New Project wp34s micro usb flash cable revisited

[matthiaspaul]

I was thinking about designing a USB to TTL serial converter to be build into the calculator myself, but with the Micro-B USB receptable (and IR LED) facing to the back of the calculator, not its side. This way, the cable gets into the way less frequently. (Compare this with the cableing of a computer mouse.)

While USB is convenient, I still have alot of RS232-based equipment as well, and therefore would have to fit in a serial port as well. In order to save space I was tinkering with the idea of multiplexing the 3.3V TTL RX/TX signals with the USB D+/D- lines, so that, by default, the calculator would route the RX/TX signals to the connector and reassign them to D+/D- only if the USB host provides 5V via VBUS. While this would continue to allow to power the calculator via USB it would rule out the possibility to power the calculator via an external RS-232 level translator. The difficult part would be to multiplex the signals while drawing zero power from the calculator itself (which is no option as it is powered by coin cells only).

The alternative to build in a "smart" interface would be to just add a connector providing RX, TX, GND and VCC (3.3V), but in this case I would rather use a female 2.5 mm 4-conductor TRRS connector instead of a USB connector. The drawback would be that a multitude of different pinouts exist for such connectors, so some protection would have to be included in order to avoid the calculator being bricked when plugging in the wrong cable by accident.

Does someone know if the calculator's RX and TX signals are ESD protected already?

Greetings,

Matthias