I was surprised by a pump indicating shutdown/interlocked (using DC blocks) when the setpoint and output were already set to the passive states. This did not make sense to me as the interlock can't accomplish anything more than is already being accomplished except preventing the output from being set to an active state if the setpoint changed.
The specific scenario is needing to block flow before stopping the pump to prevent backflow, but interlocking the pump off if the valve used to block flow is closed and the pump is running deadheaded for too long.
I realize the DC block has likely always worked like this, but I can't seem to understand why it should work that way. I'm just curious if there is some rationale or use case that explains why it works this way. I find knowing the history of things makes me more effective at applying.
In reply to Brian Hrankowsky:
Here you go. Careful what you ask for...
Andre Dicaire
In reply to Andre Dicaire:
Well looks like I have successfully waited long enough with my vacation so Andre can answer your question(s). The only thing I will add is that the DC block was designed by the Fieldbus Foundation and Emerson had to follow this design as manufacturers "could" put these block(s) in Foundation Fieldbus devices. Now this particular Discrete Control (DC) block I have personally never seen in a device (Analog bus so most discrete devices were on discrete buses AS-i, Profibus, Devicenet, etc) and I don't believe there has been any changes to this block for a very long time. It does NOT have some items that we have needed to migrate PROVOX/other control systems and it took many revisions to get a block to allow us to do things more natively in DeltaV by using the Enhanced DC block starting in v13 (more than 3 SPs, Interlocks to non-passive states, motor operated valves-stop/open/close, pulse outputs).
I don't have the time to confirm because of project activities but I think the EDC will work the same in your instance (and Michael's) but there was some changes done to allow the stop/open/close of a motor operated valve (Stop command given but the PV would show Open or Close if that was the PV) that "might" cause it to work differently in certain configurations. Maybe Andre or someone else would have time to test/verify but I don't unfortunately.
And sometimes it might be better to not take the red pill and just be the battery with some 'knowledge'...since Andre has taken us in the matrix