From: Andrei Chichak (acpmiedm_at_telusplanet.net)
Date: Fri Mar 26 2004 - 11:58:03 PST
>Well, there's a simple serial device that is as "compatible" and "universal"
>as possible: a modem. Did we _ever_ had to configure _the modem_ to
>connect to a computer using a serial port? You simply
>plug it in, turn it on, configure all serial settings from _the computer_
>and do _nothing_ to configure the modem itself (except init strings
>for performance). Do you know why?
>
>Well, it's just because the modem _can_ sense the serial line and react
>correctly to the changes. OK, it's not a printer, but I guess it serves
>to the simpler purpose of illustrating my point of view... ;)
I'll call BS on this one. A modem is not a modem is not a modem. Take an
RS232 modem and try to plug it into a PCMCIA slot or into an X25 cable or
onto the bus of a Motorola MC68332. It won't go.
All that a modem does is modulate and demodulate. The external physical
connections are a design decision. The conversation between the
host/modem/modem/terminal is not set by standards. The AT command set was
invented by a defunct company called Hayes. You can send AT commands to
sierra wireless CDPD modems but they are not the Hayes commands. You can
send AT commands to embedded modems that implement the TCP/IP stack, once
again they are not the Hayes commands.
RS-232 was defined as a 25 pin serial connection. Most PCs/Macs/Modems do
not implement a great deal of RS-232. A PC cannot handle the external baud
rate clocks that synchronous modems can generate. But it's all RS-232.
A modem cannot necessarily sense the serial line. That is something known
as autobaud, I haven't seen an implementation of that since the 80s. If you
are assuming that a modem is a PCI card, that is a completely different
ball of wax altogether.
If you take a modem to be a parallel to serial converter with an audio
converter following that, you have a better view of what a modem is. If you
take a modem to be made by 3Com and it makes your computer talk on the
phone, you are assuming a huge number of things.
Andrei (don't get me started on WinModems)
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