PLC Ladder Diagram Basics

[dannymcg]

Hi. My background is 35+ years in field instrumentation and I'm doing a college PLC project for a mixing & batch system with pumps, high/low tank level switches, agitators on timers and open/close type valves.
I'm trying to design the ladder logic diagram. With regard to the level switches, do I have them operate their own (internal ?) plc relays ? Should I use n.o. contacts on the level switches in order to operate their respective relays when reaching the high or low levels ? I read somewhere that this way gives a positive 'on' or logic '1' but it contradicts switch wiring used for high/low alarm purposes i.e. 'failsafe'.

Apologies for what looks simple enough but, I need to ask as I am a little confused.

Any other tips to include in the ladder logic would also be greatly appreciated. (Advice on agitator timer setup/wiring etc)
Thanks.

 

[Spartan13]

I'm not exactly clear on what you're doing. Typically level switches are connected to an input; if they are activated it will change the bit registered to that input to a "1", if not it will be changed to a "0".

If I understand correctly, your code will look something like this:
HL = High level switch, LL = low level switch, B3:0/0 = drain tank bit, B3:0/1 = fill tank bit.
You have to be careful because the low level switch will be covered when the high level switch is, and neither is covered when the level is low.

[XIC] HL --- [OTL] B3:0/0 & [OTU] B3:0/1 (if the high level switch trips, latch the drain on and unlatch the fill.
[XIO] LL --- [OTL] B3:0/1 & [OTU] B3:0/0 (if the low level switch is not tripped, latch the fill on and unlatch the drain.

Does that make sense? If you have questions let me know. If you want more help, try to include a few more details; what is the agitator on the timer doing? [RTO] or [TON] or [CTU] could all be useful depending on the situation.

I'd recommend looking through Kronotech's guide: http://www.kronotech.com/LadderLogic/LadderLogic.htm
for a little more information about basic commands.

Hope this helps!

 

[Lancie1]

I read somewhere that this way gives a positive 'on' or logic '1' but it contradicts switch wiring used for high/low alarm purposes i.e. 'failsafe'.

One of the first things to learn about PLC programs is that the logic in the program DOES NOT represent physical switch contacts.

Instead the PLC logic instructions look at or examine the physical inputs from physical switches, then return a logic "1" or "0", depending on the instruction used.

The above statement is not the same as thinking that each Input bit instruction in the PLC program exactly duplicates a physical switch contact. It does not. Because of this separation between physical inputs and PLC instruction addresses, you can use whatever physical switch contact you want (NC or NO), the PLC could care less. You may want to use physical switch contacts as they are normally used in your work environment. Either way, there is a PLC instruction that will make the input do what you need. The key for beginners is to design the logic so YOU understand it.

 

[xC0MMAND0x]

Someone will post this eventually...

Thanks Ron!

https://www.youtube.com/watch?v=zlsJxSK8tPE

Start there

 

[widelto]

Danny:
This books cover all the basic aspects of plcs.
http://forums.mrplc.com/index.php?app=downloads&showfile=158
It's worth a reading.

 

[dannymcg]

Gents, thanks for all the help, links and suggestions. It's all very much appreciated. As mentioned by Lancie1, I need to remember that the plc logic is not the physical switch contacts (too much time spent with old relay panels...)
Thanks again !

 

[FactoryTalktotheHand]

Lots of people will look at a plc program and call instructions things like "Coil" or "Contact." And it doesn't help when some PLC manufacturers actually do the same thing in their own literature. They're not coils, they're not contacts, they're not timing relays, they're instructions. That's one of the hardest things to wrap your head around. You're not looking at a device, you're looking at a command.

 

[dannymcg]

I just wanted to share the following for 'future' PLC amateurs like myself. The software recommended by college is called 'TRiLOGI' by Triangle Research (And is available free). They also have a very good book for download called "PLC Programming for Industrial Automation" by Kevin Collins via the following link.

http://www.triplc.com/reqplcbook.htm

 

[Lancie1]

Yes, I have the iTriLOGIC software version 6.41 and have used it some. The simulator (although slow-running) is a nice feature, allowing the user to test programs to see if they will work.

 

[dannymcg]

Lancie1,

as you mentioned previously about separating the hard-wiring from the software, this book gives some excellent representations of the hard wiring converted to software with some switching actions being opposite in software for failsafe applications.

 

[Lancie1]

One of the traps for unwary electrican-to-programmer beginners is the logic for a Stop pushbutton. After years of wiring the stop function for a motor or other equipment to a Normally Closed contact, you just naturally think that the PLC logic instruciton for that same Stop button should also look like a NC contact.

But is is the opposite. You must use the NO PLC instruction, or as Allen Bradley calls it, the "Examine If Closed" instruction to make the Stop function work correctly in the PLC logic. In PLC 1-and-0 binary logic, the NO instruction returns the same logic Output as its Input, but the NC instruction returns the Opposite of its Input.

 

Instruction   Input  Result

Logic "N.O."    1      1

                0      0       

Logic "N.C."    1      0

                0      1

 

[dannymcg]

That I did notice................