This analysis focuses on the 219 errors in this site where TSTQ blocks test blocks inappropriately. There are 209 inputs to TSTQ blocks being tested that do not have quality to test. In addition, quality is being tested on ten AO/L blocks that do not have quality on their input signal. I began this analysis in the earlier article: A System Without DBDOC - Analysis I and continue it here. Keep in mind that this is a running power plant!
TSTQ Testing Blocks Without Quality
To keep this brief, the images will show the block being tested. Be confident that it is feeding a TSTQ block input and that the intended protection for the plant is simply not present. Also, you might not be familiar with the feature of DBDOC that shows lines that can carry quality in blue, to contrast with lines that cannot carry quality in black.
It turns out that, in this system, somebody did not get the point about the TSTQ block. Half of the errors show places where the logic state of a function block is  in the alarm condition, but that state is fed into a TSTQ block instead of affecting the process by being input to an OR operation directly.
TSTALM Block 8272 output does not have quality (note that it is a black line). Therefore the TSTQ Block 8917 that tests it will never trigger due to this signal. Nor will it trigger due to the actual situation the system designer presumably wanted to detect: that of TSTALM Block 8272 being .
The two instances of testing the low output (H//L Block 9763) are just as incorrect. The result should be OR'd, not tested for quality.
In these and similar situations, operational issues will fail to be detected as intended. There are 105 instances where a TSTQ tests a TSTALM [FC69] block, which does not have quality. There two instances where a TSTQ Block tests a H//L [FC12] block N+1, which also does not have quality
Blocks Not Propagating Quality
A common misconception is that AO/L [FC 30] and DO/L [FC 45] blocks are guaranteed to show bad quality. In fact, they only propagate the quality attribute. Thus, if the input to the block does not have the quality attribute, there will be no quality to test.
There are seven instances where a TSTQ tests an AO/L block, which is not propagating quality, and three instances where a TSTQ tests a DO/L block, which is not propagating quality.
A Sublime Example
Here is a sublime example of a class of error that occurs again and again. Good engineering is clutched from the jaws of victory! Here, TSTQ and REDAI blocks validly synthesize a picture of two versions of COMPENSATED GAS FLOW. However, the use of Transfer [FC9] Block 1835 destroys the quality chain, so bad quality will not be detected by the process. The problem would not have existed if a REDAI block had been used instead to propagate this signal.
More Flawed Quality Tests
- 2 instances - TSTQ tests F(x) [FC1] which does not have quality
- 2 instances - TSTQ tests A [FC2], which does not have quality
- 2 instances - TSTQ tests H/L LIM [FC6], which does not have quality
- 2 instances - TSTQ tests SQRT [FC7], which does not have quality
- 20 instances - TSTQ tests T [FC9], which does not have quality
- 8 instances - TSTQ tests SUM(K) [FC15], which does not have quality
- 23 instances - TSTQ tests [NOT FC33], which does not have quality
- 3 instances - TSTQ tests S R [FC34], which does not have quality
- 33 instances - TSTQ tests OR (2-Input) [FC39], which does not have quality
- 1 instance - TSTQ tests OR (4-Input) [FC40], which does not have quality
- 1 instance - TSTQ tests REMSET [FC68], which does not have quality
- 4 instances - TSTQ tests M/A MFC/P [FC80], which does not have quality
- 1 instance - TSTQ tests TRIG [FC171], which does not have quality
This analysis show a plant operating with hundreds of errors DBDOC has detected by flagging TSTQ block inputs that do not have quality to test. Half of them are especially worrisome because they show incorrect engineering that was never detected, i.e. it was the original intention to test a signal, not the quality of the signal, in the first place. Was there a FAT (Factory Acceptance Test)? If so, it missed them all.