Hi,
slackraspi wrote:I have one doubt: to give power to the raspberry I'm using a power charger by Asus whose output is 5v - 2A? Is it enough?
For supplying both devices, probably yes (from mathematical sum perspective).. but the point is elsewhere... There is a current limit RasPi can provide through USB ports.
Some general text first...
"Bigger" USB devices can sink such a high amount of current, that is even above USB specification limit. For example, I have a scanner which requires 700mA, where the current limit set by USB 2.0 standard is 500mA for higher loads and 100mA for a group of smaller loads. Therefore, this my scanner is not USB 2.0 compliant, despite the fact that it is made by a very known and respected company. The reason is obviously in competition pressure (to reduce cost by not providing additional power supply and to be more user-friendly by having just USB cable).
One of the lack of the USB ports is a discrete current limiter, which would reject loads sinking currents higher than 500mA/100mA, since the device exceeding this amount of current is not USB-compliant. Instead, USB ports have just "soft" current limiter - during USB enumeration procedure the USB device "introduces" itself also with data about maximum current it "intends" to sink from USB port. If it has stand-alone power supply, then this value is 0. Many vendors cheat here, because they set this software value to 500mA or 100mA (or even to 0) despite the fact that the actual value is higher (like for my scanner). If the actual value would be revealed, then each and every computer would have to reject this device.
Since most computers don't have proper USB current detecting/limiting elements, this "introduction" is the only data they can rely on... and due to some safety margin on the computers, these USB devices can usually operate normally. (Laptops can cause some troubles to those cheaters and you can read various messages in many forums that USB device X doesn't work on laptop Y... but the problem is typically not in the laptop.)
Going back to the RasPi specifics, it has a current-limiting element, which can cut the power if the current exceeds maximum overall value. You can search here for "polyfuse" word for details.
I mentioned above about two maximum currents applicable to USB 2.0. They can be 500mA or 100mA. Each hardware vendor decides what are the capabilities of the particular product's USB port. For the RasPi, the team has decided to desin the board for 100mA USB devices (keyboards, mouses, etc., including all USB devices with their own power supplies).
slackraspi wrote:I have also used another power charger (by Samsung) whose output is 5v 0.7A, but with it the printer doesn't print at all.
700mA is enough current for RasPi model A (without any attached device). I'd not supply model B with this power supply.
slackraspi wrote:Someone wrote me that it is all because the raspberry has got unreliable usb connections?
You should forward him/her a link to this message.
As mentioned above, RasPi was designed to support only 100mA USB devices.
slackraspi wrote:Can anybody help me find a solution?
People usually add powered USB hub, which then provides required current.
If your printer doesn't have a dedicated power supply, another option is to "inject" the power into the USB cable. Some people are performing back-powering of RasPi from USB, but this flow skips mentioned fuse and I don't recommend it.
What you could do is to take Asus supply and split power cord onto two paths - connect first one to the related RasPi power connector and the other one to supply the printer.
Best wishes, Ivan Zilic.
The software I want to use is CUPS and the driver is the foo2zjs downloaded from http://foo2zjs.rkkda.com/
