PLC scan time

Ajinkya24

Member
Join Date
May 2012
Location
Mumbai
Posts
6
Hello everybody. Before placing my doubt i request you to open the link given below.
http://www.plcs.net/chapters/respdetails6.htm
It is the page "effects of response time" from Learn PLC's section of this website itself. The last example given at the bottom of page shows this diagram.[FONT=Arial, Helvetica, sans-serif][SIZE=-1]
maxot4.gif

They have said [/SIZE][/FONT][FONT=Arial, Helvetica, sans-serif][SIZE=-1] (worst case because the input is not seen until scan 2). But as can be seen from the figure the input is getting on in the 'in' mode of scan 1 itself. So how is it that input is not seen until scan 2?
Please rectify me if i am wrong in understanding the example. Thank you.
[/SIZE][/FONT]
 
During the IN time, the inputs are scanned and stored in memory. If an input changes state during the IN time, the state may or may not be read depending on if the the particular input has been scanned and stored in memory yet.
 
As a separate point, the link you posted states the worst case delay time as [FONT=Arial, Helvetica, sans-serif][SIZE=-1]2 scan cycles - 1 input delay time

[/SIZE][/FONT]I reason that this should be [FONT=Arial, Helvetica, sans-serif][SIZE=-1]2 scan cycles [/SIZE][/FONT]as the output may not be turned on until the OUT phase has completed (and the ouput concerned could be the last output) - is there a flaw to my reasoning?
 
Dear L D,

Thank you for the guidance. This helps me a lot. Does this mean that the change in state of input should stay constant during input mode of scan cycle?
 
Each input module is checked once during the period marked "IN" of each scan. If the actual state of the input device changes, the program in the PLC won't know about that change of state until the next "IN" portion of the scan. It doesn't particularly matter what the real-world device does during the "IN" phase. The state of the input in the PLC program will be its state at the instant the PLC interrogated the input module.

If the actual state in the real world changes one microsecond after the PLC interrogates the input module, the PLC won't know about the change until the next time it interrogates that input module.
 

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