S7 200 and OP73 micro

[Ingeni@rio]

hi guys,
this is my first post on this forum and hopefully not the last
I am a mechanical engineer trying to learn as much as I can abou PLCs and need your help in my learning procress...
I`m working on a project of replacing old relay logic (cca 30 relays+5 timer relays) automated hydraulic press for pressing paper moulded fiber cartons with S7 200.
I made a ladder logic for controlling the press and would like to replace start/stop switches placed on switchboard doors with OP panel.
The reason is that in that way I will need less DI(primarily for manual work mode) and also hope to learn winCC Flexible in the process.
Now I have seen in some programs that programmers use V-bit instead of M-bits for an ON/OFF function on panel.
Can somenone please elaborate what is better and why?

 

[Paul B]

In the S7-200 you have M memory and V memory. As a general rule of thumb I do not use M memory because it is not possible to upload or save the M memory. For example if you use V memory you can put the plc in stop and create data block from RAM to save all values. You can also upload the data block (the V memory) to your laptop for use in downloading to another controller, as in you have 150 of these machines to put programs in. Also the M memory is limited, only 31 bytes I think it is, while V memory is abundant (in excess of 5000 words).

Now if you think about M memory as being temporary then it makes perfect sense to use it for bits coming from an HMI device. In the end it is up to the user.

For momentary bits from the HMI I will configure the HMI to set the bit. Then in the plc code at the end of the scan (last subroutine or group of rungs called) I will enter rungs that say if the momentary bit is set, reset it. In this way if the HMI sets the bit it cannot get "hung". You would be amazed at how much trouble this prevents.

Good luck!

 

[Ingeni@rio]

Thank you very much Paul for the answer...