The HP-9810 was the successor of the HP-9100. Although it kept software compatibility, its internal hardware organisation was completely different. The core memory was replaced with volatile MOS RAM chips and the display used the new technology of 7-segment light emitting diodes (LED) instead of cathode-ray tube.
Arithmetic routines were stored in ROM and user programs in RAM. Several RAM extensions, specialized ROM modules and peripherals (paper tape reader/puncher, line printer, measuring instruments) could be connected to the basic model.
The standard version of this calculator included a magnetic card reader located directly behind the two yellow keys at the top-right in the keyboard. While a thermal printer was available as an option, it could only print numeric characters unless you had the Alphanumeric Printer ROM.
The add-on ROMs for this machine came in the form of a block that plugged into the left side of the keyboard; each had its own specialized keys. You can see the edges of the ROM block in the picture; the 3x5 array of keys at the leftmost area of the keyboard is its top. It was common to have more than one ROM extension available in a single block, since you could only plug in one at a time.
With this first generation of programmable calculators, engineers eventually became independant from main frames and computer centers. They could write programs, save, recall and print them off-line; and even easily carry the computer from a place to another... it was a true revolution at the time.
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