Analog Input Sampling time

[Garry]

I'm looking at using a V570 with a V200-18-E3XB snap on I/O and am trying to determine the sampling time for the analog input.  The documentation on the V200 states that the conversion time is synchronized to the cycle time.  If I then look at the V570 documentation, it states that the cycle time is approximately 9usec/1K of program size. 

 

Am I correct in assuming that (1) the analog input will read a new value every scan and (2) if my program is 1K, the analog input updates every 9usec?  If I am using multiple analog inputs, do each of them update every 9usec or do you have to allow 9us for each analog input?  Or am I going about this completely wrong?

[Michael_P]

Hi Gary, 

Your explanation and question is very clear, you are definitely correct in your assumptions. I will help clairify the rest, the method is successive approximation. It will read one channel, each scan.For example the scan time of the PLC which can be seen in SI0 is 2ms then for 3 channels it will take 3 x 2ms = 6ms.  I hope this is helpful. 

Thank you

[Garry]

Thanks Michael!

[Simon]

Can I make some additional comments.

 

Firstly you can monitor the actual PLC scan time using SI0.  This is only in units of ms so it can show zero if the scan time is less than 1ms.  If you want to get more resolution refer the following topic:

 

http://forum.unitronics.com/index.php?/topic/106-scan-time/?hl=scan+time

 

and in particular Emil's post:

 

 

SL20 - in V570 gives the scan time in units of 250 ns. In V130 and V350 - in units of 200 ns. In standard Vision models it's not supported.

Small tip - you can measure execution time of any part of your program via the function Interval. You can find it in VisiLogic > Utilities > Debug. It's described in Help topic On Line Test Mode.

 

You will find that a typical scan time for a V570 running with no ladder logic is about 500microseconds. 

 

I don't really know how to apply the quoted spec of "9usec/1K of program size.".  There seems to be a base scan time, irrespective of the ladder code.  Perhaps the 9us/kb is the additional scan time due to ladder code, I have never really figured that out.  However the scan can also be influenced via HMI loading.

[jrcordon]

Hello! I need to be able to change the sampling interval using the HMI so. Is it possible or can one only set it once prior to downloading the program?

Thanks,

Jose