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 9810 used a three line LED display that displayed the X, Y and Z registers in a manner similar to the HP 9100. Here is a good picture of a 9810 (picture) courtesy of Alex Knight. The 9810 measures 21" long x 17.75" wide x 5.75" tall. The 9810 used RPN and the standard model could hold up to 500 program steps and 51 data storage registers of memory and had a built in magnetic card reader. A built-in thermal printer was available as an option but it would only print numeric characters unless a ROM that contained the alphanumeric Printer ROM was added. The ROMs for the 9810 were in the form of a block that plugged into the left side of keyboard. Each block had it's own keys and key labels. Several blocks were available that contained more than one ROM.
