Convert Ladder to SFC and ST

Hello Friends

I would like to represent the start/stop of a motor in the five programming languages (Ladder, FBD, IL, SFC, ST)

I have done in Ladder, FBD and IL.

Somebody could help me to do in SFC and ST?

Thanks in advance.

ST

Another way to do it in ST would be like this:

It's has same function. This is usually how beginners tend to program (in contrast to the post above).
But they would likely check for Start first:
It's easy to read but would slightly mess up things if Start and Stop are active at the same time. So it doesn't work exactly like the ladder example.

A slightly more elegant variation of the first example in this post could be like this:
.

Pete.S. said:

ST

Code:

Motor:=(Start [COLOR=Blue][B]or[/B][/COLOR] Motor) [COLOR=Blue][B]and[/B][/COLOR] [COLOR=Blue][B]not[/B][/COLOR] Stop;

Click to expand...

The stop-pushbutton should be NC:

motor = (start of motor) and stop

I agree with Ivo. The industry standard is that the stop button is a physical N.C. contact. So it should be an N.O. in the program.

It also helps when the symbols include the information, not just "start" and "stop", but "start_PB_NO" and "stop_PB_NC".

also, in the screenshot, the parallel contact that latch the motor, points to a different address O:0.1, instead of O:0.0.

ivo.maenen said:

The stop-pushbutton should be NC:

motor = (start of motor) and stop

Click to expand...

No, look at the ladder. It's a NO push button.

JesperMP said:

I agree with Ivo. The industry standard is that the stop button is a physical N.C. contact. So it should be an N.O. in the program.

It also helps when the symbols include the information, not just "start" and "stop", but "start_PB_NO" and "stop_PB_NC".

also, in the screenshot, the parallel contact that latch the motor, points to a different address O:0.1, instead of O:0.0.

Click to expand...

I don't agree with that at all. NC for stop button was the standard 30 or more years ago when we used relays and not PLCs.

For a PLC I think it makes more sense to use positive logic whenever possible, meaning logical 1 is 24V and the state when the input/output is activated. That makes all push buttons normally open, NO.

Exceptions to that would be emergency stops and other safety circuits.

In real life I seldom use two buttons for start and stop and a self-latching circuit. Most systems I work with already have a HMI or several and are also often controlled from DCS/SCADA systems. That means a start command can be issued from many places. My preference is for an illuminated push button which I program like a toggle. Then it's easy to see if something is running (it's illuminated) and it can be easily controlled from anywhere and it doesn't take up much space.

.

And if the real world physical wire falls off the pushbutton or you have a wire break what happens in your logic if the button is wired normally open?

Steve

nehpets said:

And if the real world physical wire falls off the pushbutton or you have a wire break what happens in your logic if the button is wired normally open?

Steve

Click to expand...

As Pete wrote:

Pete.S. said:

Exceptions to that would be emergency stops and other safety circuits.

Click to expand...

Interesting discussion.

I've usually heard the argument of: "what if the wires to the stop button are broken?".

With a NO contact as a stop you then can't stop the motor unless you also have an emergency stop. I'm pretty sure I've seen a few Siemens tutorials where they use NO buttons as stop though.

I do not condone the use of SFC for this purpose but for educational purposes here is my stab at doing the start/stop in SFC (With a Normally Closed Stop Button).

To the right of the step boxes is the ST command in the step box.

Thank you all por your responses, were very useful
:site: