RllPlus Basic Q

[mrg1999]

Been programming since the 60's, everything from Basic to Assembly Language to Binary but RLL and RLLPlus syntax is throwing me.
Automation Direct's RLLPlus tutorial shows the following diagram to wait for a "START" signal from a momentary push button (see attached).
Since its an ISG, its the only stage active at start up. The way I'm reading it, is that nothing happens until the NO start switch is depressed, then the rest of the instruction on that line is executed? Stage 50 becomes active, but why the JMP S0?
Wouldn't that tell the program to go back and wait for another depress of the START switch? Shouldn't that be a jump to another stage?
And what about if I throw a NC switch into the equation?
Under normal situations, wouldn't the rest of the line with the NC switch get executed?
I'm really confused with the logic!

 

[mhw]

why the JMP S0?

I think that it is a typo. It will work but serve no useful purpose that I can think of.

Wouldn't that tell the program to go back and wait for another depress of the START switch?

Yes, it will act just like it would if it was not in a stage since there is nothing to cause the stage to be turned off.
 

what about if I throw a NC switch into the equation?

Just like it would if it were not in a stage.
Do not let stage anxiety overwhelm you. It is both simple and helpful.

[mrg1999]

Thanks for the reply.
Misprints in tutorials don't help the cause.
The one thing I don't understand is why the inputs are represented by NO or NC switches. The PLC doesn't know if the input is NO or NC it only knows the state of the switch.
To me it would make more sense if the inputs were represented as being at a HI state (+24VDC in my case) or LO (0V).
If the input is HI then execute these instructions
If the input is LO - execute those instructions

[Mike Nash]

The PLC doesn't know if the input is NO or NC it only knows the state of the switch.
To me it would make more sense if the inputs were represented as being at a HI state (+24VDC in my case) or LO (0V).
If the input is HI then execute these instructions
If the input is LO - execute those instructions

Well, as for me, please don't redesign the PLC interface.

The "PLC" doesn't necessarily know the voltage level on the input. The input module (yes, it can be a brick that has the I/O built in but...) reports whether the input is on or off but that may be a high voltage state for on or it may be a low or zero voltage for on depending on whether it is intended for sourcing or sinking devices.

N.O. and N.C. are more of a convention, but we LIKE it.

[BobO]

When we developed Do-more we tried to address this both ways. Of course we still have N.O. and N.C. contacts, and we display the status color on the basis of the effect of the input, but we also added the raw input state displayed as ON/OFF.

If I recall correctly, the Productivity team originally did status as a true value without regard to the effect in context, and pre-release users were *not* amused. It got changed before release.

[mrg1999]

Thank you for the suggestion, I will check it out.