from the needing help department...
how to make a PLC out of a standard computer
Software, including programming, OS issues, etc. topic
Posted by angel889 on 23 November, 2009 - 8:44 pm
Since I have started going back to school, I have learned a lot about electronics and I have acquired a lot of the tools and components to help me be able to learn all that I can at home. I have heard that a PLC can be made by using a home computer because essentially a PLC is a processor /power supply/ input and output devices.

So my question is does anyone know how I can make a PLC by using a old home computer. I have several old computers with the typical video audio cards and network cards. I already have the software (for Allen Bradley RSlogixs) to make my ladder logic, and I am becoming pretty familiar with how to use it for the English 500 and 5000. so the main thing I need to know is how to make my input and out puts for the computer so that I can us it to control some solenoids, switches sensors, motors and other 12v components like lights. the ideas is to be able to use the computer to control my Christmas lights and home security system and to learn more about how PLCs work. so if any one has any ideas or maybe you no where I can get directions on how to turn my personal computer into a PLC, please let me know. I have lots of parts laying around from computers, printers monitors to prox switches that I want to use.

Posted by Ananymous on 24 November, 2009 - 4:10 am

You are very much welcome here, but you aim seems only as hobbyist or amateur.

First of all u must be clear that this forum is highly professional forum, where engineers and technologist share their experience professionally.

Any how to use your Home PC as PLC, You need not to use Rs-Logix. It is more better if you develop your program in Assembly Program or similar computer language then you may be able to play with the micro-processor of you Home PC.

For I/Os handling you may purchase or develop a card which may work serial, USB, Parallel or PCIC port of Motherboard of your Home PC.

Any how this reply is not to let you down but it is only to make a clear picture for you.

Plz Make a habit to ask only one or two comprehensive questions so that so may get a good reply.

Posted by M Griffin on 24 November, 2009 - 3:20 pm
> First of all u must be clear that this forum is highly professional forum, where engineers and technologist share their experience professionally. <

Highly professional spelling and grammar appear to be optional though.

Posted by slowdrift on 16 May, 2010 - 11:21 am
use a usb port to give an i/o order to an relay which take volts order to open-close and you are ready! to open or close your usb port automatically simply use a macro recorder program which let you to do this automatically any time you want.

start-> control panel-> system-> device manager-> usb port, disable or enable.

it works for me.

Posted by giovanni on 24 November, 2009 - 5:55 am

What you want to do is a great way to learn about electronics but not so great to learn about plcs. If you want to control things with an old pc just google parallel port i/o. If you want to learn about plcs you should save some money and buy a unit from ebay, this would be the cheapest and quickest way to learn.

What ever you do PLEASE be careful when wiring things up in your home, you don't want to be starting any fires.


Posted by M Griffin on 24 November, 2009 - 4:31 pm
There are really two aspects to the entire question. One aspect is the hardware. The "cheapest" involves using the computer's parallel port and manipulating the individual output lines. You will have to get some connectors, wires, transistors, relays, and some perf board to make an interface board. There are lots of plans for these on the Internet if you use Google to search for them. I can't make a specific recommendation for a particular design though.

For more "industrial" style off the shelf hardware, look at some 6000 series I/O modules from Advantech. These are blocks of I/O that you connect to your computer with Ethernet. You can get these with built in relays. Alternatively, you can buy an I/O board that plugs into your computer. Most of these will also need some external relays (conventional or solid state), and for a small number of I/O are probably more expensive than the Advantech modules.

For either of these options, make sure you know what you are doing when you are wiring things up so that you don't burn the building down or electrocute someone.

For software you have a number of options. A PC being used as a PLC type controller is called a "soft logic system". These are software systems that allow you to program the PC just like you would program a PLC. Some examples of these are:

Classic Ladder

my own project MBLogic ""

and Beremiz ""

Alternatively, you can simply write the control program in a conventional programming language. If you are using the Python language, you may want to add on the "pyparallel" package to control the parallel port, or use the MBLibs library (one of the MBLogic packages) to control the Advantech module (via Modbus/TCP).

So, there are a number of options open to you. The "best" choice is going to depend on what you want to do and how much money you have to spend.

I will also point out that you can get a conventional hardware PLC relatively (as these things go) from Automation Direct. Some other vendors may have something comparable as well.

Posted by John Correa on 25 November, 2009 - 10:07 am
It's a good idea to bring to life again old PCs. Make them a home or industrial controller is even a better idea. But certain things you must care about: for example (1) what happens with the outputs when energy supply goes out (retain their last state or all clear to zero). (2) When energy returns how much time needs the controller to resume its functions (PC with a standard OS need several seconds even minutes). (3) Can you make a software or hardware change without interrupting the controlled process .

Also is a good idea to use "soft-logic" that emulates a PLC in a PC. There are interesting open source tools. May be that also you can use RSLogix to do PC control. The problem is how you can access the real world inputs and outputs from these programs.

Also there are several low cost PC interface h/w to connect the i/o (parallel port, serial, USB, ethernet, wireless..etc.). You must care if these interfaces can be accessed from your soft-logic. Modbus protocol is common way to make your i/o speak with your software, so your softlogic must permit to do this. Another good idea is to have a driver that allow to access your parallel printer port using Modbus, i don't know if there exist any driver like this but if this is the case you can build a very low cost home automation controller with old re- incarnated PCs.

Posted by djames on 25 November, 2009 - 4:42 pm
RSLogix will only work in the AB PLC. You need to write a in a different language if you are going to use a PC.

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