Sleep problems with HP printer

Ok, here's my problem:
I recently got a HP Deskjet 840C printer, which works fine with my Mac. The only trouble I have is that, if I leave it's USB connector plugged in (it's plugged into the keyboard), my Mac won't stay in sleep mode. When I put it to sleep, it goes to sleep mode, but directly wakes up again. It makes no difference if the printer is turned on or off.
The only thing that works is unplug the printer, put the Mac to sleep, and plug the printer back in again...but that's not really the way it should be, and it's annoying.

I have no other USB ports free, as one of the Mac's port has the keyboard, and the other has an Audio/MIDI interface.
I can't connect the Audio interface through a hub, because of audio latency problems, and I read somewhere that the keyboard should always be connected directly to the Mac, to avoid problems with the mouse (connected to the keyboard, due to it's short cable)

Any thoughts?

No ideas???

Open the HP software/preferences or something of that nature. On my deskjet 5550 there are a few options about sleeping and standby modes.

As it is quite an old printer, all I have is the Apple driver, no HP software. Did check at HP, but all they had was a newer driver, no software...
thanx anyway