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Hi im currently building an application to control over 200 device in a building. Originally i want to use an arduino and bunch of relays to control them. After asking some questions and such in this forum, people suggest me to use a PLC instead(concerning safety and such). My new problem is that, i dont really understand how PLC works for a lot of devices, what do i do if i want to control 200 relays with a PLC? most PLC only got 8-16 Relay output? how can i extend it?

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Im not asking a recommendation on which product to buy and such etc (as i know it against the sites rule), im just asking on a appropriate approach to my application. Thx for the comment and replies !

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I suggested the PLC approach in my answer to your other question.

enter image description here

Figure 1. A remote I/O (input/output) block. From left to right: network ports, power connector, input / output terminals.

Remote I/O comes in many forms from panel / DIN-rail mount to modular, snap-together, etc. The units are "dumb" in that there is no program in them. They read the status of the input signals and pass them back to the controller on request. They receive output commands from the controller and set the outputs as instructed.

Remote I/O blocks are available for

  • Digital signals - typically 24 V DC, 110 or 230 V AC.
  • Analog signals - typically 0 to 10 V or 4 to 20 mA.
  • Thermocouple and Pt100 temperature sensors.

Some output blocks can switch high currents (maybe 10 A or so) and this may avoid the use of additional relays. Sometimes the replaceable relay is preferred for long-term ease and cost of maintenance.

You might find some suitable software to control the remote I/O directly from a PC. As I suggested in the answer to the other question, keep it simple, scaleable, reliable, long-term vendor support, and, above all, maintainable by someone other than you!

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  • \$\begingroup\$ Hi, many thx again for your answers.. I do think it answers my question. So does @Marko one, but more detailed. Thx! \$\endgroup\$ – Aufa Husen Jul 20 '16 at 8:34
  • \$\begingroup\$ For some reasons, i cant get yours as the solution though... I tried two marks 2 solutions and it fails... Now i cant change them(sorry im quite new to this forum). Again, many thanks! \$\endgroup\$ – Aufa Husen Jul 20 '16 at 8:37
  • \$\begingroup\$ You can unaccept my answer and wait a day or two to encourage even better answers. Then I'm afraid you have to pick one. ;^) \$\endgroup\$ – Transistor Jul 20 '16 at 9:06
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With help of distributed IO stations, that communicate with industrial protocols: Modbus, Profibus, Profinet, ... The building automation is a way different than industrial automation, it uses Konex, EIB, LON, Bacnet network and lots of devices work indipendent, they are just remote controlled.

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  • \$\begingroup\$ im sorry, quite new to all this. Im just not getting it.. , what is the form of this IO station? im quite questioning what the hardware approach to it... intead of the communication protocol between them (i quess?). \$\endgroup\$ – Aufa Husen Jul 20 '16 at 7:42
  • \$\begingroup\$ @AufaHusen It's a device with IO modules, like inputs outputs and a communication processor for needed protocol. Try to google: "remote io system". \$\endgroup\$ – Marko Buršič Jul 20 '16 at 7:56
  • \$\begingroup\$ oh, i see.. that seems to be the thing im looking for, to confirm a few things, here are some conclusions i've made.. please see if its correct : those I/O system are modules that can be connected to a PLC with some protocols to control devices ? an I/O system can have an output of a relay can it? Thx.. \$\endgroup\$ – Aufa Husen Jul 20 '16 at 8:01
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most PLC only got 8-16 Relay output

That is not really true. While some cheap brick PLCs are non expandable, most PLCs are expandable by stacking modules into the rack or by connecting multiple remote racks. This way you can have tens of thousands of I/O on 1 PLC.

It sounds like you are new to PLCs. Don't worry about networking stuff as that's really advanced. Learn about Ladder Logic by watching youtube videos like these

Allen Bradley PLC programming

Click PLC - one of the best low cost PLCs out there. We use a lot of Click PLCs at my job. They are low cost, free programming software, easy to learn, and extremely reliable.

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