Path: utzoo!attcan!uunet!auspex!guy From: guy@auspex.auspex.com (Guy Harris) Newsgroups: comp.dcom.lans Subject: Re: RTS/CTS flow control Message-ID: <1786@auspex.auspex.com> Date: 8 Jun 89 20:55:16 GMT References: <5137@charon.unm.edu> <5187@b11.ingr.com> <1989Jun6.163942.15568@utzoo.uucp> <32466@bu-cs.BU.EDU> Reply-To: guy@auspex.auspex.com (Guy Harris) Organization: Auspex Systems, Santa Clara Lines: 35 > In the context of full duplex interfaces RTS/CTS is hardware >based flow control Well, it depends on the interface. I could easily imagine a system where the, well, *hardware* didn't support "full-duplex RTS/CTS flow control" (by which I assume people mean "if CTS is high, you may send, otherwise you may not send; if you're ready to receive, raise RTS, otherwise lower RTS - CTS and RTS cross over in the connection between the machines") but the software did. None of the serial port interfaces with which I've worked support full-duplex RTS/CTS flow control in hardware; for instance, the Z8530 SCC chip used by Sun's CPU serial ports and ALM-2 board, and the Signetics 2661 EPCI used on the Systech MTI boards, do not. They support the form of "flow control" to which Henry was referring, and which was not, as I understand it, intended as flow control but was intended as a way to control a half-duplex modem (but which is, in practice, often used for flow control). I am told that AT&T has a serial port chip that supports full-duplex RTS/CTS flow control; the person who told me that no longer remembers how it figures out that the host is incapable of receiving any more characters, and therefore that it should drop RTS. (I hope nobody's using "hardware flow control" to simply mean "out-of-band flow control", as opposed to "in-band" flow control such as XON/XOFF flow control. Said usage is quite unfortunate, as out-of-band flow control is not necessarily done in hardware (and sometimes can't be done in hardware, cf. the above chips and full-duplex RTS/CTS flow control), and, depending on your definition of "hardware", in-band flow control can be done in hardware (consider a "smart" serial port board that does it for you, and doesn't require the host's driver to do so. A better term for "out-of-band flow control" is "out-of-band flow control".)