General Question

elektra890

Member
Join Date
Jul 2003
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1
I'm doing I/O schematics for an RTU. My question is - can instrument devices be connected in series so that for, say, 10 level switches, can you connect them in series and use just one terminal in the RTU? Say, all are for discrete inputs. Can you just use one DI terminal in the RTU for all these 10 devices?
 
Certainly you can... but it sure sounds like an awful way to do it!

10 devices in series to one Input is a big "AND" circuit. All conditions must be true to turn ON the Input. If the Input is OFF, all you know is that at least one of the 10 level switches is not ON. Which one? Can't tell.

You are far better off bringing each device into a separate input. Then you can develop any kind of combinational logic that you want. You will also "know" exactly, which devices are ON, and which are NOT.
 
As Terry said, if you can use individual inputs, you really should.

One particularly difficult problem can easily be "trapped" if you do it this way - an intermittent fault.

If one of the switches opens intermittently you can build internal latching logic inside the PLC for each input so that you can easily "trap" the specific input in the act. This can save untold hours of troubleshooting and downtime.

Steve
 
Sounds Funny ! Never seen something like this on the shop floor in my entire 8 years of career in Industrial Automation :)
 
Series circuit.

Sure you can connect them in series but in order to get a input signal all ten switches would need to be made (true). Why would you want to do it that way? It kinda defeats the reason for using a PLC.
What is your thinking? :unsure:
If for some reason, that I am missing, you need them to be in series you should have them as individual(real world) inputs to the PLC then series them in your program. :confused:
Roger
 
Last edited:
10 switches in series.

Yes, we have machines with connection like that. This is for a safety purpose, the operator should close the 10 doors in able to operate the machine.
 
Joel said...
"Yes, we have machines with connection like that. This is for a safety purpose, the operator should close the 10 doors in able to operate the machine."

If it is a small machine and all of the doors are within easy reach or view, then yes, you can do it as simply as Joel said.

As a safety device, the ten-door-series-circuit should either enable or disable a physical power relay (the one that supplies power to the output side of the PLC).

And there should be at least some kind of indication that the safety circuit is not made. That is, there should be an Input that indicates "Safety Circuit is Closed". It's always nice to know WHY something won't do what it's supposed to do.

The program should, of course, condition its behavior based on the state of the Safety Circuit. That is, do not disable the power to the PLC. Instead, have the program respond to the fact that the Safety Circuit is - or has just - opened. While the open Safety Circuit causes the output power to drop out, the lost Safety Circuit Input signal causes the program to adjust itself accoringly.

When the Safety Circuit signal is restored, do you want the program to simply "resume" from where it left off?

Maybe YES, maybe NO.

Or, should the system require a re-start process of some kind?

Maybe YES, maybe NO.

The "CLASSIC" answer is... it DEPENDS!

So, all of that is fine for a wee-small system - one where you can easily "see" the "doors" (these could just as easily be E-Stops).

What if the system is not-so-wee or maybe it's friggin' huge!

It might take an hour to make the rounds to check all of the doors or E-Stops. All because you don't know which door or E-Stop is preventing the process from running.

Certainly you can have the E-Stop or door wired so that a local light indicates "ajar" or some such. But you would still have to at least begin to make the rounds to find it.

The larger the system, the more cost effective it is to have each door or E-Stop provide a single unique input to the PLC (in addition to the series circuit that controls the output power relay).

Then, if the system won't run, the status of each E-Stop and/or door can be observed on an HMI screen - or, at least, on the input indicators at the PLC.
 
Yeah, What Terry Said...

Always a plus to monitor the status of EACH switch. On larger systems with a many guards and E-stops, sometimes the customer won't cover the added costs of more wiring and inputs. In this case, each switch should at least make a 'home run' back to a terminal block. IOW, a separate 'loop' for each switch (out and back).

I hate finding machines where wires are simply run directly from switch to switch to switch. In order to figure out WHICH switch is open, you have to take apart EVERYTHING... :mad:

When each switch loop begins and ends at a terminal, it's a simple matter of checking for power at each terminal. Makes it a breeze to locate the open switch... (y)

beerchug

-Eric
 
my 2 cents

I know most are tired of hearing about the "sealer" machine I redid recently but the fact that it increased production 300% makes me proud.

I wired limit switches on the "heat bar" in series with the E-stop circuit for safety reasons. Once the machine was up and running I went back and used the secondary side of the limit switch as an individual plc input. Why? To monitor when the switch failed so operator/line tech would know.

Can you do this? YES
Should you do this? IT DEPENDS

There are 2 major considerations...
1. Safety
2. Cost

Hopefully safety has the priority but that is not always true. Only YOU know the situation involving these considerations, if you have 20 input devices and 10 inputs then you MUST make do something to make it work BUT look at safety issues.

Being a maintenance tech and having to troubleshoot alot I personally see no reason (besides safety even though it would be more convenient) that you shouldn't wire 10 switches in series....the knowledge (documented of course) that they are in series would be enough for me.
 
my small brain

Following the answer above.
Some more from my small brain....he he :D

Yes,You can use 10 levels switch ,connect in series,
but how about"Safety" and "Monitoring"
May I recommend ..by this 2 ways
1.Connect (individual) each level Switch to Aux relays. That means
10 Aux relays ..You can use several fung tion from contacts of relays.
2.Use level sw. with plc (10 Sw.=10 inputs) So,you can write program
in series as well and can do more by program ,what you need..

Platootod
:cool:
 

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