Yeah there are some funny old folks out there....
Here is another one, "I don't like set/reset", but to OR the output round the trigger logic & then use a not to reset it is basically the same thing
---| |----| |-|----|\|-----( ) O:1
O:1 |
---| |---------|
Only difference is the logic is on one rung better in my mind for debugging though.
I have seen it many times I once worked with a guy, we were doing some mods on an existing machine where there were a lot of SR instructions, he proceeded to replace them with the above as he did not like set/reset (huh!)
It was not as if the sets & resets were in different rungs.
I have been in this game for over 35 years and come across some bad logic in my time, sequence words are my favorite and have used them extensively.
My pet hate is where bits are set & reset many times through the program.
Sequence words have a number of advantages, you can enable a function (be it an output or call to another block in a range of the word i.e. enabled between 10 & 20, while enabling another output between 12 & 18 A good example is open block valve while seq is 10 to 20 start pump 12 & 18. so in essence make sure the valve is open before stepping on to start pump.
Other advantages being able to jump steps forward/back easily, and displaying process text on HMI or Scada.
I too have been "in this game" for many years, and can concur that a sequencer built using a word value as a "step number" offers more advantages than bit logic.
Here are some of them ....
1. Easy to see where you are in the sequence. You can only have one value in a single word.
2. The "step number" can be generated at functional spec writing stage, so the software corresponds to the spec.
3. You can only ever be on any one step at a time.
4. Step number values are retentive in all PLC types. Will always survive a processor restart. Many "bit-logic" sequencers will completely reset themselves at power-on unless using Set/Reset or Latch/Unlatch bits.
5. Easy to modify, especially if there are lots of "spare" steps inserted into the functional spec.
6. Jump forwards, backwards, perform iterative loops, etc., all very easy with a numerical step sequencer. Not so easy at all with bit logic.
7. The functions of "Step-On" and "Step-Jump" can be handled by a "Sequencer Engine" piece of code that is common to all sequences. The "sequence" itself is containerised in its own routine, making fault-finding and debugging a doddle
I'm sure there are more advantages, but that list will do for now ....