Two rungs writing to one coil

[Tark]

Fluoronator –

There can be instances where someone would use the same coil on different rungs. Everyone here can speculate as to how one could, why one should, or why one shouldn’t. In the end it’s best you know exactly why this was done (I’m assuming you will be the one who will have to troubleshoot the equipment one day) and no one here will be able to provide you with that answer, even with a copy of the code. Now that you know there can be reasons for this kind of logic, try getting InTouch with the programmer and ask him/her to explain it to you.

 

[S7Guy]

RMA,

This means... as soon as the logical condition is asserted, be that logical condition a limit switch connected to a solenoid, or a relay connected to the solenoid, the solenoid begins to react, at the speed of light, as soon as that logical condition exists.

Where do you get the "speed of light" thing? Electricity doesn't move at the speed of light. It's a function of the conductor and current. In fact, the latency of hard-wired components will most likely be greater than plc devices, simply because the plc I/O will react much faster.

I know what you are saying (a hard-wired relay is not delayed by logic), but in reality I would hardy call it "real time" either.

 

[Mickey]

Where do you get the "speed of light" thing? Electricity doesn't move at the speed of light. It's a function of the conductor and current. In fact, the latency of hard-wired components will most likely be greater than plc devices, simply because the plc I/O will react much faster.

I know what you are saying (a hard-wired relay is not delayed by logic), but in reality I would hardy call it "real time" either.

Yes it does:

http://www.jimloy.com/physics/electric.htm

Or maybe not:

http://www.radioelectronicschool.net/files/downloads/howfast.pdf#search=%22speed%20of%20electricity%22

 

[S7Guy]

Mickey, going back to my old college days, we learned something in between, i.e. electricity is very fast, but when measuring the real time latency, it wasn't near the speed of light. I guess my point was that there are latencies in every system. With PLCs, we are just trading off one latency for another. By the strictest definition, there is no such thing as "real time" I suppose.

 

[Terry Woods]

S7,

I said...
"If logical circumstances call for a signal to be applied to one or more output devices, then those devices become influenced by that signal at the speed of light." ...the key-word being... "influenced".

In a PLC, assuming the standard read-evaluate-write scan-scheme, the Input has to be ON before it is detected as being on (lost time = w), the input is then detected and recorded (time = x), then the Input is evaluated in logic (time = y), then the Output Table is written to the Output device terminal (time = z).

Total time = w + x + y + z

In this case, the Output device can NOT feel the influence of the Input device until the scan reaches the point where the Output is written. In general, that is just a bit less than scan-time.

In a hardwired system, if the Input is a limit switch, and it is connected to a valve-coil, then the valve coil "feels" the influence of the electrical force just as soon as (at the speed of light) the limit switch is closed.

In this case, the coil feels the influence of the electrical force as soon as (at the speed of light) the limit switch is closed. What is the time difference between the switch being closed and the coil being influenced?

Time difference = Wire distance divided by the Speed of Light.

As far as we know, practically speaking, the Speed of Light seems to be the limit in terms of electrical influence.

Being under the "Influence" does NOT imply a completed response. It only implies that the influence has been applied and that a completed response is in development.

This is as close to Real-Time that anyone can expect, without relying on a soothsayer. For all practical purposes, at least in this universe, this IS Real-Time! And it is the ONLY Real-Time that we know!

By the way, I have NEVER seen anyone say that any electrical activity can happen "instantly", as in "zero-time". I did indicate, at one point, 0.000 secs, as in zero-time, as being REAL-TIME... but the all-caps was supposed to stress the difference between Real-Time, limited by the Speed of Light, and REAL-TIME, where the Speed of Light is INFINITE!