Evolution of the HP 48gII

[Eric Rechlin]

It's been too many years so my memory may be wrong, but I think it may have been a combination of the two. The port probably put out TTL voltages, and the special cable had a level shifter in it, but the level shifter needed power from the remote device to run, meaning anything that didn't provide power wouldn't be able to interface with the 48gII.

Contrast that to the serial connector on the later 48gII and 50g, which provided its own power to run a level shifter in the cable.