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+--- Thread: HP48 / HP50G Serial Hardware Ground Connections (/thread-1298.html)
HP48 / HP50G Serial Hardware Ground Connections - TomC - 05-12-2014 06:18 PM
I would like to connect an external circuit to an HP48 (or HP50G) via the serial port. I would also like to power this external circuit from the AAA cells of the calculator (I know, this will be tedious, but a can connect to the batteries and route the wires external to the calculator). Current drain will be minimal.
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.
Also, am not aware of the serial communications circuit in the HP50G but do have a 'EricRechlin' HP50G cable that I can use. Is there a HP50G schematic available?
Thank you in advance for helpful advice,
TomC
RE: HP48 / HP50G Serial Hardware Ground Connections - Steve Simpkin - 05-12-2014 07:25 PM
(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/IssuePDFs/1991-06.pdf
I have not seen details on the HP-50G serial port configuration.