system to create momentary start bits in the PLC.  DeltaV would write to a coil and the PLC would reset the coil several seconds later.  The write only occurred on a command initiated by an expression or an operator.  Not a wired connection to an External reference.

The PLC would always win on a race condition.  The logic in both systems has to take this into account. If this is a manual comman, like an Action Block where you write the IN_D to 1 and the logic clears it after executing an assignment commnad to the register via an External reference.  You can read the External reference and only write if it needs to be written to.  Also, the DeltaV Module should not be able to repeatedly write to the register.  I would use an Input register configured as an External reference.  You could wire the value as an input to a block but you can only manipulate the value with an expression assignment statement or by user write to the parameter.

Andre Dicaire