Extra Rung?

I AM A MAINT. MECH. JUST LEARNING SOME OF THIS PLC STUFF AND HAD WHAT PROBABLY IS A VERY SIMPLE QUESTION. IN THE LEVEL APPLICATION SECTION OF THE LEARNING PLCS PART OF THIS WEB SITE THERE IS A EXAMPLE WHICH SHOWS INPUTS MAKING THE OUTPUT (INTERNAL RELAY?) 1000. 1000 IN TURN MAKES OUTPUT COIL 0500. WHY THE EXTRA RUNG? CAN'T YOU JUST CHANGE THE 1000 TO 0500?
JUST CURIOUS
BOWRIDER

You certainly can use the 500 Output directly... if you want.

But, if 1000 represents a particular state in the process, then you might want to use the 1000 output to trigger other things besides the 500 output.

Depending on the particular situation you might actually have a few other contacts in the rung for the 500 output. Those conditions also have to be satisfied before 500 goes ON.

Meanwhile, in another part of your program, output 501 is also looking to see the state of output 1000. But output 501 is using other conditions, different from those that output 500 is watching.

It's not a case of having an extra, useless, rung. Output 1000 becomes available for any number of rungs.

That's part of the magic we know as PLC's! One output can be used almost any number of times!

THANKS, I APPRECIATE THE HELP!

When I first develop a program, it is much better for me to build very simple rungs. Then, once a segment is working and if I can condense the program to have more instructions per rung without causing confusion I will do so. On fairly small programs, less than 50 rungs, I don't even bother. Back to your original question, yes you can.

Try to make it as simple and easy to understand as you can. I have recently started working for a Japanese company and find that many of the PLC programs are EXTREAMELY overdone, IE one coil picking up another, then another and so on, and for no apparent reason...If you have anyone who has to troubleshoot with just a print out of the ladder they will truely appreciate SIMPLICITY.
Bob

I HEAR THAT, WE JUST GOT THROUGH MAKING SOME PROGRAM CHANGES ON ONE OF OUR CASERS AND IT WAS JUST AS YOU DESCRIBED, ENDLESS LINKS VIA INTERNAL RELAYS TO RUNG AFTER RUNG AFTER RUNG.... TOOK ALL DAY TO MAKE JUST A COUPLE SMALL CHANGES!

I asked same question.

I was told the program was written at the office and when they came to commission the program often output Y0 and Y1 for example would be crossed. With internal coils this was easy to change. Same if relay in PLC goes down. Easy to change output or input.

But murder for the guy trying to trace the fault.

I use outputs for triggering other bits within the program but normaly the PLC is in the machine and all connected before I start writting program. But this is not the case when written by outside firms.