Siemens plc scanning speed question

Nan · Nov 30, 2015

Hi,

I am new to Siemens plc. I am using a 315-2 DP plc right now. I am wondering if there is a way to slow down the plc scanning speed to 1 second in the specific network, because I am trying to send message every second to the LED display. Thank you very much!

rollinstone · Nov 30, 2015

Nan said:
Hi,

I am new to Siemens plc. I am using a 315-2 DP plc right now. I am wondering if there is a way to slow down the plc scanning speed to 1 second in the specific network, because I am trying to send message every second to the LED display. Thank you very much!

Wouldn't it be easier to put inside timer which triggers sending message every second?

What I mean by saying timer - I'd use Cycle/Clock Memory byte which you can configure in your HW Config

Nan · Nov 30, 2015

Thank you for your response! when I put a timer on it, it will trying to send multiple message during 1 second and then stop for 1 second and so on...

rollinstone · Nov 30, 2015

Nan said:
Thank you for your response! when I put a timer on it, it will trying to send multiple message during 1 second and then stop for 1 second and so on...

I suggest You to read this one
http://wiki.hmkdirect.com/mediawiki/index.php/Clock_Memory

mk42 · Nov 30, 2015

Nan said:
Hi,

I am new to Siemens plc. I am using a 315-2 DP plc right now. I am wondering if there is a way to slow down the plc scanning speed to 1 second in the specific network, because I am trying to send message every second to the LED display. Thank you very much!

It is possible, yes. If you remove the networks from OB1, you can put them in a cyclic OB (OB35, etc). Make sure you set the cyclic OB period to 1 second in the CPU properties in the HW config.

HOWEVER, what you probably want to do is what the other posters suggested, with the timers/clock bits. You may need to use an edge detection instruction so that it only triggers once.

Nan · Nov 30, 2015

Thank you for all replies! I end up use clock memory and one shot instruction(-|p|-) to make it work... If I use DB35, it will interrupt the operation of OB1. Thanks again for all your helps!

bkottaras · Nov 30, 2015

If you use DB35 it will not do anything to the main OB1.
However if you use OB35 it will.
Hope the DB35 was just a mistake.

mk42 · Nov 30, 2015

Nan said:
Thank you for all replies! I end up use clock memory and one shot instruction(-|p|-) to make it work... If I use DB35, it will interrupt the operation of OB1. Thanks again for all your helps!

Glad you got it working! To follow up to a possible misconception:

Assuming you mean OB35, yes, it will interrupt OB1. However that interruption should only be very brief (if you are running a small amount of code), and then OB1 will pick up right where it left off. Cyclic interrupts are not required, but they can be very useful, and I've seen many systems that mix them with OB1.

DeKemp · Nov 30, 2015

mk42 said:
Cyclic interrupts are not required, but they can be very useful, and I've seen many systems that mix them with OB1.

Indeed, the thing that needs OB30-38 are for example PID controllers or other time critical functions

Nan · May 18, 2017

Thank you very much for your replies!

Siemens plc scanning speed question

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