HP-48 FRAM Cards

[matthiaspaul]

Unfortunately, external power via USB is impossible in the HP48, as the card connector does not carry the necessary supply signal. It has two unused pins (card pins 38 and 39), so it would be possible to modify the calculator to connect this to "VBAT" (battery "+" and CPU pin 145), but I guess most users wouldn't want to open their calculator for this.

Unfortunately, I have no information about the internals of the Yorke chip, in particular the circuitry between "VBAT" (CPU pin 145), "VH/V+" (CPU pin 130), "VDD/VCC" (CPU pin 125) and "VCO/VCC-ON" (CPU pin 126).

It very much depends on the internals, but there is a slight chance that it is possible and electrical safe to either reverse-power the calculator through VCO/VCC-ON (which is available on the card connector pin 1) or through the 4-pin serial connector (which is connected to VDD/VCC via 75R, so that a cable could be connected between the USB-powered expansion card and the serial connector), but unless I know for sure, I won't try, as it could easily damage the calculator as well. However, it is possible that someone has already investigated this in the past...

Another important question to answer is how much current can be drawn by an expansion card through VCO/VCC-ON. Does someone know? Is this documented somewhere?

Greetings,

Matthias