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I have external devices connected to PC which require USB 2.0 connection and a peripheral connector (Molex connector) power connection.

Since those devices are installed several meters(<3m) away from the PC, very long cables are used to connect the PC and the devices.

I want to clean up all the messy cable connections. My plan is to unify two kinds of connections into single circular connector such as GX16 or GX20. Now, I have to know how many pins I will need.

At first, I thought I need 8 pins for 4 lines from the peripheral connector and 4 lines from USB 2.0. However, I notice that some lines are duplicated.

  • peripheral connector consists of one 12V, one 5V, and two GND.
  • USB 2.0 consists of one 5V, two data, and one GND.

If I can share all duplicated lines, I will need only 5pins:

  • one 5V, one 12V, one GND, two USB data

Is this idea safe? If it's bad idea, how many pins do I need here at least?

P.S. The circular connector idea is a passthrough connector for power and signals in order to convenient cable management. Each end of the cable is split into USB and molex connectors anyway.

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    \$\begingroup\$ IDE is a 40 pin data connection, presumably you mean the one normally referred to as a "Molex"? Using non-standard cables and connections for USB isn't usually a good idea and may not work reliably. \$\endgroup\$
    – Finbarr
    Commented May 15, 2023 at 10:13
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    \$\begingroup\$ Please be aware that the two data lines in USB needs a characteristic impedance. It's not just any two wires. \$\endgroup\$
    – winny
    Commented May 15, 2023 at 10:23
  • \$\begingroup\$ @Finbarr Sorry for confusion. \$\endgroup\$
    – slyx
    Commented May 15, 2023 at 11:48
  • \$\begingroup\$ @winny Do you mean just cutting a USB cable and reconnecting it won't work? \$\endgroup\$
    – slyx
    Commented May 15, 2023 at 11:49
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    \$\begingroup\$ Depends on how long your non-USB cable in between is. \$\endgroup\$
    – winny
    Commented May 15, 2023 at 12:37

2 Answers 2

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That "IDE connector" wording initially confused me. Actually that connector is called "Peripheral Connector" in ATX12V specification, but often called "molex connector".

I can't say connecting two grounds is dangerous for sure.

But connecting 5V from USB(Vusb) and +5VDC power rail is terrible idea because:

  1. USB controller can be powered from +5VSB(stand-by 5V, always present), so when system is stopped, +5VDC can be powered from USB port.

  2. Vusb and +5VDC can have different actual voltage, thus when they are connected causing current to flow between them. As a result current can exceed 0.5A threshold and hopefully will trip over-current protection.

What I would recommend to use 5V only from molex connector.

1 - GND

2 - USB_DP

3 - USB_DN

4 - +5VDC

5 - +12VDC

6 - Vusb(optionally), should not be connected to +5VDC

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    \$\begingroup\$ Voltage difference may cause unexpected flow sounds dangerous. I'll follow your suggestion. Thank you! \$\endgroup\$
    – slyx
    Commented May 15, 2023 at 11:52
  • \$\begingroup\$ I think in reality most systems that have 5V molex connectors also use the same power supply for the USB ports. However you may find ones that don't. Maybe a 12VO motherboard used with a non-12VO PSU, so the motherboard makes its own 5V separately from the PSU which feeds the Molex connectors. \$\endgroup\$ Commented May 15, 2023 at 18:01
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Sounds like a poor idea. Not the idea to use only one set of 5V and GND itself, but:

  1. several meters cable for USB.

USB is specified to work up to 5 meters with compliant devices, cables and connectors that are guaranteed to work. Is your cable less than 5 meters and USB compliant?

  1. incompliant connector for USB.

It may not pass the data rate required and it definitely does not sequence USB power and data wires correctly, so please understand that using custom connectors not intended for high speed USB data rates or hot plugging can cause problems or even damage your devices.

  1. USB supply pin is generally current limited to some sensible amount of overcurrent. It does not come directly from power supply. USB may even come from computer 5V standby supply, so they cannot be combined. You could use either USB 5V for everything, or Molex 5V for everything, or just wire both. Please understand that the 5V directly from power supply can provide several amps of current, so maybe some protection is needed, and thick wires for the direct power supply connections.

  2. The ground pins may also need to be separate. If you only have one ground return path, the USB ground is not rated to handle other return currents from direct 5V/12V connections. If you only use direct ground from power supply, then the USB return path is not along the data wires.

So while you could do anything, without knowing what devices you plug it, how much current they consume, and how much voltagr drops during 3 meters of unknown wire, it will be best to keep everything separate.

If something goes wrong, at best it just does not work, if USB data is disturbed by poor connector or poor wiring, or if there is so much load that poor wires drop too much voltage, so there may be intermittent power loss during use (e.g. USB hard drive). At worst, you fry your USB connector, motherboard, power supply, some wiring, or the USB device.

With a non-hotpluggable connector in between, never hot plug that but only via the actual USB connectors which are hot-pluggable.

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  • \$\begingroup\$ USB2 is specified for up to 5 meters. \$\endgroup\$
    – Finbarr
    Commented May 15, 2023 at 11:17
  • \$\begingroup\$ @Finbarr it is. With compliant cables, connectors and devices. So it does not apply here. \$\endgroup\$
    – Justme
    Commented May 15, 2023 at 11:18
  • \$\begingroup\$ @Justme Yes less than 5 meter. At most 3meter it is. Since external devices only provide USB port anyway, the end of cable will be splitted into USB port and molex port eventually. Could it be still problem? \$\endgroup\$
    – slyx
    Commented May 15, 2023 at 11:42

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