HP48G Series Keyboard Hardware

[Steve Simpkin]

The June 1991 issue of HP Journal magazine discusses the HP-48SX keyboard PCB briefly starting on page 31. Some of the key characteristics are:

"The HP 48SX contains two printed circuit boards. The main logic board is sandwiched between the topcase assembly and the bottom case. The Mylar domed keyboard with carbon graphite traces is housed in the topcase assembly."

"All key lines are isolated from the main system address lines by built-in 4-kΩ carbon graphite resistors."

Page 26 also mentions:
"The keyboard is made of Mylar with tuned resistance carbon graphite traces. This allows a low profile, reduces cost, and improves reliability because of the inertness of graphite."

Page 29 includes additional details:
"The keyboard lines are multiplexed with the address lines. This scheme reduces the pin count on the IC and saves printed circuit board area, both of which lower costs.However, it requires that key line resistance be carefully controlled. The exact sheet resistance of the carbon ink was measured. This information was used to hand-tune the area of each of the 98 key line traces."

A PDF of this issue is available at:
https://www.hpl.hp.com/hpjournal/pdfs/Is...991-06.pdf