External power and usb become hot

Hello to everyone, I'm trying to control a 3-axis board with parallel interface using an arduino 2009. I report here all my steps: 1- connect directly arduino (powered by usb) to board. In this way, usb become hot because the board is protected by opto-isolator 2- Use an external supply to control the board but even in this case the usb cable become hot 3- Remove Vcc from usb cable but even in this way the cable become hot 4- use only data + and data - of usb cable but in this way windows don't see arduino

To protect microprocessor I putted a buffer between board and pins. Waiting your answer. Greeting, Fabio

If it gets hot quite simply you are trying to draw too much current from it.

Your description of your setup is not very clear. Are you trying to run it all off the USB cable?

usb become hot because the board is protected by opto-isolator

That is simply a nonsense, it does not become hot BECAUSE it has opto isolators.

You need to provide a diagram of your setup complete with where the power is coming from for each component, along with links to your various components.

Not 100% sure of what gets hot is it the atmega16u2 or the cable or the connector? Are you sure its not the voltage regulator thats getting hot?

There is a voltage in somewhere on the 3 axis board try disconnecting it from the arduino (bend the pin) then power it externaly.

Usb cable become hot ever. Thusing external powers is very strange because if I use an external power all current should getted by it.
I talked about opto-isolator because they need 60mA to work and they are the cause for the high current request. They are used to protect arduino for 3axis board.
To avoid to destroy atmega 328 for great current output requested, I putted a chip from arduino output to 3 axis board input. (74ls540)

My question is:
Why does the cable become hot using an external power? ( I removed the vcc from usb cable however it becomes hot, the hot cables, inside usb cable, are gnd and the external cable shield)

Describe your external power supply, and where it is connected. To isolate the usb power from the 5v bus, there must be a minimum of 6.7 volts on Vin (preferably 7 to 9 volts) with a current capability of 500ma.

60ma is a little high try using a 4n25 I drive them with as little as 9ma.

If the USB cable is hot is in the ground wire take your dmm and test the cable sheild (while hooked to the computer) ageanst the jack on the arduino while powered externally. See if there is a difference in voltage.

I talked about opto-isolator because they need 60mA to work and they are the cause for the high current request.

That will overload the output of your Arduino. The Arduino must not supply more than 40mA.

Do these optoisolators have a series resistor or are they just raw diodes?

You need a way to limit the current, 60mA sound way too high, are you sure you are not confusing the specification.

Now if you want real help and stop being a time waster then please answer the question I asked in reply #1.

In case you have forgotten I said:-

You need to provide a diagram of your setup complete with where the power is coming from for each component, along with links to your various components.