Inside the HP 85

The HP 85 provides a built-in interpreted BASIC programming environment, an alphanumeric CRT display, a thermal dot matrix printer, and a magnetic tape drive. The machine is implemented using a general-purpose LSI microprocessor along with a number of LSI support chips. Base RAM memory was 16K-bytes, which is enough for writing reasonably good sized programs, implemented with eight 16Kx1 dynamic RAM chips. The machine can be used as a fully algebraic (with precedence rules) calculator by simply entering the expression to be solved and pressing the "END LINE" key. The result is immediately calculated and displayed. The interpreter has quite a compliment of built-in math functions, including trig, logarithms, square root, integerize, random number generation, and more.

CRT Display of HP 85

The display on the 85 is a 5-inch diagonal CRT that can display up to 16 lines of 32 characters each. The character set is made up of a 5X7 dot array, with upper and lower-case letters defined in the character set. The display is a 'live' display, meaning that information on the display can be 're-entered' by positioning the cursor on the line and pressing the "END LINE" key. This makes program editing convenient, as a section of code can be listed on the display, and edited in place. The display has a buffer that holds 48 lines (three screenfull's worth) of display, and a "ROLL" key can scroll the view through the buffer.

The Built-in Thermal Printer

Given that the HP 85 was primarily developed for use as a computer, being able to log the output of programs (as well as being able to print out listings) was an important consideration. To meet this need, the HP 85 provides a built-in 32-character per line thermal dot matrix printer. The printer is quite fast, printing full lines at about 1 line per second. It is also very quiet. The printer has a single 5X7 dot-matrix thermal printhead that prints characters at a time as the printhead is moved across the paper. The printer prints bidirectionally, eliminating the need to reposition the printhead to the beginning of a line, saving time (and reducing noise). The content of the screen can be printed verbatim with a single keypress, convenient for capturing program output from the CRT. Of course, programs can be written to directly output to the printer.

The 85 also provides a digital magnetic cartridge tape drive for storage of programs and data files. A single tape cartridge can hold 210K bytes of data, and provides a 'filesystem' interface to the user, with files having names rather than just being file numbers like on older machines. For canned applications, on power up, the system checks to see if a tape is in the tape drive, and if so, it looks for a file on the tape called 'AUTOST', and if it exists, the program in that file is automatically loaded and executed. BASIC program statements exist to create, delete, and read and write data to/from files on the tape drive, as well as allowing program overlays and chaining to occur from the tape drive. These operations were not terribly fast, as the data transfer rate to/from the tape drive was only about 650 bytes per second.

The Expansion Slots on the Back Panel of the HP 85

The HP 85 is a tremendously expandable machine. By this time, interfacing external devices to computing systems had become more standardized, with standards like GPIB, RS-232, Centronics parallel, and others becoming much more wide-spread. Interfaces to connect to just about anything were available for the machine, including a modem that allowed the calculator to be used as a terminal, a speech synthesizer, GPIB and HP-IB interfaces for controlling instrumentation, and of course, all manner of printers, plotters, tape drives, floppy disc, and even hard disk drives. Another module that was available was a "ROM drawer" that would plug into one of the expansion slots. In the ROM drawer, up to six ROM modules that expand the capabilities of the BASIC interpreter could be installed. ROM options included an Advanced Programming ROM, a Matrix ROM, Mass Storage ROM (added support for external disk devices), as well as a machine language debugger and an assembler to allow program development at the 'machine' level rather than working through BASIC.

For much more information on the HP 85 and other HP calculators, Dave Hicks' Museum of HP Calculators provides a wealth of detailed and interesting information.