Date: Sat, 24 Feb 1996 21:15:33 +1100 From: Bruce Evans <bde@zeta.org.au> To: msmith@atrad.adelaide.edu.au, petzi@zit.th-darmstadt.de Cc: hackers@FreeBSD.org Subject: Re: 230.400 bps on serial port Message-ID: <199602241015.VAA30131@godzilla.zeta.org.au>
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>> I need a reliable data transfer rate of 230.400 bps on the serial port for >> PPP. Can that be done ? getty only has a setting for 115.200 bps. I have a >> Gigabyte 586 ATEP mainboard with FIFOs. Can I use them for 230.400 bps, and >> how ? Or should I buy a special serial card for that ? >... >Note that at the speed you suggest a 16550 UART will be generating around >15,000 interrupts per second. This will place a _serious_ load on your >system, and if you're using IDE disk, may cause you to suffer communications >problems. All wrong. - 230400 bps full speed bidirectional generates at most 23040 / 16 + 23040 / 14 = 3085 interrupts/second. - 15000 serial interrupts/second isn't a _serious_ load under FreeBSD. On a 486/33, 23040 serial interrupts/second for full speed bidirectional through a 16450 has an total (user + sys + intr) overhead overhead of about 33%. This is only a serious overhead :-). On the same system, the interrupt overhead for a slow (1.25MB/sec) IDE disk is 56% for 2500 interrupts/second, and the overhead for an 1100K/sec WD8013EBT ISA ethernet card is 21% Sys + 54% Intr for 756 interrupts/second. - IDE disks have no effect on serial communications under FreeBSD. (Bus-hogging DMA controllers may have an adverse affect if they steal too many cycles.) Bruce
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