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I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host D+ ----- Hub D+
Host D- ----- Hub D-

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host D+ ----- Hub D+
Host D- ----- Hub D-

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host D+ ----- Hub D+
Host D- ----- Hub D-

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this questionthis question, in which the answers suggest that just connecting the ground lines would be enough, and this onethis one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host D+ ----- Hub D+
Host D- ----- Hub D-

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host D+ ----- Hub D+
Host D- ----- Hub D-

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host D+ ----- Hub D+
Host D- ----- Hub D-

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host D+ ----- Hub D+
Host D- ----- Hub D-

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host D+ ----- Hub D+
Host D- ----- Hub D-

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host D+ ----- Hub D+
Host D- ----- Hub D-

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

Tweeted twitter.com/#!/StackElectronix/status/237564172840361984
Fixed data pin terminology to D+/D- rather than the Rx/Tx I had incorrectly used.
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Ian Renton
Ian Renton
  • 121
  • 3

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host TxD+ ----- Hub RxD+
Host RxD- ----- Hub TxD-

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host TxD+ ----- Hub RxD+
Host RxD- ----- Hub TxD-

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host TxD+ ----- Hub RxD+
Host RxD- ----- Hub TxD-

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host D+ ----- Hub D+
Host D- ----- Hub D-

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host D+ ----- Hub D+
Host D- ----- Hub D-

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host D+ ----- Hub D+
Host D- ----- Hub D-

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.

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Ian Renton
Ian Renton
  • 121
  • 3

Drawing 100mA / common ground for a bus-powered USB hub

I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.

My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:

PSU 5V  ----- Hub VCC In
PSU GND ----- Hub GND In
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:

PSU 5V  ----- Hub VCC In
PSU GND ---=- Hub GND In
Host GND--/
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:

PSU 5V  ------ Hub VCC In
PSU GND ----=- Hub GND In
Host GND -=/
          |
         50R
          |
Host 5V -/
Host Tx ----- Hub Rx
Host Rx ----- Hub Tx

In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.

Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?

Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.