3 ms·

They should have used different pins for source and sink power.

deleted 8y ago
[deleted]
mort96 8y ago
A display port cable is ghe same in both ends. Maybe they should've designed display port to have different connecrors for source and sink, but I must imagine they had compelling reasons to not do that.
- gizmo686 8y ago
  They could crossover in the cable. So both sides provide power on pin 20, and leave pin 21 notconnected, then have the cable wire pin 20 to pin 21. Then, when a device needs to draw power, they connect pin 21 as the source and leave 20 notconnected. If both devices supply power, then both power circuits are open. This still isn't ideal, as extension cables would need to be non-crossover and you would have a problem if you ever tried using an extension cable in place of a normal cable.
  - DSMan195276 8y ago
    > This still isn't ideal, as extension cables would need to be non-crossover and you would have a problem if you ever tried using an extension cable in place of a normal cable. I don't think that's really a big problem: 1. If your device has a female port (most devices), supply power on pin 20, source power on pin 21 2. If your device has a male port ('dongle' type devices), supply power on pin 21, source power on pin 20. 3. M-F (extension) cables are non-crossover 4. M-M and F-F cables/adapters are crossover With that configuration I don't think there's any way to plug something in incorrectly or accidentally use the wrong cable. You can't use an extension cable in place of a normal cable without using a M-F adapter, which would do the crossover for you. It gets a bit fuzzier when involving converters to other port types, but I think it's actually less complicated then the current model in that case. As long as you just supply or source power on the right pins for the DP port type you have (male or female), then there is nothing else to worry about when creating a converter. At the very least, I can't imagine it would be any worse then the current situation. With the current setup extension cables should have pin-20 connected (Though it sounds like they might not), but M-M/F-F cables should not, which IMO is much messier then the above.
    - gizmo686 8y ago
      That seems like it should work, although it is a different (and better) proposal than what I had. Essentially, your proposal is to have the invariant that males supply power on pin 20 and source on pin 21, while females supply on 21 and source on 20. It is easy to see that this invariant is maintained by all of your proposed cables. And, since the only way to plug ports together is male-female you are guaranteed to always be matching supply with source. Since this invariant is maintained at every terminal, there is, as you identify, no added difficulty in creating converters, as they would only target one gender of the port anyway. Out of curiosity, is there any port that takes this approach (either for power or data pins)?
      - cesarb 8y ago
        "Traditional" USB does that: USB-A ports source power, USB-B ports sink power. (Also, USB-A ports are always towards the host, which controls the bidirectional serial link.) The standard cable has a USB-A plug on one end, and a USB-B plug on the other end. There can be no USB-A to USB-A (or USB-B to USB-B) cable, since they're forbidden by the standard, so you can't connect two sources (or two sinks) together. And of course it didn't work: there are many standard-breaking USB cables with identical plugs on both ends, and standard-breaking devices which require them.
        DSMan195276 8y ago
        That's not quite what I described. With my design either port can source or sink power, they just do it on opposite pins. And it wouldn't be USB-A vs USB-B, but male vs female So for USB, you would need a 5-pin port, where two are devoted to power. And both USB-A and USB-B would have the same wiring, but male ports source and supply on opposite pins as the female ones. Then M-M or F-F cables crossover the power lines.

deleted 8y ago

[deleted]

A display port cable is ghe same in both ends. Maybe they should've designed display port to have different connecrors for source and sink, but I must imagine they had compelling reasons to not do that.

gizmo686 8y ago
They could crossover in the cable. So both sides provide power on pin 20, and leave pin 21 notconnected, then have the cable wire pin 20 to pin 21. Then, when a device needs to draw power, they connect pin 21 as the source and leave 20 notconnected. If both devices supply power, then both power circuits are open. This still isn't ideal, as extension cables would need to be non-crossover and you would have a problem if you ever tried using an extension cable in place of a normal cable.
- DSMan195276 8y ago
  > This still isn't ideal, as extension cables would need to be non-crossover and you would have a problem if you ever tried using an extension cable in place of a normal cable. I don't think that's really a big problem: 1. If your device has a female port (most devices), supply power on pin 20, source power on pin 21 2. If your device has a male port ('dongle' type devices), supply power on pin 21, source power on pin 20. 3. M-F (extension) cables are non-crossover 4. M-M and F-F cables/adapters are crossover With that configuration I don't think there's any way to plug something in incorrectly or accidentally use the wrong cable. You can't use an extension cable in place of a normal cable without using a M-F adapter, which would do the crossover for you. It gets a bit fuzzier when involving converters to other port types, but I think it's actually less complicated then the current model in that case. As long as you just supply or source power on the right pins for the DP port type you have (male or female), then there is nothing else to worry about when creating a converter. At the very least, I can't imagine it would be any worse then the current situation. With the current setup extension cables should have pin-20 connected (Though it sounds like they might not), but M-M/F-F cables should not, which IMO is much messier then the above.
  - gizmo686 8y ago
    That seems like it should work, although it is a different (and better) proposal than what I had. Essentially, your proposal is to have the invariant that males supply power on pin 20 and source on pin 21, while females supply on 21 and source on 20. It is easy to see that this invariant is maintained by all of your proposed cables. And, since the only way to plug ports together is male-female you are guaranteed to always be matching supply with source. Since this invariant is maintained at every terminal, there is, as you identify, no added difficulty in creating converters, as they would only target one gender of the port anyway. Out of curiosity, is there any port that takes this approach (either for power or data pins)?
    - cesarb 8y ago
      "Traditional" USB does that: USB-A ports source power, USB-B ports sink power. (Also, USB-A ports are always towards the host, which controls the bidirectional serial link.) The standard cable has a USB-A plug on one end, and a USB-B plug on the other end. There can be no USB-A to USB-A (or USB-B to USB-B) cable, since they're forbidden by the standard, so you can't connect two sources (or two sinks) together. And of course it didn't work: there are many standard-breaking USB cables with identical plugs on both ends, and standard-breaking devices which require them.
      - DSMan195276 8y ago
        That's not quite what I described. With my design either port can source or sink power, they just do it on opposite pins. And it wouldn't be USB-A vs USB-B, but male vs female So for USB, you would need a 5-pin port, where two are devoted to power. And both USB-A and USB-B would have the same wiring, but male ports source and supply on opposite pins as the female ones. Then M-M or F-F cables crossover the power lines.