Measures per minute

Hello everyone,

i'm new to plc programming and i´m trying to develop a project in a S7-300 Siemens PLC, wich consists in a measurement system of dimension and weight. My problem is that i need to show on my SCADA application the rate of measures per minute and i have no idea how can i make the PLC to give me the values i need. I´ve been trying to use a timer and a counter but it don´t seems to work because i can´t define a maximum time for each measurement. I´m programming with LAD. Can anybody give me a help?

Best regards

Félix Vaz

You could set up an interupt OB for every 1 second (1000 ms) and read the difference between this OB call and the last.


Is this too simple for your application??

I´m not sure if i understand what you ment about the OB call. Is that some kind of external timer to let me know how much time has passed since my last measure? Could you explain in a more elaborate way what you were trying to say? Remember that i´m relatively new to this matters. Thanks.

Siemens uses different 'blocks' to divide the program into logical sub-units.

OB = organisation block's
FC = function (like FB but without an DB)
FB = function block (FB needs it's own DB)
DB = datablock

OB1 is the most important because this organizes how the siemens plc will run your program that you have written in FC's, FB's and DB's. You can not call OB1, but OB1 must be programmed. Other OB's, like OB1 are called direct from the cpu and you can not program a 'CALL'-instruction for OB's. What you can do, for some OB's is define when they are called. FC's/FB's are used the divide the logic of your entire proces into subprocesses. DB's are used to store bigger amounts of data.

There are also other OB's then OB1.

OB35 is the cyclic interrupt OB. In the HardWare configuration (CPU properties/Cycle interrupt) you can define the time-interval (ms) for wich this OB35 will be executed. Example; if you define 1000 ms, then OB35 will interrupt the program of OB1 every 1000 ms. After OB35 is executed, OB1 will resume it's program. If at that moment lets say FC1 was active, then FC1 will continue.

This link should help you also:

http://www.automation.siemens.com/fea/html_76/down_module.htm