HP48 / HP50G Serial Hardware Ground Connections

[Steve Simpkin]

(05-12-2014 06:18 PM)TomC Wrote:  My question is - Is this practical? I realize that the HP48 uses a rather novel scheme to generate the RS232 voltages, and I am concerned with connecting the ground of the external circuit to the ground of the HP48 serial connector may cause a sneak path from the calculator B+ and cause problems.

Your concerns are valid. The "Ground" connection on the RS232 connector of an HP-48 series calculator is actually at a voltage of +4.3V relative to the calculator battery ground. Connecting them together would be bad. See page 36 of the June 1991 HP Journal for more information.
http://www.hpl.hp.com/hpjournal/pdfs/Iss...991-06.pdf

I have not seen details on the HP-50G serial port configuration.