Siemens S7-200 anomaly?

The second rung of a small program:

| I0.1 I0.0 v50.0
|--][------][------( )
| |
| v50.0 |
|--][----

I0.1 is a reset P/B, I0.0 is the e-stop circuit.

If the main switch is turned off with v50.0 latched, I find it is still latched on power resumption. I had to use the first scan bit to kill it on power-up.

Is this peculiar to this particular PLC, or do all PLC's act in this way?

If the data is retentive this will happen. I've just started using the 200 series and I am not sure what the default is but you can set as much of the data retentive as you like I think. Most PLC's have some if not all retentive data, several can adjust what is retained and what is not.

Hi
DesertDog's right. Open up the 'System Block' and you can adjust what's retentive. I guess that the EStop is healthy on power up?

pp

Thanks, I'll try that tomorrow.

Strange that a PLC would default to a method that could result in a dangerous situation on power-resumption, I'd have thought?

(btw the e-stop circuit has no relay so would be healthy immediately on power-up)

GP2004 said:

Strange that a PLC would default to a method that could result in a dangerous situation on power-resumption, I'd have thought?

Click to expand...

It's not the PLC that's powering up into a 'dangerous' state, it's your program. The PLC doesn't know what it has attached to it's inputs or what you are using it's memory bits for, it just knows that it has to have X bits as retentive.

This is the sort of information you need to find out when planning your program to avoid these dangerous situations occuring

Paul

P.S this post is not meant to be critical of you

PLucas said:

It's not the PLC that's powering up into a 'dangerous' state, it's your program. The PLC doesn't know what it has attached to it's inputs or what you are using it's memory bits for, it just knows that it has to have X bits as retentive.

Click to expand...

Hmm, sounds like the sort of point that would make for an interesting poll

"Which method would be safest?"

1/ immediately going to a "last-powered-state" position, by default, at power resumption
2/ mimicking traditional hard-wired control and maintaining everything off at power resumption, and making retentive the non-default setting

btw anyone know if AB's act like this? I don't recall the old 5/25's being this way, although it's been a while...

I like GE's way of handling this. %Mxxxx bits are the main bits for logic, %Gxxxx bits are intended for global comunications (but can be used for anything). Default is off for %Mxxx bits and on for %Gxxx bits. Any %Mxxxx bits that you need to be retentive can be set so by selecting a diferent a retentive output symbol instead of the regular coil output. %Gxxxx bits are always retentive.

GP2004 said:

Hmm, sounds like the sort of point that would make for an interesting poll

"Which method would be safest?"

1/ immediately going to a "last-powered-state" position, by default, at power resumption
2/ mimicking traditional hard-wired control and maintaining everything off at power resumption, and making retentive the non-default setting

Click to expand...

The method that I have always considered safest, either from a Power-Up, or going into/coming out of an ESTOP or SafeStop condition, is this one:

3/ There shall be no movement of any kind that is not operator initiated during system powerup or entering/exiting ESTOP/SafeStop.

I'm very fond of three position, return to center selector switches, and manual (contact wrapped) seal-ins, and position sensors on cylinders and the like. I also like master pneumatic and hydraulic dump valves.

First scan after a powerup, or return from ESTOP looks at the current position of everything, and sets the state accordingly (or forces it to a safe state if required).

I do leave configuration bits and things retentive, but anything that can cause motion is forced to a zero-motion state.

Does that make sense?

btw anyone know if AB's act like this? I don't recall the old 5/25's being this way, although it's been a while...

Click to expand...

AB doesn't use retentive bits, it uses kind of retentive instructions: a latch/unlatch instruction is retentive.

This is more like the old-fashioned (?) hardwired relay logic used to be: a latch/unlatch relay keeps it's state even during power-off, while a classic start/stop circuit is always in it's resting state at power-on.

Although I'm basically a Siemens man, I do like the way AB handles this matter better.

Kind regards,