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SubjectRe: serial.c (half duplex support)
On Thu, 21 Oct 1999 wrote:

> Date: Wed, 20 Oct 1999 18:36:16 +0100 (GMT)
> From: Riley Williams <rhw@MemAlpha.CX>
> Ted, I know you're against adding variables to the trees,
> but to me, this sounds like some sort of "Enable half duplex serial
> support" variable would be the best way to go, as that would provide
> the best of both ways:
> 1. If (as it normally would be) the kernel is compiled with
> the said option set to "n", then the serial driver would
> remain as it currently is.
> 2. For those (like Kees) who need it, the kernel could be
> compiled with the said option set to "y". In this case,
> it would define an additional pair of routines to handle
> the switching from send to receive and back again.
> Since these additional routines are neither defined nor called in the
> first case, it would be possible for those needing half duplex support
> to develop it without impacting on those who don't need it.
> Given that in my experience almost everyone who turns this on is going
> to need to modify serial.c to make the half-duplex support routines work
> correctly given the vagrencies of their half-duplex devices --- which
> remember are NOT standardized --- I don't see the point of adding the
> variable in the tree.
> You're going to have to muck with serial.c after the fact anyway, and I
> don't want to deal with the support questions from clueless users that
> try to turn on the feature, watch it fail, and then send me mail
> complaining about the fact. Sorry, but that's a support nightmare that
> I'd rather not have to deal with....

I suggest serious 1/2 duplex users use a bit in the printer port to
do the switching anyway, rather than using a modem control line. It
has low delay and therefore does not usually clip off data when

In any event, you need to know when the last character was trasmitted
before you switch this bit. I modified the TX-routine to switch the
bit when the UART shift-register is empty. When any TX characters are
to be sent, the bit is set on. When the TX shift register is empty
(last byte has been sent), it is set off.

No other modifications are necessary -- and the serial port continues
to work as usual for regular RS-232C.

Dick Johnson

Penguin : Linux version 2.3.13 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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