Date: Sat, 21 Jun 1997 14:05:10 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: louie@TransSys.COM (Louis A. Mamakos) Cc: terry@lambert.org, adam@veda.is, joerg_wunsch@uriah.heep.sax.de, freebsd-current@FreeBSD.ORG Subject: Re: getty modem control Message-ID: <199706212105.OAA27805@phaeton.artisoft.com> In-Reply-To: <199706211830.OAA22108@whizzo.TransSys.COM> from "Louis A. Mamakos" at Jun 21, 97 02:30:55 pm
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> > Connect speed is evil. The data rate should be driven with the > > RS232C external clock for modems (described in the RS232C, Bell 103C, > > and Bell 212 standards). The negotiation for speed should be done > > at the carrier recognition, and not between the UART on the modem > > and the UART in the computer. > > Yes connect speed is evil. The data rate should be fixed at some > higher rate, and you should use CTS/RTS "hardware" flow control. It's > unclean and non-EIA standard, but the only thing which actually works for > contemporary modems which use RS232, and link-level compression and > reliability. You can't reliably run the computer-modem rate at a higher rate that the modem-modem rate. Conversely, you can't reliably run the modem-modem rate at a higher rate than the computer-modem rate. This is because of issues of bufferring, and when common UART chips trigger interrupts relative to their FIFO size. The last modem you could reliably run at differential speeds was the original MNP Microcom modems; they had huge buffers. Link-level compression is evil anyway; compression belongs on the host side of the host UART so that the datarate is not limited by the serial port rate. > > This is an evil perpetrated on honest, God-fearing people by Intel > > and National Semiconductor. > > You can't blame Intel for *this* particular evil. Sure I can! If they had ripped off the Zilog design, there wouldn't be a problem. 8-). > The folks that got this stuff right was Telebit. Their original modems > had two serial interfaces on the connector, so you could chat with the > modem out-of-band to configure and query it. The original Hayes modems had external clocking. It's not necessary to go sync to utilize an external clock. 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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