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5.20 Why does an unused serial port consume cpu?

Perhaps you've got a (probably fairly long) unshielded serial cable attached to it, with either nothing at the other end or a powered-off device at the other end. EE's call this an antenna. It's probably picking up most of the radio stations in your area, which the serial chip is interpreting as a continuous stream of garbage bytes, which it feeds to getty, which tries to interpret them as login attempts.

How do you avoid this problem?

• leave the device at the other end switched on (even when it's not transmitting, it will assert a voltage that overrides the noise)
• unplug the cable from the next when you're not using it
• use 'kill -STOP' & 'kill -CONT' to stop and resume the getty process as needed
• buy an adequately shielded serial cable

• (continue with 5.21 How to adjust MegaPixel Display brightness and focus?)