The ADM-3A was one of the first computer terminals manufactured by Lear Siegler, first produced in 1975.^{[citation needed]} It had a 12 inch screen displaying 12 or 24 lines of 80 characters.

Originally priced at $1195, a DIY kit later sold for $995. The original model, the ADM-3, only displayed capital letters; this was quickly supplanted by the more advanced version with both lower and upper case. Versions of the terminal were available where the display showed only twelve (rather than twenty-four) rows of eighty characters—in those days,^{[citation needed]} RAM was expensive, and halving the display size halved the RAM requirement (and likewise all uppercase required only six bits per character to be stored rather than seven). Further optional add-ons included a graphics card enabling it to emulate a Tektronix 4014^{[citation needed]} and an extension port which would allow daisy chaining several ADM-3As on a single RS-232 line.^{[citation needed]}

The ADM-3A's overall setup was controlled by 32 DIP switches under the nameplate at the front of the machine, beside the keyboard, including speed from 75 to 19,200 baud. The advanced configuration options allowed split speed connection, sending at one rate, and receiving at another.