Safety PLCs with low cycle time

rakeshkhawas10

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• Jun 29, 2018

• #1

Hi All,

I am looking for a Safety PLC which can execute the I/Os and logic in around 25mSec. This is for about 1000 I/O points. Any suggestions?

I have the following in mind, but am not sure about them:
1. Hima
2. Siemens
3. Allen Bradley

I have figured that Triconex and Honeywell's Safety Manager can't do 25msec.

Rakesh

 

mk42

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• Jun 29, 2018

• #2

Modern PLCs can do a lot in 25ms, but safety can add a fair bit of overhead as well. I'm not sure I've ever seen a system with 1000 safety IO. I'm also not sure I've ever seen a system that actually needed a 25ms reaction time.

What are you trying to do?

 

rakeshkhawas10

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• Jun 30, 2018

• #3

Hi mk42,

Thanks for your reply. Do you know which SIL3 Safety system(s) could be qualified to execute around 1000 I/Os in around 25mSec?
Thanks.

 

L D[AR2P#0.0]

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• Jun 30, 2018

• #4

Say you perform a mix of instructions for each data point.


20 booleans
10 words
2 floating point


For a Siemens S7-1518F CPU the execution time per data point would be:


20x1 + 10x2 + 2x6 = 42nS


Double it for the safety overhead. Double it again for any unforeseen stuff that gets added later.



With 1000 points the execution time would be 168 microseconds.


Adjust your instruction mix and select a CPU using the data sheet: https://w3.siemens.com/mcms/program...0/cpu/Documents/s7-1500_techn_data_cpu_en.pdf

 

Last edited: Jun 30, 2018

rakeshkhawas10

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• Jun 30, 2018

• #5

Hi L D,
Thanks for the clear explanation. Looks like Siemens S7-1518F will clearly fulfill the requirement.

 

the_msp

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• Jun 30, 2018

• #6

Beckhoff Twincat 3 has integrated safety option (TwinSafe) and depending on processor/IO can get down to 12microseconds scan time

 

mk42

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• Jun 30, 2018

• #7

L D[AR2 said:

Say you perform a mix of instructions for each data point.


20 booleans
10 words
2 floating point


For a Siemens S7-1518F CPU the execution time per data point would be:


20x1 + 10x2 + 2x6 = 42nS


Double it for the safety overhead. Double it again for any unforeseen stuff that gets added later.



With 1000 points the execution time would be 168 microseconds.


Adjust your instruction mix and select a CPU using the data sheet: https://w3.siemens.com/mcms/program...0/cpu/Documents/s7-1500_techn_data_cpu_en.pdf

Click to expand...

https://support.industry.siemens.com/cs/us/en/view/93839056

This spreadsheet is a better reference for safety code. It includes timing for each of the safety instructions for each PLC model, as well as IO overhead.

 

jstolaruk

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• Jul 1, 2018

• #8

1000 safety I/O? or regular I/O with safety sprinkled through out?

 

rakeshkhawas10

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• Jul 1, 2018

• #9

Mostly Safety I/Os. Sprinkled with some regular I/Os.

 

jstolaruk

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• Jul 1, 2018

• #10

Allen Bradley's newer 1756-L8x series has extremely fast program scan times, on the order of 10 to 15 times the 1756-L7x, but I think in the end it will be I/O RPI that will be the limiting factor. Which fieldbus network will you be using (or are using)?