Re: RS-232, No short No send?
From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 10/07/04
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Date: Thu, 07 Oct 2004 09:48:52 GMT
"boki" <bokiteam@ms21.hinet.net> wrote in message
news:4c3c095a.0410061709.323acf54@posting.google.com...
> "Roger Hamlett" <rogerspamignored@ttelmah.demon.co.uk> wrote in message
news:<dgV8d.591$K95.102@newsfe2-gui.ntli.net>...
> > "Boki" <bokiteam@ms21.hinet.net> wrote in message
> > news:ck14g8$bfl$1@netnews.hinet.net...
> > > Deaer Roger,
> > > Yes, I connected pin.3(TXD) to oscilloscope
signal
> > and
> > > pin.1 connected to oscilloscope ground.(didn't swap lines)
> > > but there is no signal in oscilloscope. If I short pin.2 pin3, it
works.
> > >
> > > I am confusing why it didn't send out signals when pin.2 pin.3 are
not
> > > short. ( I floting all other pins.,.except signal and ground.)
> > >
> > > Best regards,
> > > Boki.
> > OK.
> > One thought, is that if the receive input is disconnected, the code
will
> > be seing a continuous 'break'. It may be that this is disabling the
> > transmission. Try just pulling the RX line low, and then seeing if TX
> > starts. The handshake lines should have a suitable signal available
(RTS
> > or CTS).
> >
> > Best Wishes
>
> Sorry, there is no 'break' can be detected by my program, because
> I floating all other pins, I only connect TX, RX, Ground, to scope.
>
> I check it works or not by its TX and RX pins. If I short
> pin.2(TX) and pin.3(RX) then I can see the RX signal received the
> signal I send from TX( or say, there is a signal send out from TX), if
> I open TX and RX, the TX is no output signal anymore....@@ (program is
> still sending signal)
You are missing the point here. You are not talking directly to the port,
but to quite a bit of interface electronics and code in between. Some
UART's, will disable their output, if they see a 'break'. The 'break'
signal in RS232, is so called, because it corresponds to a 'broken wire',
with the incoming RX pin not being pulled to the 'idle' state. Having the
RX pin disconnected, generates this signal. I suspect your hardware, is
turning off the TX, when it sees a 'break'.
Best Wishes
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