You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

Reply
 
Thread Tools Display Modes
Old November 27th, 2007, 10:59 AM   #1
Miika
Member
Finland

Miika is offline
 
Join Date: Aug 2004
Location: Etelš-Suomi
Posts: 50
Accuracy of the clock memory byte in Siemens S7-300

Does someone know how the clock memory in Siemens works? I tried to search specifications from S7 programming manual, but there was no information of the function of clock memory. Is it sycnhronized with real time clock?

In my application I need an accurate one second pulse. Now I'm comparing the seconds of the real time clock to make a pulse, but it's not very ***y way to do it.
  Reply With Quote
Old November 27th, 2007, 11:54 AM   #2
STL???
Member
United Kingdom

STL??? is offline
 
Join Date: Sep 2005
Location: UK
Posts: 879
I believe that the Clock memory is very accurate, the problem comes when you have additional program scantime added between each pulse.
  Reply With Quote
Old November 27th, 2007, 03:52 PM   #3
Miika
Member
Finland

Miika is offline
 
Join Date: Aug 2004
Location: Etelš-Suomi
Posts: 50
Quote:
Originally Posted by STL???
I believe that the Clock memory is very accurate, the problem comes when you have additional program scantime added between each pulse.
Sorry, my question wasn't clear enough.

I my case the clock pulse is used to count seconds in over 24 hours surface treatment, so more important is a minimum discrepancy comparing to real time clock in the end of the treatment. Starting and ending time are saved on scada screen at the beginning of the treatment. Remaining and elapsed time is also shown in the screen.

So if in clock memory pulse is 1 millisecond lag to real time clock, it makes 4 seconds lag in a day. It doesn't look good on the screen.
  Reply With Quote
Old November 27th, 2007, 04:00 PM   #4
S7Guy
Member
United States

S7Guy is offline
 
Join Date: Nov 2003
Location: Dayton, Ohio
Posts: 1,250
The clock pulse (or any other S7 timer) is not accurate enough to make a 24hr clock. What is wrong with using the real time clock for this? That's what it is there for.

Also, there are two ways to access the clock. The first is via an SFC, and the other is to directly address the time in OB1. Unless you care how much time has elapsed since OB1 started (and in your case, you don't), then the OB1 time will work fine. Do a search right here, and you'll see it has been discussed several times already.
  Reply With Quote
Old November 28th, 2007, 04:04 AM   #5
Ken M
Member
Scotland

Ken M is offline
 
Join Date: Mar 2004
Location: .
Posts: 1,136
You haven't mentioned which CPU you are using, but real time clock accuracy isn't a major feature of most PLC CPUs in any case. In the S7-300 series I've just checked a spec which says the 312 clock variability is "<15 seconds / day" and the 319 clock variability is "<10 seconds/day".

There is nothing to suggest what influences accuracy (temp, power-supply irregularities ...?) or what the repeatability is. Also, I assume these are worst-case limits. There is no indication I've seen of what typical figures may be achieved.

Don't go seeking levels of accuracy which are unachievable. How "real" does the real-time need to be?

Ken
  Reply With Quote
Old November 28th, 2007, 05:19 AM   #6
chud
Member
South Africa

chud is offline
 
chud's Avatar
 
Join Date: Oct 2007
Location: KALAHARI
Posts: 989
If ur using a scada system why not use the scadas time functions. Pretty sure it is linked yo your operating systems time, which is very accurate
  Reply With Quote
Old November 28th, 2007, 05:46 AM   #7
Miika
Member
Finland

Miika is offline
 
Join Date: Aug 2004
Location: Etelš-Suomi
Posts: 50
Quote:
Originally Posted by chud
If ur using a scada system why not use the scadas time functions. Pretty sure it is linked yo your operating systems time, which is very accurate
Real time clock in PLC (CPU 314) is synchronized via NTP once in every minute. So the real time clock in PLC is very "real time". For communication with scada I use cp 343-1 lean ethernet card.
  Reply With Quote
Old November 28th, 2007, 11:58 AM   #8
RMA
Member
Scotland

RMA is offline
 
RMA's Avatar
 
Join Date: Sep 2004
Location: North of Hamburg, Germany
Posts: 2,052
Quote:

There is no indication I've seen of what typical figures may be achieved.




My experience with a 317 - 2 DP and a number of 315 - 2 DPs suggest that the typical value is not far removed from the 10 - 15 sec / day quoted. I could get the time reasonably accurate used the adjustment feature in CPU properties, but sooner or later the time would start drifting off again. In the case of 317 the CPU was in an air-conditioned control room, so I wouldn't have thought it was responding to changing temperatures - no idea what the reason was, I eventually gave up and let it drift.
  Reply With Quote
Old November 28th, 2007, 01:31 PM   #9
manmeetvirdi
Member
India

manmeetvirdi is offline
 
Join Date: Oct 2004
Location: India.
Posts: 750
How OB35 will respond if it is only made to count seconds?

A M 0.0//start
JCN qw1
L 1
L MW 50
+I
T MW 50
L 10//if OB 35 set at its default value
==I
JCN qw1
L 1
L MW 52
+I
T MW 52
L 0
T MW 50//seconds over here
qw1: NOP 0


But Ithink execution time of OB35 can make it drift over a period of 24 hours.

Ok how about synchronizing with nuclear clock via cp343-1 lean;-)

Last edited by manmeetvirdi; November 28th, 2007 at 01:37 PM.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Siemens S7 300 rocket1010 LIVE PLC Questions And Answers 11 August 16th, 2006 08:27 AM
S7: indirectly addressing Byte within DB? Johnny T LIVE PLC Questions And Answers 9 September 4th, 2005 07:14 AM
Siemens S7 ethernet protocol fzhang LIVE PLC Questions And Answers 3 May 19th, 2004 01:06 PM
Siemens S7 300 sparky64 LIVE PLC Questions And Answers 3 May 5th, 2003 04:50 AM
SLC 5/03 data loss andydaoust LIVE PLC Questions And Answers 1 October 10th, 2002 07:38 PM


All times are GMT -5. The time now is 05:15 AM.


.