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I have two printers and one computer. I want to print to both printers the same document at the same time... Thus when I press print on my document both printers will print the file simultaneously.

I don't want to install any software on my computer – it should be able to work on any computer.

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up vote 4 down vote accepted

No. Unlike a parallel port, Universal Serial Bus cannot be split like this; each device has to negotiate with the others for speed, power, and capabilities, and even if multiple devices are similar or the same, there will be low-level differences that will confuse the bus. You would need a hardware splitter that would reprocess the data signals and direct them appropriately, and it would have to speak the appropriate protocol(s).

In this instance you're best off getting a print server that you can configure to send jobs to both printers once it has received them from the print client.

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Thanks! I think I understand the speed problem. Let's say you DO split it. From there one leg of the Y goes to the printer and the other leg goes to a someplace else where speed is not a problem. Is the power problem really insurmountable? (I can't vote your answer up since I don't have enough reputation) – ISJ May 27 '12 at 9:22
It's not "speed and power are problems", it's "speed and power must be negotiated with the host". You can't get around that. Even if both devices negotiate the same speed and power, the low-level differences (e.g. device serial number) will still rake you over. – Ignacio Vazquez-Abrams May 27 '12 at 9:24
Hmmm, so it's not just a bunch of binary digits that can be intercepted? – ISJ May 27 '12 at 9:26
They can be intercepted. But you need something in the middle that can intercept it; it isn't enough to just split each of the four wires and route them to each device. – Ignacio Vazquez-Abrams May 27 '12 at 9:28
Ah ok, Thanks! What is that thing in the middle called? Is it something like this: link – ISJ May 27 '12 at 9:31

Theoretically, 127 devices can operate on a single USB interface, although in practice, they would require far more bandwidth in total than available. This means the communication has to be a lot more advanced than sending a constant stream of instructions back and forth. You don't want mouse movements picked up by a storage device and interpreted as a write signal, for instance.

Whenever a device is connected, the host sends a SETUP signal, followed by a 11-bit address, four of which are reserved for defining the device function, hence the limit of 127 devices. All subsequent requests to transmit or start receiving data, will mention this address. That's why simply splitting the cable won't work. In fact, it is often split internally already. Both printers will receive the instructions, but only one of them will listen.

Tricking the printer into listening to the address assigned to its neighbour, won't work either. Both slaves would reply simultaneously and the resulting signal will be ambiguous and/or gibberish. Therefore, no simple logical splitters exist that can link two printers the way you want. You'll need to install software that duplicates the print job before it gets sent to the USB controller.

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