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Re: Problems with serial port communication (voltage level of serial port)
From: Cubic <email@example.com>
Date: Wednesday, January 17, 2001 12:54
Subject: Problems with serial port communication (voltage level of serial
>I have a program which runs very fine on pc and which compiles and
>executes aswell on the axis board.
>The problem is when I write on the /dev/ttyS0 it's okay and I can see
>that something is transmitted from the device which is connected to
>ttyS0, but the select on the filedescriptor when reading in my program
>doesn't get anything.
>So my question is what voltage level is used for the serial port of the
>axis board, is it the same as on pc ports ???
The COM ports uses drivers that comply with the 232 standard,
but they are driven by 3.3V and has a voltage trippler,
so the resulting voltage is slightly lower then what other 232
drivers might generate (should still be within the spec though).
I haven't seen this kind of problem before.
>What other problem could you think off ?
It might be a software problem if you don't open/configure the port
right, and the tty holds the data until a newline or something.
Try using the cfmakeraw function (see 'man cfmakeraw') if you don't
You can also try the hwtestserial program.
>Another program with another serial device connected to the serial port
>works on the axis board, but when this program runs connected to the
>"problem device" it's the same stuff. I can say this because this device
>returns an echo, everything which is written should come back, but I
>can't read anything, something should happen, the only thing I get is a
>To my mind this all sound to a voltage level problem of the serial
Hope this helps