PLC Information and Links

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PLC (Programmable Logic Controller)
/>http://en.wikipedia.org/wiki/Programmable_logic_controller

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  Wiki:

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures.

Computing Library:

Programmable Logic Controller hardware
(PLC) A device used to automate monitoring and control of industrial plant. Can be used stand-alone or in conjunction with a SCADA or other system.
(1997-02-11)

Power Point Example of PLC Ladder Logic (Link from Wiki)
/>http://claymore.engineer.gvsu.edu/~jackh/books/plcs/powerpoint/ch01.ppt

PLC History (http://www.plcs.net/chapters/history2.htm)

In the late 1960's PLCs were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.

When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome. Were there problems? You bet!

These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.

In the mid70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903.(ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.

Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!

The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay!

The 90's have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980's. The latest standard (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time! PC's are also being used to replace PLCs in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.

What will the 00's bring? Only time will tell.

PLC Links and Information

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  [*

]http://www.plcs.net/contents.shtml - Lots of PLC information, various diagrams, etc.
[*]http://www.allaboutcircuits.com/vol_4/chpt_6/6.html - PLC Ladder Logic information.

Code Listings, Examples, Emulators

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]http://www.codeproject.com/KB/miscctrl/PLC_simlutor.aspx - MFC C++ PLC Emulator
[*]http://www.codeproject.com/KB/aspnet/ModBusDeltaDVPSeriesPLC.aspx - C# Communication example with Delta DVP PLC
[*]http://www.plcsimulator.org - ModBus PLC Simulator

I am in no way a PLC person, or coder, I figured I'd just help get some info in this section to help get it started. Everything linked above was found with Google. Hope this helps anyone interested in this subject.

[Teddy Rogers]

There is a software package called Unity developed by Schneider Electric (for Schneider PLC's). It has a built in PLC simulator, debugger, supports quite a few languages and is a good piece of software to get in to PLC programming...

/>http://www.schneider-electric.us/products-services/products/programming-software/unity-pro-software/

Ted.

[rokko]

hehe funny, that's my proffession.

i work with siemens s7/wincc, rockwell & mitsubishi in different

languages like ladder, structured text and mostly awl (like assembler).

if u have questions, pm me.

[Pofadder]

Very interesting subject indeed.

We have 25 CNC machines which all but one are using Fanuc controls. The other one is a Siemens. I do have a fair knowledge on how PLC ladder diagrams are working and would like to learn more about it. I also know that you don’t tamper or play around with ladder diagrams on CNC machines unless you are looking for big trouble. These machines move fast and you don’t take chances with them. For example: We had to remove a checking fixture from one machine that had to go for repairs. Now, the machine won’t start without the fixture being plugged in. At this point I would like to trace through the ladder diagram where it shows the device has been plugged out and then bypassing it by changing the ladder so that the machine think the device is still plugged in. You can make changes on the machine itself or I can use a software that can open, change or write PLC’s.

I noticed the link in Teddy’s post is not working anymore. Is that software still available for download?

Thank you.

[Teddy Rogers]

CNC's are usually custom machines dedicated to do a specific task which is why there will be limited flexibility with the logic unlike PLC's. Having said that it is like comparing apples with oranges, they are both fruit but completely different

Schneider moved the product page for Unity Pro, you can now find it here along with a trial download link...

/>http://products.schneider-electric.us/products-services/products/plc-programming-software/industrial-end-user-oem-programming-software/unity-pro-software/

Ted.

[SteveS1]

I hate PLC programming which they tried to teach me at College along with COBOL launguage ..i'd always rather write in assembler or C code

Anyway, interesting

Steve