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CPU execute instructions

Created by: allayahassine at: 5/29/2014 6:12 PM (2 Replies)

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allayahassine Posts: 133 Rating: (0)	OK, another question: when chosing a CPU from the list of HW Config, among the properties there is a value followed by ms/kinst (ex: 0.3 ms/kinst). Is that means that the cpu execute 1000 instructions in 0.3 ms ? if so, this property is for machine language (after the translation of LADDER) ? Finally, how i choose a CPU having the suitable value ms/kinst ?
allayahassine Posts: 133 Rating: (0)	Last edited by: O_Moderator at: 5/31/2014 12:36 PM new subject after splitting from /tf/WW/en/Posts/109114#top
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hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 1/8/2025 Posts: 6858 Rating: (1366)	allayahassine OK, another question: when chosing a CPU from the list of HW Config, among the properties there is a value followed by ms/kinst (ex: 0.3 ms/kinst). Is that means that the cpu execute 1000 instructions in 0.3 ms ? if so, this property is for machine language (after the translation of LADDER) ? Finally, how i choose a CPU having the suitable value ms/kinst ? It is not recommended to count all instruction in a program and calculate the whole required time for execution. I always support the idea of a real test with the maximum computational load such as activation of all communication, putting all loops in auto and activate all possible logics to see what is maximum time is required for execution.With all test, the max value shown in module properties is only a rough value for process reaction time. Selecting a CPU with a proper computational capacity includes some rules of thumbs.There is no tool for proper selection.For small machines using options in Micro Automation or small S7-300.For medium sized machines use more complex S7-300 series or small S7-400.For plants and complex plant use S7-400.There are other options to have the best selection: Size of process image Being able to process safety programs Redundancy and changeover reaction time I hope this brief explanation helps you! Hamid Hosseini
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5/29/2014 6:52 PM

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Ayman Elotaify

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allayahassine
OK, another question: when chosing a CPU from the list of HW Config, among the properties there is a value followed by ms/kinst (ex: 0.3 ms/kinst). Is that means that the cpu execute 1000 instructions in 0.3 ms ? if so, this property is for machine language (after the translation of LADDER) ? Finally, how i choose a CPU having the suitable value ms/kinst ?

Hello

A very good question.

A short answe, this value is for the number of bit operations that this CPU can handle in certain time.

Normally each CPU execute instructions according to technical data.

Let's take an example of CPU 315-2DP "6ES7315-2AH14-0AB0"

According to technical specification;

6ES7315-2AH14-0AB0technical specification

CPU processing times
for bit operations, typ. 0.05µs
for word operations, typ. 0.09µs
for fixed point arithmetic, typ. 0.12µs
for floating point arithmetic, typ. 0.45µs

So this CPU take 0.5 micro sec. for operation of a bit instruction.

In HW config of CPU 315-2DP, it's written "6ES7 315-2AH14-0AB0

Work memory 256KB; 0.05ms/1000 instructions"

That's the same number provided above. It means that this CPU will take 0.5 msec. to execute 1000 bit operations.

CPU execute instructions

Created by: allayahassine at: 5/29/2014 6:12 PM (2 Replies)

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CPU execute instructions

Created by: allayahassine at: 5/29/2014 6:12 PM (2 Replies)

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