cctalk Digest, Vol 70, Issue 3

Brent Hilpert hilpert at
Thu Jun 4 17:37:44 CDT 2009

Tony Duell wrote:
> > If there were one thing about RS232 and (smart) modems that I wish had been
> > differently, it would be that the hayes modem standard had permitted some
> > technique to switch between data mode and control mode without having those 2
> > second guard periods. That was (is) so annoying waiting for dial-up software to
> > get the modem into control mode.
> The point weas that the Smartmodem would work using only the TxD and RxD
> data lines (it didn't use the control lines, simply because you couldn't
> rely on them even existing on some RS232 interfaces) _and_ it could not
> be fooled if the special sequence of characters to go to command mode
> occured e.g. in a file download. It was assumed (I think quite
> reasonably) that you'd not get the characters _and the pauses_ in a download.

It's no mystery what the original point was.

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