Hello All,

Do PLC's use logic gates or Logic Ladders?

I've been asked to explain two different types of stop/start/retain relay control circuits for suitable for either the AC or DC

I'm going to compare an electro-magnetic relay system with an electronic logic or PLC system but I'm not sure if I'm getting a PLC and Electronic logic system mixed up or they are both the same?

Replies would be gratefully appreciated.

 

Do you mean the software representation or the internal logic?
The software is based on boolean logic.
And is a direct representation of hard wired control logic.
Unclear as to the question.
Telemecanique wiring diagrams
Max.

 

Thanks for the reply Max.

I'm unsure of the question also, isn't electronic logic PLC?. If a PLC uses Boolean logic then I presume so.

Isn't ladder logic converted to Boolean logic for a PLC?

 

Usually there are two methods of programming PLC's, one is the typical schematic representation in order for maintenance personnel that are familiar with hard copy schematics to view and trouble shoot on screen.
The other is some form of flow chart or boolean expressions directly.
Some use the term PLC for any microprocessor logic, but it is more familiarly used to describe industrial control systems that bear the title, 'PLC controller' as described above.
Have you used or come in contact with a Industrial PLC at all?
Max.

 

I have seen them but I've never used one.

I'm writing an assignment and the question is strange in my opinion its for motor control

"Compare the construction and operation of two different types of stop/start/retain relay control circuits for suitable for either AC or DC


Guidance: For example you may compare an electro-magnetic relay system with an electronic logic or PLC system

 

The Telemecanique PDF contains these typical stop/start/retain rungs, they are in schematic format but are typically identical to those portrayed in a PLC. e.g. page 7 etc.
The differences between AC and DC are in the power supply of course, and the method of coil suppression for the contactor/relay, AC will be a RC snubber and the DC version will be a Reverse biased diode rectifier.
The only thing now considered wrong is the use of the overload contact shown on the RH side of the coil, it should be on the LH side.
Max.

 

Thanks Max I understand the difference between AC and DC circuits.

Ill have to read more about PLCs I think. This has got me thinking .

 

The original question appears to be about the difference in logic, where did the PLC issue/question come in?
A little on the history http://library.automationdirect.com/history-of-the-plc/
Max.

 

Where did the PLC issue/question come in?.

For guidance it says "you may compare an electro-magnetic relay system with an electronic logic or PLC system"

 

So it is basically the comparison of the hard wired logic and the internal programed rung of a PLC, which of course still requires the physical input/output devices, just that the logic is an internal process on the latter.
Max.

 

Thanks Max.

I've got this now .