Modify/force variable in STEP 7

mikas_m

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Join Date
Feb 2007
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Hello, I'm having problems with modifying variables in STEP7. I'm trying to force memory variable via VAT (variable table) but no succes.
For example, let's say I have the following code:
A I 0.0
= M 3.0
Now, obviously value of M3.0 will depend on state of input I0.0. For the purpose of testing I'd like to modify M3.0 to some value (1 or 0) no matter what I0.0 is.
Is this possible to do via VAT Modify option?
I really need to have a way of forcing M3.0.
How this can be done?

Thanks
 
Forces only work on i/o.
There is no forcing of M and DB addresses. You can modify them in a VAT table, but that does not stop the PLC program from changing them.
 
Oh, about how to force the M and DB addresses.

I dont know exactly what you want to achieve, but I have some "simulate" bits that anable me to run my PLC programs without the real i/o being connected (for example with PLC-SIM).

When a simulate-bit is active it does 2 things:
1. Read in of inputs and read out of real outputs are blocked. This so that no harm can be done in case the real i/o IS connected.
2. A simulation of the "outside world" is activated.
This means that my programs will run as if it had the real i/o and everything connected.
It also means I can toggle "inputs" that do not have simulation code attached.

Is that what you want to do ?
 
Hello Jesper
Well, not quite, that I0.0 is just simple example. In reallity I have some more complicated logic that sets or resets M3.0. M3.0 is then passed to another logic. I need to test sequence of motor drive without some initial conditions during maintenance. I want to set M3.0 at will and want that simulated value to be processed further. I can do the same thing without simulation, but I need to bypass a couple of contacts in the field. The reason I want to force this merker memory is because I don't want to change program logic, because it there is always possibility to leave something changed and not returned to previous state.

Thanks anyway
 
Hi again.

It is not possible without changing the existing program.

You should program maintenance functions so that they are always available, without having to change the PLC program.
 
mikas_m said:
Hello Jesper
Well, not quite, that I0.0 is just simple example. In reallity I have some more complicated logic that sets or resets M3.0. M3.0 is then passed to another logic. I need to test sequence of motor drive without some initial conditions during maintenance. I want to set M3.0 at will and want that simulated value to be processed further. I can do the same thing without simulation, but I need to bypass a couple of contacts in the field. The reason I want to force this merker memory is because I don't want to change program logic, because it there is always possibility to leave something changed and not returned to previous state.

Thanks anyway
Click to expand...
Dear mikas
I have the same probelm exactly. if you find the way please let me know.
 

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