From owner-freebsd-hackers Sun May 11 12:56:46 1997 Return-Path: Received: (from root@localhost) by hub.freebsd.org (8.8.5/8.8.5) id MAA10791 for hackers-outgoing; Sun, 11 May 1997 12:56:46 -0700 (PDT) Received: from phaeton.artisoft.com (phaeton.Artisoft.COM [198.17.250.50]) by hub.freebsd.org (8.8.5/8.8.5) with SMTP id MAA10782 for ; Sun, 11 May 1997 12:56:42 -0700 (PDT) Received: (from terry@localhost) by phaeton.artisoft.com (8.6.11/8.6.9) id MAA06176; Sun, 11 May 1997 12:49:35 -0700 From: Terry Lambert Message-Id: <199705111949.MAA06176@phaeton.artisoft.com> Subject: Re: sio flow control problems with 2.2? To: tom@sdf.com (Tom Samplonius) Date: Sun, 11 May 1997 12:49:35 -0700 (MST) Cc: brian@awfulhak.org, luigi@labinfo.iet.unipi.it, hackers@FreeBSD.ORG In-Reply-To: from "Tom Samplonius" at May 11, 97 11:56:05 am X-Mailer: ELM [version 2.4 PL24] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-hackers@FreeBSD.ORG X-Loop: FreeBSD.org Precedence: bulk > ... > > It's theoretically impossible to give the modem too much data - > > the next character is only sent on receipt of an interrupt (maybe > > you've got something else causing interrupts ?) that says the > > transmit buffer is empty. > > Not quite. Generally you drive a modem at 115200 bps, while the line > rate is only 28800 bps (plus some extra for compression). When the > modem's internal buffer fills up, it asserts flow control. If the > computer doesn't see it, it will keep pumping data to the modem at 115200, > which the modem will likely discard. This is why there are stone tablets in a cave on Mt. Ararat which (loosely translated) say: "Thou shalt not set the port speed higher than the committed data rate for the communications device unless thine flow control is active and trustworthy". I think it's the one immediately after: "Thou shalt not use in-band flow control unless thou art a blithering idiot or a slow witted VT3xx or VT4xx series terminal from DEC". And immediately before: "Thou shalt use the RS232C standard specified external clock for thine strange-ass baud rates, lest though be judged to be non-compliant with the strict definition of RS232C". NB: Most modem and UART manufacturers are evil heretics. Sorry if I'm mis-quoting, it's been several years since my UNIX serial I/O pilgrimage to this holy site... Regards, Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers.