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Hewlett Packard HP-28C and HP-28S

• Type: Programmable RPN Electronic Calculator
• Size: 6 x 3.75 inches / 15 x 9.5 cm
• Size (open): 6 x 7.5 inches / inches / 15 x 19 cm
• Serial number:
  • 28C: 2722A00081
  • 28S: 3006A01620
• Date of Manufacture:
  • 28C: May 1987
  • 28S: January 1990

Accessories:

• Hewlett Packard 82240A Infrared Thermal Printer

Both units with box and manuals. Case considered superfluous.

This was quite a departure from the previous state of the art. In many ways, the functionality derives from the 41 series, but the physical differences are obvious. Look at that keyboard. Two sections? A hinge? (OK, OK. The left-hand keyboard sends signals over a three-line serial bus to the processor on the right-hand side. The bus lines do not bend; there are two sections that run the length of the hinge, that twist rather than bend. 180 degrees of torsion is a lot less stressful than a 180-degree bend. HP claims they've tested the hinge motion for two million repetitions without a failure. I still have a ways to go.

I bought a 28C to replace a dead Texas Instruments TI Programmable 59, but soon found myself constrained by the 2k of RAM in that unit. So I passed it along to a friend and picked up the 28S. (The present 28C is a recent eBay purchase.) The 28S featured 32k of RAM and a heirarchical system of storage, to keep your variables sorted out and happy. Both calculators featured an infrared output port to send bytes to a printer.

The calculator is programmed with a language called (internally) RPL ("Reverse Polish Lisp"), an abstracted combination of LISP and FORTH, with an arbitrarily-deep stack (keep pushing until you run out of memory!). The abstraction layer lets you push objects of many different types without the user needing to keep track of how much memory each item requires. Got a complex number? Push it on the stack. Got a matrix? Ditto. Heck, push a whole program on the stack!

Ultimately, the hinge (and the lack of expandability and I/O options) made people nervous, and HP dropped this model in favor of the HP-48 series.

A delightful range of photos of boxes, manuals and a scan of an actual sales receipt can be found at the ancillary scans page!