Date: Sun, 11 May 1997 12:49:35 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: tom@sdf.com (Tom Samplonius) Cc: brian@awfulhak.org, luigi@labinfo.iet.unipi.it, hackers@FreeBSD.ORG Subject: Re: sio flow control problems with 2.2? Message-ID: <199705111949.MAA06176@phaeton.artisoft.com> In-Reply-To: <Pine.NEB.3.94.970511115255.22294A-100000@misery.sdf.com> from "Tom Samplonius" at May 11, 97 11:56:05 am
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> ... > > 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.
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