A MODulator/DEModulator translates digital information from your computer into
acoustic tones that can be sent and received, from modem to modem, via
standard telephone lines.

Atari produced several modems for use with the 8-bit Atari computers:

Atari 830 Acoustic Modem:    ( = Novation 'CAT' )
- a stand-alone, acoustically coupled, frequency shift keying (FSK) modem
- up to 300 bits per second
- Bell 103/113 modem compatible
- requires Atari 850 Interface Module or equivalent

Atari 835 Direct Connect Modem:   (by Racal-Vadic?)
- 300 bps
- Bell 103/113 modem compatible
- pulse dialing
- 2 SIO ports
- packaged with the TeleLink II cartridge

Atari 1030 Modem with ModemLink Telecommunications Program:  (by Penril)
- 300 bps
- Bell 103/113 modem compatible
- built-in ModemLink software.
- 2 SIO ports

Atari XM301 Modem:
- 300 bps
- Bell 103/113 modem compatible
- with XE Term disk software (developed by Russ Wetmore for Atari)
- permanent SIO cable, must be at end of SIO chain
- draws its power from the computer via SIO

Atari SX212 Modem:
- SIO & DB25 RS-232 serial ports, must be at end of SIO chain
- 1200 baud
- Bell 103/113/212A modem compatible
- SX Express! disk software (developed by Keith Ledbetter for Atari) to be
  sold separately, packaged with an SIO cable.
- Key engineer/designer: Jose Valdes at Atari