About Siemens Plc Scan time Calculation

Unregistered

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I have a setup in GE Plc, where PLC receives data from GBC(GENIUS BUS CONTROLLER) module,which inturn gets data from BIU module.These BIU modules gets data from the corresponding I/O MODULES linked to it, in the field.Similarly, I have ABOUT 30 BIUs,which finally is linked to a single GBC.My configuration is for about total of 2000 I/Os. Hence, after calculation it has fuond that GBC apporximately takes 30milliseconds to update itself with latest datas.
So,can anyone tell me how can we implement this kind of set up in SIEMENS OR MODICON OR AB plcs.Also, I want to know the calculation of scan time with which, the main interfacing modules gets updated with latest information from fields.
Can anyone help me or give me idea of SCAN time calculation.

Thank you.
 
Hi Unregistered Guest,

For Siemens S7-300 and -400 PLCs, the main scan cycle is done by Organisation Block 1, OB1. Each time OB1 executes there are a number of temporary declarations whose values are automatically updated by the operating system. These include OB1_PREV_CYCLE, OB1_MIN_CYCLE, OB1_MAX_CYCLE. These are three integer values with units of milliseconds. They measure OB1 execution from start to next start, including I/O update and any other interrupt routines which may have taken place along the way.

For things like Profibus I/O of course, the actual exchange of data is usually much more frequent. Check the 'Network Properties' in a STEP7 project and you'll see that the 'Ttr' and 'Ttr typical' calculations for a Profibus configuration. The 'T'oken 't'target 'r'otation time shows how frequently the DP master controller chip exchanges data with the slaves. I would guess that for most applications this will be 8-12 times faster than the PLC scan time. But of course this may result in the same value being exchanged with the I/O 8 or 12 times during a single scan until the PLC logic changes it.

Regards

Ken
 
'unregistered'

Genius Bus is GE Fanuc's proprietary network standard.

Are you planning to connect an existing or planned Genius Bus system to Siemens, AB or Modicon PLCs ?
That may pose a problem, as Genius Bus isnt supported by the other manufacturers.
On the above website, it is stated that Genius bus is "Supported by many third-party interfaces, including Variable Frequency Drives, Operator Interfaces, Human Machine Interfaces, and many other devices". I would check up on that claim.

If you are free to select which bussystem you want, then it is much easier.
For Siemens the natural choise will be Profibus, for AB it will be DeviceNet or ControlNet, and for Modicon ... I dont know, someone else may tell.
 
I have some systems with about 100 stations, of which 80 are micromaster drives, and the Profibus update time is about 25ms, while the PLC scan time is about 9ms. This is not a typical situation, but the profibus scan time is always worth at least checking out.
 

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