Xref: utzoo comp.unix.internals:2542 comp.unix.programmer:1576 comp.unix.wizards:24881 comp.unix.questions:30382 Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!ucsd!ucbvax!mtxinu!ed From: ed@mtxinu.COM (Ed Gould) Newsgroups: comp.unix.internals,comp.unix.programmer,comp.unix.wizards,comp.unix.questions Subject: Re: Hardware flow control for TTY ports under V.3 Message-ID: <1991Apr14.202750.22010@mtxinu.COM> Date: 14 Apr 91 20:27:50 GMT References: <451@frcs.UUCP> Reply-To: ed@mtxinu.COM (Ed Gould) Distribution: comp Organization: mt Xinu, Berkeley Lines: 38 >There is no portable way to do hardware handshake under UNIX. The >termio(7) man page author didn't think of it (this is the only >explanation I have for that omission). There is exactly one reason that the termio(7) page doesn't describe portable hardware flow control. That reason is that it doesn't exist. The man pages document *existing* systems. They are not specifications for something that doesn't exist. There is also a reason why the termio software doesn't specify or support hardware flow control. When that software was being developed, there was a wide variety of types of serial ports available. Some had provisions, for RTS/CTS flow control, but they were by far the exception, not the rule. Many ports had no modem-control support at all, some had only carrier detect, and some had full support for the signals. However, what "full support for the signals" meant was that the *software* could look at them. This is all consistent with the long-since-adopted EIA RS232C standard (note - industry standard, not one person's idea of how it should work). RS232C (while it may be outdated) does not have the notion of flow control, per se. It knows about line turnaround for half-duplex systems. That is what RTS (request to send, sent from the DTE - in this case the computer - to the modem) and CTS (clear to send, from the DCE - modem - to the computer) were designed for. Hardware flow control came along and coopted those signals for other purposes. There still exist half-duplex systems in the world. Would you suggest that hardware manufacturers abandon the standard that supports them in favor of an ad-hoc solution that works in some situations? That's not what I think standards are for. -- Ed Gould No longer formally affiliated with, ed@mtxinu.COM and certainly not speaking for, mt Xinu. "I'll fight them as a woman, not a lady. I'll fight them as an engineer."