USB Printer Chooser Switch

Posted on Jun 19, 2012

With a DPDT switch, I can switch the two data lines. I tried connecting the shields from the two computers together, the grounds two computers together, switching the D+ and D- lines with the DPDT switch, and leaving the VBus lines float, but for some reason it didn't work. The Lexmark printer seemed to need to see the +5 Volts. It had been many years since I read the USB standard (It was USB 1.0 back then) and I did not recall a function for the VBus beyond providing power to the peripheral, but in true experimentalist fashion, I added a transistor switch to supply VBus to the printer from whichever computer was connected, and the printer started working.

USB Printer Chooser Switch
Click here to download the full size of the above Circuit.

I used a pair of transistors to "OR" the Vbus voltage from the two cables that go to the computers, but a while after building it, I realised that the "off" transistor could turn on because the backwards transistor (collector and emitter swapped) could still work, but with most transistor types with a lower gain. Modifications to prevent the "off" transistor from turning on would have resulted in a lot of parts, so, abut two years after building the switch, I tried a pair of Schottky diodes as David Hogg, a reader of this web site. I used 1 Amp diodes instead of the lower current diodes because it will stand up better to abuse, and the 1 Am diodes will have a lower voltage drop than one with a lower current rating at a given current. The diodes work just fine in my office. While on this subject, a fellow by the name of Jim Glasgow build a switch before he came across this web site. His switch used a four pole break-before-make switch. I think this is a superior method, if you can find such a switch. One possible hitch to switching the ground, is that there is no way to be sure that the ground connects first, so the data or Vbus conductors could connect first, thereby possibly subjecting the circuits in the printer or one of the computers to hazardous voltages. Its best to tie all the grounds together and only switch data and Vbus. The common admonishments apply to this circuit: Keep the wires as short as practical,...

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