The HP 9810, 9820 and 9830 were all announced simultaneously by HP in the December 1972 HP Journal (picture). The 9810, 9820 and 9830 were all four bit machines and all of them used a serial bus internally.
The 9820 used a formula oriented language called HPL (Hewlett Packard Language). HPL was HP's version of the then widely popular APL language. HPL allowed subroutine nesting, flags and multi-dimensional arrays of up to the size of the system memory. But the HP 9820 had very limited capability unless optional ROMs were added. For example, a Mathematics ROM had to be added in order to added in order to perform Sine, Cosine, and Tangent functions. There were three ROM slots in this model and they were located on the top left hand side just above the display. There were three groups of keys on the left hand side of the keyboard that were dedicated for use with the ROMs. Each ROM came with an overlay (picture) that showed the functions assigned to each key. The standard 9820 model came with 173 registers of memory, a 32 character LED display, a built in 16 character thermal printer and a magnetic card reader.
