HP joined the microcomputer craze in 1979 with their Series 80s computers. These were usual and actually more like an followup to their calculators. They utilized a custom 8-bit processor made by HP. It had nifty non-standard features. For one thing lots of registers (64 of them) and the system was designed to work with BCD just like a calculator. It supported natively 12 digits with exponents to 499 (IIRC). The first release (HP 85) used a built in tape drive. Again very unusual. When diskette support was released, they chose HP-IB (GP-IB or IEEE-488 nowdays) instrument bus.
This example was picked up at an University of Kentuck Surplus auction. It is an HP 86B with 3 memory cartridges and a ROM expansion cartridge. Hence its total RAM is 448K. Not bad for the early 80s. Too bad it only ran HP series 80s software. Also it would not run the HP 85 binaries! The HP series 80s computers were to some extent incompatible with each other. The series was discontinued in 1984.
EDIT: I believe the only thing I have used it for was as a calculator.
Monitor was bought at Goodwill
Memory expansion was by cartridges like many other systems of the time.
ROM expansion used this carrier with places for individual chips. Very much aimed at the engineer market.