# Circuit Diagrams versus Logic Diagrams

Why do we sometimes neglect resistors and power sources when we draw circuits and other times not?

Is there different terminology for diagrams that are only concerned about the logic involved and diagrams where the voltage and resistance is accounted for?

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phinds
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Why do we sometimes neglect resistors and power sources when we draw circuits and other times not?

Is there different terminology for diagrams that are only concerned about the logic involved and diagrams where the voltage and resistance is accounted for?
Circuit diagrams are circuit diagrams and logic diagrams are logic diagrams. You can study the electronics involved in a logic diagram but generally it is not of any interest, else you would have started with a circuit diagram. Generally, when you draw a logic diagram with things like NAND gates you really don't care what the implementation methodology is, you just care about the logic.

donpacino and Schfra
Averagesupernova
Gold Member
I guess I would need some hard evidence. Examples. Many times partial schematics are scribbled out omitting the obvious parts. That is not to say a proper published schematic is missing parts simply because it is assumed that whoever is looking at it will know they are supposed to be there.

Circuit diagrams are circuit diagrams and logic diagrams are logic diagrams. You can study the electronics involved in a logic diagram but generally it is not of any interest, else you would have started with a circuit diagram. Generally, when you draw a logic diagram with things like NAND gates you really don't care what the implementation methodology is, you just care about the logic.
If you’re designing a complex circuit, would you not start with logic diagram and then make the circuit diagram?

phinds
Gold Member
2019 Award
If you’re designing a complex circuit, would you not start with logic diagram and then make the circuit diagram?
No, not if it's just a logic circuit. I used to design VERY complex logic circuits and I didn't give a hoot or a holler how the gates were implemented (actually I KNEW how they were implemented but it was irrelevant unless there were timing issues).

Why do we sometimes neglect resistors and power sources when we draw circuits and other times not?

Is there different terminology for diagrams that are only concerned about the logic involved and diagrams where the voltage and resistance is accounted for?
Do you have example drawings of what you mean?

One possible take on the question .... many years ago, a programmable logic controller may have used something like this in IL (Instruction List).
Code:
LD I1
AND I2
OR I3
OUT Q1
which is the as this logic expressed in FBD (Function Block Diagram)

and as LD (Ladder Diagram).

Ladder diagram, substituted with the appropriate switch and relay coil symbols, constitutes a wiring diagram as well as expressing the logic involved.

It isn't necessary or desirable to include everything in a circuit diagram, indeed, without an appropriate level of abstraction an electrical schematic would become practically unreadable. The logic examples above were generated using Siemens Comfortsoft which was designed to program their Logo! series of intelligent relays. A user's understanding circuit operation would not be improved by including a component level drawing of a Logo! logic controller.

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krater
Svein
No, not if it's just a logic circuit. I used to design VERY complex logic circuits and I didn't give a hoot or a holler how the gates were implemented (actually I KNEW how they were implemented but it was irrelevant unless there were timing issues).
Yes - but in the end someone has to implement them (and deal with the electronics involved).

A former colleague of mine was an expert in this transformation. After the logic was finished and simulated (in VHDL), his job was to inspect the gate layout and make sure that the power was supplied to all gates, that the circuit ground was distributed in such a way that no spurious feedback would occur and so on. He was the one that converted successful theory into successful practice.

Ladder diagram, substituted with the appropriate switch and relay coil symbols, constitutes a wiring diagram as well as expressing the logic involved.
If I could like this twice I would.

I integrate both systems into wiring/logic details... To explain, I show a conventional wiring diagram with all the value's etc or at least a key reference if it would clutter the diagram, where they terminate to a PLC it is just ID'd as it's part code. All logic maps, timing values, boolean states etc etc are supplied as separate sheets and like in the logic program shown above on the Logo, you can label/tag your inputs/outputs and other process boxes to whatever you wish to if you choose, as these programs are often unique and can get around control issues others may have struggled to overcome then giving too much info can work against you, I password protect my PLC's and only give enough info to show input and output tags, timer values and other key values that may be required for maintenance, if I supply the full logic details then I will Macro some of the more complex stages to protect my interests.

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berkeman
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berkeman
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