Labelling of pins with arrows

[rs20]

Legend:
 |  is the edge of the board (to help illustrate which edge of the board we're on)
 o  is the pin header in question (0.1" or whatever)
 -> is the arrow label

You'd think that the meaning of an arrow on a PCB is obvious. Even I would've thought this was a dumb question a month ago. But I managed to confuse myself with a PCB that had the arrow drawn between the pins and the edge of the board, because I was suddenly unsure if the arrows were being drawn "inwards vs outwards" with respect to the pin (i.e., moving the arrow to the other side of the pin would require flipping it to stay points towards or away from the pin), or if the arrows were being drawn with respect to the board edge.

Does anyone with board layout experience have some rules of thumb to go by here? I personally feel like it would be reasonable to assert that arrows ideally shouldn't be between pins and the board edge, to absolutely avoid any possible ambiguity.


(And no, coppercone2, I like being inside )

[HwAoRrDk]

Forgive if this is a stupid question, but what are the arrows supposed to represent? Whether a pin is an input or output? Direction of current flow?

Logically, regardless of position of pin or label with respect to board edge or whatever else, I would expect that an arrow pointing away from a pin is an output/source, and an arrow pointing towards a pin is an input/sink.

[rs20]

Forgive if this is a stupid question, but what are the arrows supposed to represent? Whether a pin is an input or output? Direction of current flow?

Not a stupid question at all! In the poll, I'm asking which drawing would represent an output pin. (That's what I meant by driven/transmitting, but the word output might have been clearer.)

[Ysjoelfir]

I personally voted for o -> |, but the results of the poll show exactly my thoughts. On one hand I aggree with hWaoRr[...] that the arrow would point away from the corresponding pin, but on the other hand you could argue that an arrow pointing from the circuit to the pin ( meaning -> o | ) is showing that the singnal is "flowing" FROM the circuit TO the pin and from there OUT of the board. So I only voted for 0 -> |, as this is the way I do it and if there is no space available on the board, I just don't write arrows just to not confuse the user.

[SiliconWizard]

I don't see a difference between 1 and 3 really, so it's all a matter of style. 3 would look more common, but I don't think 1 would be ambiguous either.

[thinkfat]

It would very much depend on the context and the usage of said pins. But I'd say, if a board-to-board connector is involved, anything that points towards the nearest board edge is "output", anything that points towards the center of the PCB is "input".

[tooki]

I don't see a difference between 1 and 3 really, so it's all a matter of style. 3 would look more common, but I don't think 1 would be ambiguous either.

It would definitely be ambiguous, then, if you're saying you see 1 as an output. With the arrow pointing towards the pin, to me, it'd clearly be an input.

[SiliconWizard]

I do not agree with that.

[Ysjoelfir]

I do not agree with that.

Don't worry. I do So what do we learn from that?

[SiliconWizard]

I do not agree with that.

Don't worry. I do So what do we learn from that?

Something obvious: even though there seems to be some common convention, arrows can still be interpreted in different ways by different people, with a clear, but different, rationale.
Mine was not to stick to the pin itself, but consider the signal flow on the board.  Others will strictly think about the pin itself outside of its context (the board).

So in the end: just don't use arrows for this. Use IN and OUT (I and O if you don't have enough area left is also OK), and call it a day. Just my 2 cents.

[tooki]

I do not agree with that.

... thus confirming what I said: it IS ambiguous!

[rs20]

Thanks everyone for voting, I suspected the results would be mixed but I was surprised by exactly how the results fell. This will influence my future PCB design for sure. I put together a little infotard summarizing the results:



Just in case people wonder about inconsistencies:

• Someone voted for both option 3 and option 4. Since that makes absolutely no sense, I considered that a non-vote.
• And yeah, I'm conflating "arrangements you'd interpret as output" with "arrangements you'd consider acceptable". DW.