5.3) What should I know about using fast modems with the Atari?

While Atari never marketed a modem faster than the 1200 bit/s SX212, the Atari
is capable of supporting modem bitrates of up to 19200 bit/s, depending upon
circumstances described here.

Several factors come into play in using modems faster than 2400 bit/s with the
Atari.

Firstly, the top bitrate supported by a given software application on the
Atari can be the determinative limiting factor.  Many programs on the Atari
may not support, or may not be able to keep up with, bitrates greater than
1200 bit/s or 2400 bit/s.

Assuming the use of an Atari program that supports higher speeds, it will be
useful to have an understanding of data flow control.  Here is a definition of
flow control from: http://preview.tinyurl.com/p8muzmp

Often, one modem in a connection is capable of sending data much faster than
the other can receive.  Flow control allows the receiving modem to tell the
other to pause while it catches up.  Flow control exists as either software
(XON/XOFF) flow control, or hardware (RTS/CTS) flow control.  With software
flow control, when a modem needs to tell the other to pause, it sends a
certain character, usually Control-S.  When it is ready to resume, it sends a
different character, such as Control-Q.  Software flow control's only
advantage is that it can use a serial cable with only three wires.  Since
software flow control regulates transmissions by sending certain characters,
line noise could generate the character commanding a pause, thus hanging the
transfer until the proper character (such as Control-Q) is sent.  Also,
binary files must never be sent using software flow control, as binary files
can contain the control characters.  Hardware, or RTS/CTS, flow control uses
wires in the modem cable or, in the case of internal modems, hardware in the
modem.  This is faster and much more reliable than software flow control.

Some/later 2400 bit/s modems, and probably all modems with 9600 bit/s speed
capabilities and up, normally use V.42 standard error correction and V.42bis
standard data compression.  Subjectively, V.42/V.42bis are nice at 2400 bit/s,
important at 9600 bit/s, and essential at any speeds beyond 9600 bit/s.

V.42bis requires hardware flow control (and V.42 error correction).  But with
Atari equipment (except the MIO and Black Box) hardware flow control is not
supported so V.42bis cannot be used and should be disabled.  Standard Hayes
modem command to disable V.42bis data compression: AT&C0

While V.42 error correction can technically work with either software or
hardware flow control, for reasons described above it is typically only used
when hardware flow control is available.  Since Atari equipment (except the
MIO and Black Box) does not support hardware flow control, V.42 should
generally be disabled.  Standard Hayes modem command to disable V.42 error
correction: AT&K0
Note that disabling V.42 also has the effect of disabling V.42bis.

Finally, hardware ports on the Atari have their inherent top bitrate limits.
The serial ports of the Atari 850, for example, support a top bitrate of 9600
bit/s.  Other modem interfaces for the Atari may support bitrates of up to
19200 bit/s.

Clay Halliwell offers a tip on achieving 9600 bit/s through the 850 Interface:
On 11 Feb 1996, Marc G. Frank said:

> I'm having problems getting a modem attached to my Atari 850 to
> communicate at 9600 baud.  When I set my communications program to 2400
> baud, everything works fine.  However, when I set it to 9600 baud, the
> modem echoes my characters but doesn't act on them.

The problem with the 850 is that some of them (like mine) don't produce a
PERFECT 9600 baud signal.  As a result modems can't train on it, and while
they will echo characters back, for some nitpicky reason they won't pick up
on the "AT" attention code.

The solution is to do all your dialing at 2400 baud, but set the S37
register to force the modem to try to connect at 9600.  Then switch your
Atari to 9600 after connecting.
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