All the time i see "printer.c: usblp1: error -150 writing to printer" in dmesg.
What does it mean? On usblp1 I have printer HP DeskJet 840C.

It is the problem in the usb printer device class driver. Try to remove the bidirectional parameter from the config file.

Where can I remove the bidirectional parameter? Where is config file?
I'm using CUPS.

CUPS on your router, or CUPS on your client (laptop/desktop)?

You need to change the setup on the router. p910nd does most if not all you need though...

You have not written this information. I took what i knew:

It is not possible to change the cups backend behavior (without a recompilation).

You can try to add the proto_bias=1 (the unidirectional protocol) parameter in the /etc/modules.d/60-usb-printer (after the word printer in White Russian or usblp in Kamikaze) for the usb printer class driver. This way you can disable the bidirectional capability of the driver/printer. Notice that you will also disable using a better protocol for other devices.

There were several printers that advertised the bidirectional capability but required the unidirectional handling for the proper work. These printers are included in the driver's exception table and should be handled properly (including your 840C).
Just for the curiosity, can you tell me the vendor/device information? The included exception is for the device vendor=0x03f0, device=0x0604 (HP DeskJet 840C).

Thanks for reply.

CUPS is on router. Asus WL-500GP with Kamikaze 7.07 kernel 2.4.

I tried p910nd and I had lots of problems. I'm using 3 printers and it was messy if they were turned on in different order than it was in setup. Additionally p910nd has problems with handling errors. On Win XP often the docs stayed in queue for hours.
So in my particular situation I needed something more complex and CUPS seems to be just fine.

So if I'm using 3 printers then after this setup all of them will use unidirectional handling? If yes then I have to look for another way.

Why do you need this?

I still don't know what does it mean "printer.c: usblp1: error -150 writing to printer". Anybody can explain this error code? Is it something that should bother me or should I ignore?