The PR-40 is a 5 by 7 dot matrix printer. That means it can print five dots across by 7 dots high...basically creating each character using different combinations of 35 possible dots (see example pictures). The printer could print a basic 64 character ASCII set, which includes 26 letters (upper case only), 10 numbers, and 28 special characters like punctuation or equals sign.

Instead of 8 1/2 x 11 inch paper that you might be used to, the PR-40 printed on a roll of paper that was a standard 3.875 inch wide and was commonly used by adding machines at the time and was readily available.

The printer was popular because it was compatible with many of the early computer systems of the 1970's including but not limited to the SWTPC 6800, the Altair 8800, and even the Apple 1.

In October of 1976, Steve Jobs wrote about connecting the Apple 1 computer to this printer. It was a good fit because the Apple 1 output to a video screen was exactly 40 characters...same as the PR-40.