I have 2 arduino Uno R3. when I plug them in to the computer. The first one (lets call it A) only has yellow light near the pin 13 blinking. It does not have any green light at all. The second one (called B) only has green light on, it does not have any blinking yellow light at all. And this B arduino board is not recognized by the IDE (port gray out under tool tab).
Are they both defective?
Thank you very much
Do either pass the Loopback Test (see top of Installation & Troubleshooting forum)?
It's possible they have programs on them already - first one sounds like Blink, 2nd one who knows.
You say that B is not detected by the computer. What about A?
Press reset button on the board - see if you get a series of three blinks. If you do, the '328 is working and bootloaded. I suspect that both of the boards will pass this test.
Also, look for hte largest chip near the USB port - it'll either be an atmel atmega 16u2 (for official, and faithful clones), FTDI FT232, or WCH CH340G. If it's something other than the 16u2, those are the drivers you need (google finds them).
Do the boards show up in device manager? Do you get the "something was connected" sound when you plug them in?