3 ms·

So how’d you safely solve having cables of (widely) varying resistance? Smartphones already do this. They essentially measure the resistance of the cable by me

So how’d you safely solve having cables of (widely) varying resistance? Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly. That would require twice the number of contacts for the same number of physical wires, which would make the plugs even larger. No. Look at the pinout. It's not symmetrical. They could've made it perfectly symmetrical.

dvdkon 1y ago
That's not true, see this random pinout diagram: https://acroname.com/sites/default/files/shared/alt_mode_-_displayport_pinout.png https://acroname.com/sites/default/files/shared/alt_mode_-_d...
lxgr 1y ago
> It's not symmetrical. They could've made it perfectly symmetrical. Without internally connecting all corresponding pins to the same wire? To what benefit? The parts that need to be symmetric for passive legacy devices already are.
amluto 1y ago
> Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly. Not sufficient. It’s easy to have a cable where the long conductors can safely carry a lot more current than the terminations. Or one could have a 1 foot thin cable connected in series with a 10 foot thick cable. For that matter, a short low-ampacity cable would look just like a long high-ampacity cable.
- lxgr 1y ago
  On top of that, a slightly imprecise voltage source could also easily look like a very low resistance cable.
  - userbinator 1y ago
    It's differential impedance that they measure, not the absolute voltage.
    - lxgr 1y ago
      What would they measure that for?
      - userbinator 1y ago
        To determine how much current they can draw.
        lxgr 1y ago
        They can only measure combined voltage sag of the source and resistive losses in the cable though that way, no? Doesn’t help with the concern of high-resistance spots somewhere in a low-current-rated cable.
        NetMageSCW 1y ago
        They aren’t trying to deal with poorly manufactured out of spec cables.
- ajb 1y ago
  But resistance in series is additive - the resistance of the combination is dominated by the resistance of the worse cable: 0Ω + 1Ω = 1Ω Not sure what problem you're thinking of.
  - lxgr 1y ago
    Most of the resistance occurring in a small part of the entire cable, potentially causing a fire, seems possible, no?
    - NetMageSCW 1y ago
      No. You could also suppose a dead short in the cable that causes a fire, or just a cable with a length of detcord in the middle.
  - Tharre 1y ago
    It's not resistance per se that you care about, it's how much the cable is heating up which is mostly dependent on resistance per meter. Imagine 2 cables with the same resistance, one 0.5m long and the other 2m. At a set amount of amps, the 0.5m long cable would need to safely dissipate 4 times as much heat than the 2m cable over the same distance. And you don't know how long the cable actually is (and they make 10cm usb cables after all) so you can't make any decision based on resistance that doesn't risk some cables going up in flames.
    - userbinator 1y ago
      Fortunately copper and aluminium, the two most common metals for cables, have resistance that increases with temperature. Ultimately what I'm saying is that the endpoints have or can measure all the information they need to adapt, and this is even more accurate than requiring a cable to self-identify which is worthless if it's lying (and that can happen unintentionally if it's damaged or the connector is worn.)
      - amluto 1y ago
        The copper and aluminum will not become resistive enough to make a difference at temperatures low enough to prevent the rest of the cable from becoming crispy.
shadowpho 1y ago
>Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly. No! Where did you hear this? Smartphones usually either rely on type-c pd nowadays or do far less standard Samsung/Apple resistor on usb line check. Some power banks also do terrible thing where they increase current pull until they see voltage starting to collapse but that’s to check for OC limits on load switch. However that’s really starting to go away because type c just works
- lxgr 1y ago
  > Some power banks also do terrible thing where they increase current pull until they see voltage starting to collapse If I’m not mistaken, that was explicitly allowed under USB Battery Charging. But chargers have to opt in to that via signaling on D+/D-.
- userbinator 1y ago
  It's been common for many years. Look for a document titled "Mediatek Pump Express Introduction". Another useful phrase to search for more information: "cable impedance detection". Apparently several patents on it too.
  - shadowpho 1y ago
    >Mediatek Pump Express Introduction Yep read it. It does even more terrible things (modulating current to change voltage), but no monitoring for voltage drops. It’s also rarely supported, and looks like they just switched to pd3 (the sane protocol)
    - userbinator 1y ago
      but no monitoring for voltage drops Did you not see: "Do cable imp. Calculation. Protect cable from burnout" on page 12 "Adjust Vbus according to cable impedance/quality" on page 14 and "Cable Impedance Measurement" on page 15? Pump Express is their fast charging protocol, but even before that, in the days of feature phones, they were already doing the measurements and adjusting charging current.
theamk 1y ago
I've had a phone like this - it'd increase current until voltage start to sag. It was horrible. Never again. The difference between high-current and low-current cable was very small, so once cable got a little bit worn out, the detection was basically random. You plug cable, and get 1A charge. Unplug-replug, and it's 0.5A now. Unplug-replug and it gets 2A. I am very glad that the system has died out and we have cables that simply report their capacity directly. (that said yes, it could have been simpler, like having a symmetric pinout. But this would be a marginal improvement at best, as USB-IF seems to be incapable of producing simple designs. It'd still be thousands of pages of docs)
- userbinator 1y ago
  You just had a bad implementation. cables that simply report their capacity directly. ...which won't true when worn or the manufacturer is lying.
  - theamk 1y ago
    I don't think it is possible to have "good implementation" of cable impedance sensing, given the inherent properties of the connectors. Wetting current is a thing, after all, and it can be quite big for work connectors. Cable-reported capacity does not really change over cable lifetime - the currents used to sense resistance are miniscule, and would not be affected by cable being worn. For example, extra 0.5 ohm of cable resistance completely ruins the current sensing, but make absolutely no difference when measuring 5K or 1K resistance. And if manufacturer is lying, that's annoying but it's 100% deterministic - you know the cable is bad, you can throw it away. With impedance sensing, the same cable can be good or bad, depending on the mood, and that's why it should not be used ever.

dvdkon 1y ago

That's not true, see this random pinout diagram: https://acroname.com/sites/default/files/shared/alt_mode_-_displayport_pinout.png https://acroname.com/sites/default/files/shared/alt_mode_-_d...

lxgr 1y ago

> It's not symmetrical. They could've made it perfectly symmetrical. Without internally connecting all corresponding pins to the same wire? To what benefit? The parts that need to be symmetric for passive legacy devices already are.

amluto 1y ago

> Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly. Not sufficient. It’s easy to have a cable where the long conductors can safely carry a lot more current than the terminations. Or one could have a 1 foot thin cable connected in series with a 10 foot thick cable. For that matter, a short low-ampacity cable would look just like a long high-ampacity cable.

lxgr 1y ago
On top of that, a slightly imprecise voltage source could also easily look like a very low resistance cable.
- userbinator 1y ago
  It's differential impedance that they measure, not the absolute voltage.
  - lxgr 1y ago
    What would they measure that for?
    - userbinator 1y ago
      To determine how much current they can draw.
      - lxgr 1y ago
        They can only measure combined voltage sag of the source and resistive losses in the cable though that way, no? Doesn’t help with the concern of high-resistance spots somewhere in a low-current-rated cable.
        NetMageSCW 1y ago
        They aren’t trying to deal with poorly manufactured out of spec cables.
ajb 1y ago
But resistance in series is additive - the resistance of the combination is dominated by the resistance of the worse cable: 0Ω + 1Ω = 1Ω Not sure what problem you're thinking of.
- lxgr 1y ago
  Most of the resistance occurring in a small part of the entire cable, potentially causing a fire, seems possible, no?
  - NetMageSCW 1y ago
    No. You could also suppose a dead short in the cable that causes a fire, or just a cable with a length of detcord in the middle.
- Tharre 1y ago
  It's not resistance per se that you care about, it's how much the cable is heating up which is mostly dependent on resistance per meter. Imagine 2 cables with the same resistance, one 0.5m long and the other 2m. At a set amount of amps, the 0.5m long cable would need to safely dissipate 4 times as much heat than the 2m cable over the same distance. And you don't know how long the cable actually is (and they make 10cm usb cables after all) so you can't make any decision based on resistance that doesn't risk some cables going up in flames.
  - userbinator 1y ago
    Fortunately copper and aluminium, the two most common metals for cables, have resistance that increases with temperature. Ultimately what I'm saying is that the endpoints have or can measure all the information they need to adapt, and this is even more accurate than requiring a cable to self-identify which is worthless if it's lying (and that can happen unintentionally if it's damaged or the connector is worn.)
    - amluto 1y ago
      The copper and aluminum will not become resistive enough to make a difference at temperatures low enough to prevent the rest of the cable from becoming crispy.

shadowpho 1y ago

>Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly. No! Where did you hear this? Smartphones usually either rely on type-c pd nowadays or do far less standard Samsung/Apple resistor on usb line check. Some power banks also do terrible thing where they increase current pull until they see voltage starting to collapse but that’s to check for OC limits on load switch. However that’s really starting to go away because type c just works

lxgr 1y ago
> Some power banks also do terrible thing where they increase current pull until they see voltage starting to collapse If I’m not mistaken, that was explicitly allowed under USB Battery Charging. But chargers have to opt in to that via signaling on D+/D-.
userbinator 1y ago
It's been common for many years. Look for a document titled "Mediatek Pump Express Introduction". Another useful phrase to search for more information: "cable impedance detection". Apparently several patents on it too.
- shadowpho 1y ago
  >Mediatek Pump Express Introduction Yep read it. It does even more terrible things (modulating current to change voltage), but no monitoring for voltage drops. It’s also rarely supported, and looks like they just switched to pd3 (the sane protocol)
  - userbinator 1y ago
    but no monitoring for voltage drops Did you not see: "Do cable imp. Calculation. Protect cable from burnout" on page 12 "Adjust Vbus according to cable impedance/quality" on page 14 and "Cable Impedance Measurement" on page 15? Pump Express is their fast charging protocol, but even before that, in the days of feature phones, they were already doing the measurements and adjusting charging current.

theamk 1y ago

I've had a phone like this - it'd increase current until voltage start to sag. It was horrible. Never again. The difference between high-current and low-current cable was very small, so once cable got a little bit worn out, the detection was basically random. You plug cable, and get 1A charge. Unplug-replug, and it's 0.5A now. Unplug-replug and it gets 2A. I am very glad that the system has died out and we have cables that simply report their capacity directly. (that said yes, it could have been simpler, like having a symmetric pinout. But this would be a marginal improvement at best, as USB-IF seems to be incapable of producing simple designs. It'd still be thousands of pages of docs)

userbinator 1y ago
You just had a bad implementation. cables that simply report their capacity directly. ...which won't true when worn or the manufacturer is lying.
- theamk 1y ago
  I don't think it is possible to have "good implementation" of cable impedance sensing, given the inherent properties of the connectors. Wetting current is a thing, after all, and it can be quite big for work connectors. Cable-reported capacity does not really change over cable lifetime - the currents used to sense resistance are miniscule, and would not be affected by cable being worn. For example, extra 0.5 ohm of cable resistance completely ruins the current sensing, but make absolutely no difference when measuring 5K or 1K resistance. And if manufacturer is lying, that's annoying but it's 100% deterministic - you know the cable is bad, you can throw it away. With impedance sensing, the same cable can be good or bad, depending on the mood, and that's why it should not be used ever.