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In a schematic, if a designer chooses to use arrows on pins of a component, should they always point in the direction of current, or in the "perceived direction" of the signal? Or this a made-up designation with no real integrity?

I'm using Altium Designer and the pins on a component can be denoted as several specific types:

  • Input
  • I/O
  • Output
  • Open Collector
  • Passive
  • HiZ
  • Open Emittor
  • Power

Component Pin Editor

I will use a connector as a specific example, however I'm interested in the use of pin arrows across devices.

Here is a connector with 8 pins of the type correlating to the list above. Observe the first three pins have arrows based on their pin type:

Altium conector

I'm wondering if there are specific standards/conventions for arrows on pins.

I have a 4 pin connector in my circuit. From the reference point of my circuit, the 4 signals are:

  1. Power (12V+ from my circuit to connector, supply is from my circuit)
  2. Ground (supply is from my circuit)
  3. PWM "Out" (Device that plugs into connector has a pullup resistor and reads via GPIO, my circuit will periodically pull it to ground)
  4. Tachometer "In" (Device that plugs into connector periodically pulls this to ground, my circuit as a pullup resistor and reads via GPIO)

Here is the system schematic of my setup:

system schematic

Going back to the image of the hypothetical 8 pin connector, since the "Input" type shows an arrow going in to the connector, the type is clearly from the reference point of the component.

Using the connector as the reference point:

  • I assume I will want "Power" to be an input. That's easy, because conventional current flow into the connector.
  • With Ground, current flows back out of the connector. So that must be an "output".
  • As for PWM "Out", it's an "input" to the connector, however current flows out. I still assume "input" is the proper type.
  • The tachometer is just the vise-versa of PWM "Out".

Are my assumptions correct?

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    \$\begingroup\$ The arrows in this case are denoting the signal flow, not the current. So in these terms power has no direction. \$\endgroup\$
    – Eugene Sh.
    Commented Sep 7, 2018 at 18:41
  • \$\begingroup\$ @EugeneSh. - I am cautious when using bare words like "signal", because power is a signal too, at least in some aspects in some fields. But I of course grasp what you are getting at. \$\endgroup\$
    – Bort
    Commented Sep 7, 2018 at 18:49
  • \$\begingroup\$ Power is kind of special... You probably can put some kind of arrows to make it clear who is the source and who is the consumer. But the arrows on signals are really useful when you have complex system with chained components, extension and splitter cables documented on different schematic sheets. Then it is very useful to have these arrows to track down where, say TX and RX are going and that these are not swapped somewhere. We used even to put the arrows on the lines themselves on the schematic to make it clearer. \$\endgroup\$
    – Eugene Sh.
    Commented Sep 7, 2018 at 18:58

2 Answers 2

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No your assumptions are not correct. Inputs and outputs are generally reserved for ICs. An input has the arrow pointing into the component. An output has the arrow pointing out of the component. Power and ground pins of ICs should be set to "Power". Generally arrows should not be used on connectors. Connectors are passive, so select "Passive" as the pin type for all of the pins on the connector.

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  • \$\begingroup\$ Shoot, I am wondering about ICs too (general use of the arrows was the main focus). Maybe I should edit my question. Helpful nonetheless. \$\endgroup\$
    – Bort
    Commented Sep 7, 2018 at 18:36
  • \$\begingroup\$ This still leaves one open item: If the connector were an IC, should "PWM Out" have an arrow pointing into the IC, even though current is drawn out from that pin? \$\endgroup\$
    – Bort
    Commented Sep 7, 2018 at 18:42
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    \$\begingroup\$ As I noted under your question, these denote the logical direction of a signal, not current. \$\endgroup\$
    – Eugene Sh.
    Commented Sep 7, 2018 at 18:45
  • \$\begingroup\$ As a general rule all inputs are on the left side and all outputs are on the right side, so in the diagram there is continuity of signal flow. In OrCAD to me power is an input, so they are on the left side of the part symbol. \$\endgroup\$
    – user105652
    Commented Sep 7, 2018 at 19:07
  • \$\begingroup\$ @Sparky256 That can be only accomplished is you "take apart" your connectors on the schematic, which is not always conforming the in-house conventions. \$\endgroup\$
    – Eugene Sh.
    Commented Sep 7, 2018 at 19:11
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For what it's worth these are the IEC\IEEE standards for digital logic.

enter image description here Source: www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf

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  • \$\begingroup\$ OrCAD follows these conventions, plus the option to put an over-bar on a pin name (IC) to indicate an 'active low' input or output. \$\endgroup\$
    – user105652
    Commented Sep 7, 2018 at 19:50

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