PLC cpu

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Anonymous Poster	PLC cpu 01/31/2008 10:53 AM
	Hi... Can any one explain to me the working of a siemens PLC cpu. well if u can provide me some good sites then it will be a lot helpful. Thank u.
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JohnDG Guru Join Date: May 2006 Location: Reading, Berkshire, UK. Going under cover. Posts: 9684 Good Answers: 468	#1 Re: PLC cpu 01/31/2008 11:09 AM
	It's like any PLC: Read inputs Do power-up initialization etc (first time only) Process (ladder/function block) logic Set outputs Check communications (if any) for I/O operations Go to (1) See the Siemens site for specific details of ladder instructions, interrupts etc. __________________ "Love justice, you who rule the world" - Dante Alighieri
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Kilowatt0 Guru Join Date: Aug 2007 Location: Earth - I think. Posts: 2143 Good Answers: 165	#3 In reply to #1 Re: PLC cpu 01/31/2008 4:49 PM
	That sequence is changing. The new Allen-Bradley ControlLogix line has scheduled I/O updates, just not scheduled with the program scan. The state/value of an input could change anytime in the program scan. Good reason to stick with the ages old, "best practice" of buffering your I/O. __________________ TANSTAAFL (If you don't know what that means, Google it - yourself)
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JohnDG Guru Join Date: May 2006 Location: Reading, Berkshire, UK. Going under cover. Posts: 9684 Good Answers: 468	#4 In reply to #3 Re: PLC cpu 01/31/2008 6:24 PM
	Agree 100% - I invariably read into a chunk of memory at start of scan - then often XOR it with a mask (because half the time I don't know which way up the inputs are going to be when I design the logic). Saves hours of changing XICs into XIOs etc. I do the same kind of thing on the way out. I put the input & output images (and the XOR masks) in the CIF (Common Interface File (I think?)). I've written a set of routines (VB) using DF1 to read/write the CIF (N7 in M'logix 1000, N9 in most others, as I recall). Means you don't have to stump up the ridiculous sums they ask for RSLINX. __________________ "Love justice, you who rule the world" - Dante Alighieri
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Blink Guru Join Date: Aug 2006 Posts: 4484 Good Answers: 246	#2 Re: PLC cpu 01/31/2008 11:11 AM
	This Wikipedia article will give you the basics. Follow as many of the links at the end of the article as time allows. Beyond that, you will need the manufacturer's literature for the specific model. You might also look up "ladder logic" on Wikipedia. It is implemented in many PLCs, and even if it is not implemented on your make and model, it gives you some insight into how PLCs think. __________________ There is more to life than just eating mice.
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