Master Control Relay confusion

[mikeking]

I'm reading the book Programmable Logic Controllers published by Newnes. I'm finding the section on Master Control Relays a little confusing. I have quoted the text from the book and interleaved my questions with it. The text explains the IL code at the bottom of this post.

"With M100 switched on but M101 off, the sequence is: rungs 1, 3, 4, 6, and so on."

How can rung 4 be evaluated? Also, if rung 4 is evaluated then shouldn't rung 5 be as well?

"The end of the M100 controlled section is indicated by the occurrence of the other MCR, M101."

What does "the other MCR" mean? There's only one, right? Under what circumstance does rung 5 get evaluated?

"With M101 switched on but M100 off, the sequence is: rungs 2, 4, 5, 6, and so on. The end of this section is indicated by the presence of the reset. This reset has to be used since the rung is not followed immediately by another MCR."

MCR just delimits the end of Master Control Relay and does not change any state, right?

LD X400 (* rung 1 *)
AND X401
OUT M100

LD X402 (* rung 2 *)
AND X403
OUT M101

MC M100

LD X404 (* rung 3 *)
OUT Y430

MC M101

LD X405 (* rung 4 *)
OUT Y431

MCR M101 (* rung 5 *)

and so on (* rung 6 *)

 

[todster]

Will this help?
http://www.pdhonline.org/courses/e117a/PLC-module6.doc

More here
http://www.google.com/search?q=mcr+...0XA-TbTPJoP68AaJ9InNCg&sa=N&aq=f&aqi=&aql=&oq=

Just change the MCR to any other acronym such as TON, OSR, NEQ...... for more. Don't use the name of the instruction as the results will be fewer and less related. There's tons of free stuff out there.