My WP-34S : USB to Serial cable prototype

[Marcus von Cube]

My setup is the following:

• The original cable by HP with the blob and two push buttons
• An FTDI USB to 3.3V TTL serial
• External 3V power supply
  

The problem with the original design is that it uses the calculator 3V VBatt supply on the connector to feed the level shifters in the blob. This draws power even while the calculator is turned off. Add to this the SAM-BA boot loader which runs at full processor speed without any user visible sign of life and you know why this is a battery killer setup.

So I removed the electronic guts in the blob and just left the switches connected. They connect directly to the pins in the calculator without any electronics involved. Sometimes things are simpler than anticipated...

The FTDI cable is connected inside the blob to the RX, TX and GND pins on the calculator end. The power supply is connected to GND and VBatt, all inside the blob. Before I connect a calculator to this setup, I just remove the batteries.

MySamba runs in a virtual XP setup on my iMac. The virtual machine "owns" the FTDI USB device. There is a small setup program available at FTDI which allows to modify the characteristics of the FTDI chip in the cable. I used it to change the USB identification string to "HP Calculator Programming Cable". This makes it easier to identify the USB device to attach to the virtual machine.