Why Are Short Circuit Lines Used in RSLogix 500 Ladder Logic?


Thread Starter


Hello All,

I'm currently converting Rockwell logic from RSLogix 500 to Siemens logic in TIA Portal V13 SP1. Parts of the original Rockwell logic illustrate a short circuit line in parallel with a boolean switch. Does that not defeat the purpose of the switch, since it is being bypassed by the short circuit in the ladder logic? TIA Portal does not accept that kind of logic and I'm not sure if I can simply ignore it and retain the switch. Could anyone please spare some insight? To make the question more clear, here I'll outline the context of the logic. It refers to a metal bending machine: Whether the machine is in manual mode or not (the short circuit is parallel to manual bit), check if the clamp on push button is pressed to stop the metal sheet from being fed into the machine any further. If you could provide other examples and an explanation of why a short circuit was used, I would be very grateful.

Thank You,
I see this kind of technique used all the time; it's a quick and dirty workaround during troubleshooting/commissioning. The intention is to "delete" that contact temporarily, as the controls guy is not sure if he might want it back later, so he leaves the contact there and just branches a short circuit around it so that he doesn't forget what was "deleted".

Sounds like in this example the stop button may have been intended for manual mode. But on second thought, Stop is a function that you would always want to be able to do! Do you really want to restrict the function of that button to ONLY manual mode?

If you want the machine to function exactly as it did before, just omit the contact that was shorted around.
A shorting branch is used to bypass logic. I've seen it used as a troubleshooting tool, but I think it's bad practice generally to leave it in there when you're done troubleshooting. If it's been there a while and the machine runs as intended, and it's safe, I would just eliminate the code that's bypassed.
Most of the times it is done to check the actual toggling status of the bit while commissioning the system or during any switch checking.

If the bit is forced at IO table, its toggling will not be reflected at the rung.

Also, it is sometimes done when you are not sure of the cross reference between B bit and I bit, as we do in out Rockwell 5/40 & 5/80 series PLCs as online editing is very easy in the PLCs.
I've also used a technique where a small set of bits (in PLC-5 a reserved "B" file of maybe 5 words, in ControlLogix a DINT of 2 or 3 words) where it is identified as a "test" bit. With multiple programmers we use some unique ID (say, tags "Russ_test" vs. "Tom_test") so we don't step on each other's bits. That way we can turn on and off each bit independently to do the tests. Sometimes these bits remain but we add a CLR instruction so all the bits are off when we leave. Other times we simply search and delete each instance when we are done; it depends on the application and customer.

Richard Pickrell

Back in the day many PLCs like the TI 550 and 5 TI would not let you use a shorting branch around a contact. To get around this you could branch around a contact the place a contact of the same address but of the negated or opposite examine state in the branch. You could (can) place an "opposite" contact in series with an element to temporarily disable it.

I still use this method even though I dont have to; it helps me remember what was there, and serves as my "signature"- any of our guys who see it knows I was there.



You're right about the stop function, but the automatic mode logic is parallel with the manual mode logic, so automatic mode will override the PLC even if the button is pressed. There is a universal reset switch though. I appreciate the input. Sorry I responded so late.