The styling looked like vaguely like a toaster oven. It was boxy, with a "widescreen" aspect ratio. HP had determined that the combination of a standard 4:3 aspect ratio with the 25 line by 80 character display that was the standard of the time required the characters to have a very high profile. HP's response was to specify a CRT with an aspect ratio designed around the desired character shape instead of the other way around. Of course, this also mandated rather high manufacturing costs as standard parts could not be used.
HP took pains to further improve the rendering of displayed characters via half-pixel positioning of individual lines within each character. Although the character cell was only 7 horizontal by 9 vertical dots, half-pixel positioning effectively doubled the horizontal resolution to 14 dots , giving the characters very smooth outlines. (The initial sales literature referred to it as using a 7x9 matrix generated in a 9x15 dot character cell).
All of this resulted in an extremely easy to read display with the dot-matrix nature, and the scan lines, almost invisible.
The keyboard had flat tops, similar to the HP 9800 series desktop computers rather than the curved contours now considered to be ergonomic. It featured three keypad areas: Alphabetic, numeric, and an array of cursor positioning and editing keys somewhat similar to modern PC keyboard layouts. There were also a number of smaller function and feature control keys arrayed in two rows above the normal keypad areas. The keyboard chassis was separate from the main body, connected via a thick cable. The keyboard used a bit-paired layout (similar to that on a Teletype machine) rather than the typewriter-paired arrangement on DEC's VT100. Although large, users loved the keyboard because "it had a key for everything".
Similar to the HP desktop computers, it had a number of F-keys (F1 through F8) placed close to the screen. Paper templates were available for some application programs to which placed legends for these keys on the keyboard. Later models arranged these across the top row, and provided for screen labels close to their respective keys. Terminal configuration in the 262X series was done entirely through the screen labeled function keys rather than dedicated keys and through escape sequences sent from the host computer. The on-screen labelling of the eight function keys, pioneered by the HP 300 ("Amigo") computer, was one of the first applications of a hierarchical menu which allows accessing many functions with a small number of keys. This arrangement is now common on TI graphic calculators, and automated teller and gas pump machines, though no longer used in GUI user interfaces.
Internally, the electronics used a motherboard with plug-in daughter cards. The microprocessor, memory, serial interface card, and various optional functions were each on separate cards. This permitted easy field maintenance, upgrades, and reconfiguration. For example, more memory (providing larger scrollback capability) could be easily added, the serial interface could be changed from RS-232 to current loop, etc. The optional tape drives of the 2645 model were interfaced via another plug-in card. The plug-in card capability strongly resembled the later Apple-II expansion architecture.
The manufacturing area was across from R&D cubicles in the Data Terminals Division in Cupertino. The testing area was dubbed "beepland" because it had racks of 500 terminals, with the end of the test ending in a beep.
The HP 2640 introduced "block mode", similar to the IBM 3270 (although the IBM 3270 did not work for ASCII standard serial communications). The escape sequences Esc-[and Esc-] defined unprotected areas, but it didn't have to take up a visible space. It acted much like a web page, disconnected from the host until the SEND key was pressed. The fields could screen for alphabetic or numeric characters, a feature beyond Windows Forms today. This would be supported by programs such as DEL/3000 and VIEW/3000 which would map form data into runtime variables and databases. It also supported teletype character mode like a standard ASCII terminal, and did not need specialized communications like IBM.
The hardware was radically different from most "dumb" terminals in that the characters were not stored in a simple data array. To save memory, which could extend over several pages, characters were allocated as linked lists of blocks which were dynamically allocated. Display enhancements were encoded as embedded bytes in the stream. Software enhancements which did not affect the appearance such as dim or underline, but protected and unprotected fields were also coded with embedded bytes. The display hardware was capable of reading this unusual data structure. When the cost of memory came down by the 262X series, this was changed to a "parallel" structure with one bit for each enhancement code, but the logic required to emulate previous behaviors was complex. Inserting a code for underline would "propagate" to the next display enhancement, while deleting such a code would also have to be propagated to the next display byte. This data structure would inspire the sparse matrix data structure for the Twin spreadsheet.
Users learned to use the offline key to take the terminal offline, edit a line in the display buffer, and then retransmit it. This gave the effect of command line recall and editing even if the operating system did not support it. For example, when working at an operating system's command prompt, an erroneous command could quickly be corrected and re-sent without having to retype the entire line. This was possible in many terminals of the day, but the HP 2640 was smart enough to only retransmit the line from the first character typed by the user, omitting, for example, the operating system's command prompt. This was later implemented as "line mode".
In-house developers ported TinyBASIC to the HP 2645A, as well as developing several games in assembler (most notably "Keep On Drivin'", Tennis and Reversi).
The great over-reach was a color graphics terminal that cost more than the HP 2647 monochrome graphics workstation that sold very few units but cost a huge effort to develop.
Eventually, HP ended up selling essentially a low-cost version of the HP 2640. Today, many terminal emulators by companies such as Attachmate still implement the late 1970s feature set of these terminals on common PCs.