tutorial: response time concerns

DESCRIPTION OF SCAN DIAGRAM:
Now let's consider the longest time for an output to actually turn on. Let's assume that when a switch turns on we need to turn on a load connected to the plc output.
The diagram below shows the longest delay (worst case because the input is not seen until scan 2) for the output to turn on after the input has turned on.
The maximum delay is thus 2 scan cycles - 1 input delay time.

I don't understand the "worst case because the input is not seen until scan 2" Isn't the input shown activated in scan (1)?

imagine this..you are on the beaches in Fla or Southern Cal and there are many beautiful women around BUT out of the corner of your eye you noticed a woman, so you have to turn and look to get a close look.

Actually a plc scan isnt like that but it sounded good. Think about it like this...when you read a book you do it line by line from top to bottom, a plc is similar except it reads the book over and over very fast. So if it doesnt "see" something on scan 1 it will on scan 2 which in plc time is/can be very fast...milliseconds or less depending on plc. There are numerous threads on scan times and the process on this site if you search for them.

It does look like the Off-to-On transition of the input signal should be further to the right in order to correspond to the text.

In the example, the worst case would be when the input changes state immediately after the the CPU interrogates the input module. When that happens, you have to wait through a logic solution, an output service, an input service (when the changed status of the input gets detected), and another logic solution before arriving at the output sevice when the real-world output gets commanded to turn on.

Comments Phil?

CONFIRMATION

Thank you for your reply and confirmation. I wil most likely have several more (hundred) questions in the near future.

B.G.Pavlik

Steve Bailey said:

Comments Phil?

Click to expand...

Hi Guys,

here's the image the post refers to:


During scan 1 I tried to draw the fact that the input was missed during that scan. I drew it to the left to illustrate the 'max time' during that part of the scan. But yes, it could have(should have?) been drawn further to the right, I just tried to emphasize where the '- 1 input response time' came from.

Please note that 'realistically' many plcs have an input filter of generally 10ms (to eliminate bounce) enabled by default. But the theory is still EXTREMELY important...

By the way Steve, thanks for the plug elsewhere on 11/23/02. I always appreciate such gestures
Enjoy,

tutorial: post time concerns illusttation

Thank you for your reply and thank rsdoran for his reply


Bernard