or anything specific to one company), but early/economy models often supported only capital letters (such as the original ADM-3, the Data General model 6052 – which could be upgraded to a 6053 with a lower-case character ROM – and the Heathkit H9) RS-232 serial ports (25-pin, ready to connect to a modem, yet some manufacturer-specific pin usage extended the standard, e.g. for use with 20-mA current loops) 24 lines (or possibly 25 – sometimes a special status line) of 72 or 80 characters of text (80 was the same as IBM punched cards). Later models sometimes had two character-width settings. Some type of cursor that can be positioned (with arrow keys or "home" and other direct cursor address setting codes). Implementation of at least 3 control codes: Carriage Return (Ctrl-M), Line-Feed (Ctrl-J), and Bell (Ctrl-G), but usually many more, such as escape sequences to provide underlining, dim or reverse-video character highlighting, and especially to clear the display and position the cursor. The experimental era of serial VDUs culminated with the VT100 in 1978. By the early 1980s, there were dozens of manufacturers of terminals, including Lear-Siegler, ADDS, Data General, DEC, Hazeltine Corporation, Heath/Zenith, Hewlett-Packard, IBM, TeleVideo, Volker-Craig, and Wyse, many of which had incompatible command sequences (although many used the early ADM-3 as a starting point). The great variations in the control codes between makers gave rise to software that identified and grouped terminal types so the system software would correctly display input forms using the appropriate control codes; In Unix-like systems the termcap or terminfo files, the stty utility, and the TERM environment variable would be used; in Data General's Business BASIC software, for example, at login-time a sequence of codes were sent to the terminal to try to read the cursor's position or the 25th line's contents using a sequence of different manufacturer's control code sequences, and the terminal-generated response would determine a single-digit number (such as 6 for Data General Dasher terminals, 4 for ADM 3A/5/11/12 terminals, 0 or 2 for TTYs with no special features) that would be available to programs to say which set of codes to use. The great majority of terminals were monochrome, manufacturers variously offering green, white or amber and sometimes blue screen phosphors. (Amber was claimed to reduce eye strain). Terminals with modest color capability were also available but not widely used; for example, a color version of the popular Wyse