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Why OB35 is preferred in PCS7

Created by: manmeet at: 7/14/2012 10:35 PM (11 Replies)

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7/14/2012 10:35 PM	Rate (0)
manmeet Gold Member Joined: 1/3/2006 Last visit: 2/19/2024 Posts: 726 Rating: (71)	HI Why by default blocks are inserted in OB35 in PCS7? Why not OB1. I know you can change the insertion point but question is why OB35 is preferred?

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7/15/2012 1:47 PM	Rate (1)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/18/2025 Posts: 12330 Rating: (2700)	Hello Manmeet; The OB3x are cyclic interrupts that can be set at specific scan times; this is called the "run sequence" of the program blocks (CFC, SFC charts). By default OB35 is available in all S7 CPUs and it is set at 100 ms (adjustable by the user). The S7-400H have at least 4 different OB3x, the S7-417H has all of them (max 8), to spread the CFC blocks in different calling sources with different cycles. Using OB1 with PCS 7 is not recommended as its cycle is not set at a specific and verifiable value, which is required for the determionistic calling of the CFC blocks. You can find a lot more information in the various PCS 7 manuals, such as: SIMATIC Process Control System PCS 7 Engineering System (V8.0) 8.12.3.6 Runtime Properties of the Blocks Introduction This section describes some of the basics required to understand the runtime properties of blocks.Runtime propertiesThe runtime properties of a block decide how the block is included in the run sequence within the entire structure of the CPU. These properties are decisive for the response of the target system in terms of reaction times, dead times, or the stability of time-dependent structures, for example, closed loops.The runtime properties of the blocks have default settings but these can be adapted individually for each block.When it is inserted, each block is assigned default runtime properties. It is therefore installed in a run sequence in a task (OB). The tasks form the interface between the operating system of the CPU and the S7 program. Blocks can also be installed in runtime groups that are themselves installed in tasks (OBs).NoteWhen you create new chart, a runtime group is created automatically in which all the blocks of this chart will be installed. SIMATIC Process Control System PCS 7 CFC for SIMATIC S7 (V8.0) Chart installation pointer Each program contains its own global "chart installation pointer". This pointer determines the following: · The task into which the runtime group of the chart created next is to be inserted · The position at which the runtime group of the chart created next is to be inserted, or after which block at the task level. If the task does not yet contain a runtime group, this position is "empty". The pointer refers to the actual task. This pointer is customized only in the runtime editor (not in the CFC Editor). OB 35 is the default chart installation pointer. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 7/15/2012 1:48 PM
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7/15/2012 9:58 PM	Rate (0)
manmeet Gold Member Joined: 1/3/2006 Last visit: 2/19/2024 Posts: 726 Rating: (71)	Dear Daniel, Thanks for the inputs. Also please correct me with following understanding. PCS7 is a DCS system which normally involves in handling technological functions for example PID loop and for PID to give correct response it needs to be called at specific well defined intervals and hence they are placed in OB35. Also inputs to PID can be adjusted by defining PIP settings in hardware. Same can be said for other technological functions. Now if in case PCS7 is used for a solution which does not involve any such technological functions then whole logic can reside in OB1. Correct?

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7/15/2012 10:13 PM	Rate (1)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/18/2025 Posts: 12330 Rating: (2700)	Hello manmeet; Themostevidentdifference I faced between S7/WinCC projects and a PCS 7 project is that for PCS 7 projects there was no OB1. Every block and chart was mapped to a OB3x block, whether it was a PID logic or any other instruction. When I asked why at the time it was explained that this is how PCS 7 maintained a deterministic runtime sequence of the blocks. There are others here with more experience with PCS 7 and they are welcome to have an input to this discussion if they do not agree with what I am saying here. Hope this helps, Daniel Chartier

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7/17/2012 10:32 AM	Rate (1)
dmertens Bronze Member Joined: 10/3/2007 Last visit: 6/25/2024 Posts: 188 Rating: (34)	Hi Manmeet, Many of the PCS7 blocks use the attribute S7_sampletime where the compiler automatically replaces the value at the input with the sample time in seconds of the executing OB (e.g. 0.1 for OB35). The internal timers use this value in stead of using S7 timers as they are limited ressources and global variables which is not usefull in the type - instance concept of PCS7 where many instances of a specific FB can be present and these FB's can have a multitude of timers. If there blocks where to be used in OB1 then the value of S7_sampletime can not be established as it depends on the size of the program, this would result in incorrect timing behaviour which is not workable. However if you have blocks that do not use this functionality (e.g. your own blocks) then it is perfectly possible to create charts in OB1. Just move the first block you place in the chart in the runsequence from OB35 to OB1 and all subsequent blocks in this chart will automatically be inserted in OB1. You can also set the this chart as predecessor for subsequent charts and then all new charts you create will be in OB1. However keep in mind that this is not the standard way PCS7 works and if there is no specific bindingreason to do this, then don't. Hopoe this helps, David

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7/17/2012 10:39 AM	Rate (0)
Oreca Platinum Expert Joined: 3/14/2008 Last visit: 1/17/2025 Posts: 2161 Rating: (973)	Note that for the CEMAT library, almost all blocks are inserted into OB1, one of the exceptions. Why OB35, OB32 etc is used over OB1? If programmed in OB1,itdoesn't guarantee that the cpu processes OB1 within the set time.. If you have a PID control and stuff like that, it is not a good thing that the control logic is not executed. With OB35, 32 etc. you know the CPU processes the control logic within that time frame.
	If I could help you, you can use the RATE button. Thank you. Prospact Netherlands...your PCS7 and PCS neo Expert Topic: PCS7 Complete Overview
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7/17/2012 4:30 PM	Rate (0)
manmeet Gold Member Joined: 1/3/2006 Last visit: 2/19/2024 Posts: 726 Rating: (71)	So OB1 isn't a pariah in PCS7 as Oreca pointed out. But you should know what you are doing, avoid attributes like S7_sampletime as pointed by David, when you are playing in OB1.

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7/18/2012 12:13 PM	Rate (1)
Silent Bob Posts: 836 Rating: (100)	for PCS 7 stuff.... just don't put blocks in OB1 please. Always use cyclic interrupt blocks. Any block like PID, or timers NEED to be called in a cylic interrupt block.
Silent Bob Posts: 836 Rating: (100)
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7/18/2012 1:29 PM	Rate (0)
manmeet Gold Member Joined: 1/3/2006 Last visit: 2/19/2024 Posts: 726 Rating: (71)	What if stuff like PID and others which needs cyclic calls are not there? As pointed out by Oreca CEMAT library does needs OB1 calls.Also refer to 3rd post

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7/18/2012 2:27 PM	Rate (0)
Silent Bob Posts: 836 Rating: (100)	I don't know the CEMAT libriary. If the blocks of it arecalled in OB1, thats fine. If a block needs a cyclic call, its written in the help file for the block (should be). Usual timecritical blocks like PID (despite CEMAT libriary) need the interrupt, because the internal calculations depend on the sampletime of the blocks. Thats the reason they have the attribute S7_sampletime (usually hidden input). The compiler writes the sampletime of the cyclic block to that attribute. So the blocks know, he is called e.g. every 100ms. The calculation internally are depending on that value (integration time, whatever...). The point is, the block needs a deterministic runtime behavior. If it is called in OB1 it don't has this deterministic behavior, because nobody knows in wich time it is called (OB1 is interrupted by cylcic OBs and failure OBs, and so on). Maybe a PID would work in OB1, but it could happen that it dosn't. Blocks like TIMER_P wouldn't work correct in OB1 (it would have the wrong time at its outputs).
Silent Bob Posts: 836 Rating: (100)
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