Using PLC for more than 200 relays application?

Question

I'm currently building an application to control over 200 devices in a building. Originally I wanted to use an Arduino and bunch of relays to control them. After asking some questions and such, people suggested to use a PLC instead (concerning safety and such). My new problem is that I don't really understand how PLCs work for a lot of devices. What do I do if I want to control 200 relays with a PLC? Most PLCs only have 8-16 relay outputs. How can I extend it?

I'm not asking for a recommendation on which product to buy and such, etc. (I know that is against the site rules), I'm just asking on a appropriate approach to my application.

Answer 1

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

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!

Answer 2

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.

Answer 3

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.