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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, 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 and an extension port which would allow daisy chaining several ADM-3As on a single RS-232 line.
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.
Compared to the ADM-3, the ADM-3A was also much more controllable. It supported control codes to move the cursor around on the screen, and directly position the cursor at any point in the display. It did not, however, support “clear to end of line” or “clear to end of screen”, or other more advanced codes that appeared in later terminals, such as the VT52 and VT100.
The 5×7 dot matrix characters were displayed in amber, green, or white phosphor on black (the cursor was 7×9). The keyboard had 59 keys. The 12-inch monochrome CRT was mounted in the top half of the clamshell case, and was typically made by Ball Brothers.
Unlike later terminals, such as the VT100, the ADM-3A did not use a microprocessor in its implementation, but instead used TTL. It did, however, use RAM chips, rather than the Circulating Memory used by earlier terminals, such as the Datapoint 3300.
The shorthand notation of using the tilde character for the "Home" directory in the C shell, bash and other command shells on Unix and Unix-derived systems derives from the ADM-3A's having the tilde symbol and the word "Home" on the same key, and it was the dominant terminal in the Computer Science and Engineering department at the University of California, Berkeley when Bill Joy originally wrote the C shell.
Both the arrows and the "home" correspond to the functions of the corresponding control characters Ctrl-H, Ctrl-J, Ctrl-K, Ctrl-L, and Ctrl-^ (identical to Ctrl-~) when sent to the terminal, moving the cursor left, down, up, right, and to the "home" position in the upper left-hand corner of the terminal, respectively. (The Ctrl-H and Ctrl-J functions were standard, but the interpretations of Ctrl-K, Ctrl-L, and Ctrl-^ were unique to the ADM-3A.)
Finally, the control key was located above, not below, the shift key—in the same place where most modern PC keyboards put the Caps Lock key. Many standard Unix key combinations were designed with the QWERTY layout and the ADM-3A's original Ctrl key placement in mind. Many of those key combinations are still in use today, even on non-Unix operating systems. Seasoned computer users familiar with the original layout often claim that the different position of the Ctrl key on modern PC keyboard layouts makes the use of Ctrl key combinations more cumbersome. Solutions exist for many operating systems to switch around the Caps Lock and Ctrl keys in software, thus making the PC keyboard layout more closely resemble the ADM-3A's keyboard layout.
- ADM-3A maintenance manual.
- Tenth Anniversary ADM 3A Dumb Terminal Video Display Terminal User's Reference Manual, p. 1-5 (13 of 54).
- Very early PC keyboards also had the Ctrl key located above the shift key, just like the ADM-3A's keyboard.
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