Anita, as you see, the answer to your question is "It Depends".
Mike, thanks for the good description of how Mod 300 handles fail open valves.
I worked for Taylor Instruments (now a small part of ABB) when Mod 300 was developed and the issue of fail open (air to close) valves was handled. A little background:
In the analog controller days Taylor (and I think other) controllers would reflect the valve operation in the controller action. That is, a flow loop with a fail open valve would be set to direct action.
There was always a way to indicate to the operator the position of the valve rather than the level of the signal. Early pneumatic controllers had stick on OPEN and CLOSE labels; later electronic controllers had slide switches that would invert the operation of the output meter and manual open and close buttons.
In the late 70's the early computer control mini-computers were connected to analog output boxes and would send up or down pulses to analog boxes that would generate the 4 - 20 ma signals. The analog boxes had reverse action functions to decrease the current when an up pulse was received. So from the stand point of the computer -- the PID algorithm, the operator displays, and other functions -- a flow loop was always reverse acting. That made the software configuration much simpler. To configure the PID action you looked only at the process. In
a complex loop (override, etc.) the signal was in terms of percent valve position.
In the early 80's we carried the same philosophy into Mod 300. A function block handles the analog output and is configured with such things as the channel number and whether or not the signal was inverted. The output of the PID function block represented the valve position. That PID output is displayed to the operator. The color of valves on graphics, etc. is based on the output of the PID without regard to the valve action.
Personally, I think this is much simpler, particularly in the more complex loops. 100% is always open (even if the analog output converts it to 4 ma.)
I have heard that most, but not all, DCSs use the same scheme as the Mod 300.
There has been some confusion among customers and engineering firms that worked with other types of controls that used the older analog form. (I have received more than a few calls at home at night from sites going through startups). I have a drawing illustrating the two methods of handling fail open valves that I used when I was at ABB. It is on my web site at
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I do not know how the new 800XA handles the issue. I think that there is more Bailey (recently acquired by ABB) than Mod 300 influence. I will soon find out about how 800 does it.
This is an important issue (nothing can mess up a control loop more than having the wrong action - and, with complex control loops, correcting it can be time consuming during a startup!). Unfortunately, it is very difficult to ask a DCS vendor and get an explanation. I don't think IEC 61131 addresses the issue.