The printer matched the footprint of the Atari 810 disk drive so that it could be stacked on top of one if the user wanted.  The bottom of the 820 case is actually the top/bottom case of the Atari 810. Atari wisely designed the case so it could be used to make the entire Atari 810 case, but also be used as the bottom of the Atari 820 printer as well.

The printer was a 40 column printer and used ink Ribbons.   Its print head would impact against the ribbon and form the letters in ink on the paper roll.   The print could be very useful for printing out simple program listings or used for an Accounting program, calculator or other adding program.

It had a built in power supply and 2 Atari SIO bus connectors on the back to allow it to connect to the Atari 400/800 computers without the need of any kind of interface.  It would also allow other devices such as disk drives to plug into and "Daisy Chain" out of the back of it so more devices could be added onto the Atari computers.