S7-PLC, Set a bit and reset it some where else

R.Mati · May 27, 2012

Hi all,

It is a Siemens question again

I want to turn a bit ON in Ladder logic to be used as trigger to run a VB script.
Then VB code will turn OFF that bit.

The thing is I can not turn the bit OFF because it is ON from the ladder logic, "the ouput bit is coil".

I want to make a bit ON without using a Coil so I can turn it OFF using VB code.

Is there a way to work around this?

Thanks so much.

John Gaunt · May 27, 2012

It is the clasic Motor Run Stop using Start and Stop pushbuttons.

R.Mati · May 28, 2012

John Gaunt said:
It is the clasic Motor Run Stop using Start and Stop pushbuttons.

I want to set a bit and reset the same bit in the VB code, but as it already set in Ladder it does not allow me to reset it in the VB code.

bara_hence · May 28, 2012

Have done this and it shouldnt be a problem..

Are you using a pulse to set the bit so the conditions to set the bit isnt true when VB tries to reset the bit?

uptown47 · May 28, 2012

Can you not just 'move' a 0 into the Word (or Byte) that contains that bit? (I may be mis-understanding your question to be honest...)

R.Mati · May 28, 2012

uptown47 said:
Can you not just 'move' a 0 into the Word (or Byte) that contains that bit? (I may be mis-understanding your question to be honest...)

my bit is just like that DB100.DBX10.0 which is not a part of a word.

MarkNightingale · May 28, 2012

bara_hence said:
Are you using a pulse to set the bit so the conditions to set the bit isnt true when VB tries to reset the bit?

That is the way I would do it.

Mark

uptown47 · May 28, 2012

R.Mati said:
my bit is just like that DB100.DBX10.0 which is not a part of a word.

It is part of the Word DB100.DBW10 (or DBW9 for that matter).

You could also move a '0' into DB100.DBB10 (Data Block Byte) which includes bits 0..7 inclusively.

Or, you could just do the previous suggestion of 'pulsing' the ON condition so that when the OFF condition comes then it can be turned off again.

;-)

(Or have 2 mutually exclusive bits which reset / set each other for the ON and OFF conditions....)

wimpiesplc · May 28, 2012

You could also move a '0' into DB100.DBB10 (Data Block Byte) which includes bits 0..7 inclusively.

that is not helping. you still need to "pulse" the bit like bara_hence said. else the ladder will still make it "on". quick example -
bad:

Code:

|  M0.1                 M0.2
|---][-------------------( )---
|

as long as M0.1 stays high, M0.2 is also high.
you can not assume the VB program will keep/set the M0.2 bit low because for example updates from "outside" like HMI's happen asynchronously with the PLC cycle scan.

"good":

Code:

|  M0.1    M0.3          M0.2
|---][------(P)-----------(S)---
|

first we need a pulse (positive flank detection), then we set/latch M0.2 so it can be reset/unlatched from outside
M0.1 = PLC wants VB action
M0.2 = communication to VB, action needed
M0.3 = pulse of M0.1

correct me if I'm wrong

R.Mati · May 29, 2012

Thanks to all of you.

________________________________

wimpiesplc said:
"good":

Code:

| M0.1 M0.3 M0.2 |---][------(P)-----------(S)--- |

first we need a pulse (positive flank detection), then we set/latch M0.2 so it can be reset/unlatched from outside
M0.1 = PLC wants VB action
M0.2 = communication to VB, action needed
M0.3 = pulse of M0.1

correct me if I'm wrong

That sounds good to me, I will try it and let you know.

R.Mati · Jun 7, 2012

wimpiesplc ....

Thanks so much, all sorted.

wimpiesplc · Jun 7, 2012

glad it helped

thanks for the response

S7-PLC, Set a bit and reset it some where else

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