Serial Howto suggestions (was Re: Trouble reading from my serial device...)

[Chuck Messenger <chuckm@rochester.rr.com>]

OK, thanks for the thorough explanation, Ed -- it all makes sense now...

As far as the documentation goes -- I'm sending this to the 
maintainers/writers of the Linux Serial Howto, and the Linux Serial 
Programming Howto.  I had trouble sending to both before, so I'm 
cross-posting here in the hopes they might see it...

1) In my experience, open() does not hang if DTD is not asserted, even 
without O_NONBLOCK.  Instead, open() returns, but read() hangs instead. 
  If DTD is not asserted, and you open with O_NONBLOCK, then open() 
returns (as per the documentation), but all read()'s fail right away 
(not in the documentation).

2) It would be very helpful to point out in the documentation that you 
_must_ assert DCD -- in hardware -- or it will be impossible for you to 
operate the serial port, no matter what software flags you use.  It 
would be nice to point out the trick of attaching DTR to DCD, too.

For reference, I'm running Mandrake 9.1, with kernel 2.4.21-0.13mdk.

     - Chuck

Ed Vance wrote:
> Hi Chuck,
> 
> Yep, I'm surprised too about open. Also, in theory, the read should not have
> hanged, because you had previously set CLOCAL. Just a bug or two, here. Oh
> well. For hysterical .. oops .. historical reasons, the default termio(s)
> settings have CLOCAL off. The idea was that if you were using a modem, the
> port would hang in the open until the phone line and modem connection was
> made, as evidenced by detection of carrier tone from the remote modem. (DCD
> = carrier detect). This standard behavior has frustrated people who were
> just trying to connect local devices for many years. Some systems have a
> "soft carrier" option outside of termios that causes a port to act as if DCD
> is always asserted. 
> 
> You can accomplish the same thing by looping back DTR (pin 4) to DCD (pin
> 1). Open will assert DTR, and that will drive DCD high for you. That's what
> most folks do.
> 
> Cheers,
> Ed
...