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SubjectRe: Serial Port voltage drops with Linux
On Mon, 17 Jan 2000, Jens Benecke wrote:

> But now we have a somewhat different problem.
> The situation is this: When we connect the hardware to the serial port, the
> signal amplitude breaks down to lower than +3V (which is miniumum for the
> PC to detect a "0"). With DOS, the signal amplitude varies from -10..+10 V
> (approx) for 1/0 signals. With Linux, it's -10..<+3V, and that is not
> enough.

It is possible, but highly unlikely, that the UART has been left in
a strange loop-back mode. In particular, the DLAB (Divisor latch access
bit) ONLY must be set when the baud-rate divisor is set (on some UARTs),
or the UART will be left in a strange state once DLAB is reset.

Maybe Theodore Ts'o could check. I see in the code where UART_LCR_DLAB
is ORed with 'cval'. This could cause a problem.

Dick Johnson

Penguin : Linux version 2.3.39 on an i686 machine (800.63 BogoMips).

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