Positive edge triggers

[Ron Beaufort]

Greetings Siemens people ...



first of all, I know NOTHING about Siemens - but I’d like to ask a question ... feel free to simply ignore me ...



suppose we use the program in Post #15 ...

suppose that switch M0.0 is OFF in the field ...

suppose that switch M1.0 is OFF in the field ...

suppose that the processor is powered down ...

suppose that while the processor is powered down, M0.0 and M1.0 are both turned ON in the field ... (for example: an operator manually moves a box in front of each of two proximity detectors) ...

now suppose that the processor is powered back up again ...

question A: will M0.2 give a pulse? ...

question B: will M1.2 give a pulse? ...



like I said, I know nothing about Siemens - but in an Allen-Bradley system, these questions could be food for thought ... and ... the answer MIGHT be different depending on whether we’re talking about a PLC-5, or an SLC-500, or a ControlLogix system ...



possible moral of the story: sometimes things work fine in “normal” operation - but may cause surprises when the system is powered off, or faults, etc. ...



just curious about how Siemens handles things like this when compared to Allen-Bradley ...



thank you ...

 

[L D[AR2P#0.0]]

Both circuits will behave the same and produce a pulse.

 

[Ron Beaufort]

thank you ...

 

[L D[AR2P#0.0]]

.... of course there is more to the story than this. What if the photo eyes are ON when you power down - will another pulse be produced on power on ? - this time it depends on the retentive properties of the CPU and the M flags used for detecting the positive edge....

 

[Ziemens]

Which is the trigger bit in my example below ?
Both networks produce the same results.

M0.2 and M1.2 are the trigger (one scan pulse) bits.
never use M0.1 anywhere else in the program because it is used by the plc to memorize the last rlo status.

on the other hand, with siemens plc, you can define which memory bits range is retentive or not at power up. but usually for safety reasons, i make sure that all memory bits are off on the first scan.

 

[L D[AR2P#0.0]]

M1.1 (your Maux bit) is also used to memorize the last RLO status and the same limitation applies in that you cannot modify M1.1 anywhere else in the program.

 

[Ziemens]

M1.1 (your Maux bit) is also used to memorize the last RLO status and the same limitation applies in that you cannot modify M1.1 anywhere else in the program.

you are right. but i prefer it this way. it is clearer and i can add parallel and series conditions as i like.