Need help please! Two PLCs to sense the state of one bank of switches

simple

Good morning, Brian. First off, questions of this type usually receive a more enthusiastic response on sci.engr.control -- they're the PLC folks (PLC = programmable logic controller, a specialized computer with built-in "real-world" I/O such as relay outputs and optocoupled inputs).

Your question is lacking a lot of the information you need to decide what to do.

First, PLC inputs are almost always optocoupled, with the optocoupler and the current limiting resistor internal to the PLC. Some inputs are current sinking (NPN) type, where a contact closure to GND or the negative end of the power supply turns on the input. These are called NPN because a standard NPN transistor collector which has the emitter grounded will act the same, turning on the input when it turns on. Some PLC inputs are current-sourcing (PNP) type, which means a closed contact to V+, or the collector of a PNP transistor whose emitter is connected to V+ will turn on the opto input. Some have AC input, which means an AC signal will turn it on (an optocoupler with back-to-back LEDs are used). Some also have "logic level" input modules.

The second big question is whether the PLCs both have the same input power supply, or have different power supplies with the commons connected together. It's almost always a bad idea to connect two PLC commons together unless it was made that way or you really know what you're doing. It is an especially bad idea to tie together commons of two PLC internal sensor power supplies.

I'll assume you've got two PLCs with DC current sinking (NPN-type)

24VDC inputs for all of the following, because that seems to be the most common setup. IF YOURS IS DIFFERENT, NONE OF THE FOLLOWING APPLIES TO YOU.

If your PLCs are both running off the same sensor/input power supply, or if the two power supply commons are tied together, you can use steering diodes (1N4002) to turn on both inputs, like this (view in fixed font or M$ Notepad):

PLC1 PLC1 PLC1 Com In1 In2

^ ^ ^ 24V|+ | | --- | | - | | | V V | - - | | | PLC2 | | | | o----------------|--| In1 | | | | | | | | | | '----. .---o--| In2 | | | | o o | ^ | |ON-(OFF)-ON | '---o | | PLC1 | | 0V '-------------o-----------------> PLC2 0V created by Andy´s ASCII-Circuit v1.24.140803 Beta

If you have separate power supplies (even if they're the same voltage), you should do something like this:

PLC1 PLC1 PLC1 Com In1 In3

^ ^ ^ | | | | | | | | | PLC2 | | .----------------------------------> In1 | | | | | | | | | | | | PLC1 | | | | ------> In2 24VDC |+ |H11G2 | | H11G2| --- 1| | 5 1| | 5 - | ~ |/ | ~ |/ | V ~ -| V ~ -| | - |> - |>

| | | | | | 2 | | 4 2| | 4 PLC2 | | '--------------------------o-------> 0V | | | | '-----------. .--------' | | | | o o | ^ | |ON-(OFF)-ON | | | '--o | | | | | | '-------------------' created by Andy´s ASCII-Circuit v1.24.140803 Beta

I like to use H11G2 optocouplers with PLCs, because they're high gain darlington transistors and have a V(ceo) of 35V. You have a margin on current gain that way, and if you don't have to worry about transistor saturation voltage, it's pretty much ideal. Just leave pin 6 (base connection for the optodarlington) open.

Read this twice and evaluate your system carefully before you start doing any wiring.I'd really feel better if you could more accurately describe your PLCs and input modules before you take any advice, particularly mine. If you don't know what you're doing, ask first. It keeps the smoke from getting out, and most PLCs aren't cheap. If you have any other questions, feel free to post them on this thread.

Good luck Chris

Do the switches do any thing else, except communicate with the PLC (is the PLC monitoring some other operation, or are the switches dedicated to the PLC)? If they are dedicated, there is no need for isolation. Just connect each switch to two input bits of a digital input card (left position, right position). If neither bit is activated, the program can call that the middle position indication. Power the common terminal (I assume the switches have 3 terminals, each) by what ever voltage the digital input card is designed for. The other side of the supply goes to the card common terminal.

For example, if the Input card were designed for positive 24 volt DC inputs, one switch would be wired like this: (view in fixed width font like courier)

24 volt positive-------+ | O sw common terminal sw left position O off O sw right position | | | +---+ | | ...|.|.............. 24 volt negative--O O O O O O O O O 8 bit input card ******************** C I I o n n (read vertically) m p p m u u o t t n 0 1

-- John Popelish

you should be able to simply tie the inputs together with out a problem. you can isolate them if you wish, i really don't think is will make any difference. the PLC inputs are normally always inputs at all points.

Hi Group,

Sorry for the delay in replying, been off for a few days. Thanks for the replys, food for thought for sure. However, I think I have found a simple way to do this. They are two separate PLC from different companies, with separate power supplies. I plan on using independent switch contacts mounted on Telemecanique XB2-2 switch bodies. This way there is no electrical connection between my PLCs, the switch will operate both sets of contacts. What do you think?

Thanks

Bauschman

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That's fine. A DPDT switch is a good idea.

Chris