# Thread: computer 1 usb 1<====>usb 2 computer 2

1. Originally Posted by kryptkat
5v + 5v = 10v what happened to voltage doublers ? or voltage multiplying ? or adding two voltages together ? if you tried to read or measure the voltage on the power line of two usb connected together from two separate power supplies from two different computers would you read 10v assuming that the power supplies were 5v ?
Nope... you'd probably read 0, because you'd be damaging stuff. On the off chance it survived you have two current sources in parallel not series, so you would only read 5 volts.

what about the circuitry ? voltage regulators ? or clipping diodes like the zener ? that would send any volts over 5v to ground leaving the 5v. so then you would only measure 5v ? assuming both power supplies have the same 5v regulator circuit.
The one thing you do not do is back feed power into a current source... it lets all the smoke out and the part stops working.

And.. in this case the current source is the USB controller chip... not the power supply directly. The USB standard allows for 500ma (1/2 amp) per connector... 1/2 amp at 5 volts is 2.5 watts.

it is the current that blows parts. you can have a potential difference of charges extremely high as long as there is no current flow ie capacitor. same cap with a change of volts current will flow. ie sin or on off . the rating on the component says what it can take.
And what about that first instant when power is turned on but the capacitor is discharged, it's effective resistance is almost 0... there can be a huge burst of current... A 1uf electrolytic capacitor going abruptly from 0 to 10volts with a 1 ohm source resistance can pass a momentary burst of up to 10 amps!

so the data + and data - are one way ? like full duplex ? the data + transmits while the data - receives ? or is it data + is a control line only two way and the data - is two way data only ?
No it's a differential pair... when data+ is going positive data- is going negative at the same time. It's half-duplex communication.

2. was not pulling values out of the air. it was calculated with the tolerance numbers in the example.

ok "same time" that is what was missing. that is redundant. but i see how that would keep better integrity of the signal quality. both data lines go high or both go low and the signal goes back and fourth between devices as necessary. like a sin 180 degrees for one data line mirroring the other data line. that "differential signaling" wiki page has to be the worst page yet for explaining things.

walking in a store and see a male usb connector on both ends of the cable and think "null modem". future computers could be made to detect the presents of another computer like a device by setting it up like a client and server. the usb on the computer would start in client mode. if nothing is detected switch to server mode. but that would slow down the initialization process. a detector on the 5v + keeping the voltage at 0 and waiting for another computer. when it detects voltage act as a client or other device. if after a moment nothing then send 5v + in server mode. some day it could work like that.
for now i understand why it did not work.

did you see penny as a cat ?

3. Problem with that is, if the device is an unpowered slave, like a USB driver, it can't talk to you if you don't give it power.

There is already a protocol for that. It's the On-The-Go extension to USB 2.0.