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
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
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
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 <a
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.