Adding Wired Serial Communication
For this expansion, I was recommended the following products from Pololu.
Begin by soldering the 1x6 header onto the programmer. We use the PGM03A AVR programmer from Pololu. If you wish, you can use masking tape to hold the header in place while you solder, but be careful not to melt or burn it. The image below of the programmer's underside shows where to solder the header.
Next, solder the 1x2 header onto the PD0 and PD1 pins of the 3pi. They are located near the center of the base. The image below of the 3pi's underside shows where to solder the header.
Lastly, use two male-male jumper wires to connect the TX and RX lines from the programmer to the PD0 and PD1 pins of the 3pi. If you wish, tape the wires together or label them so you don't inadvertently connect the wrong pins.
When communicating via wired serial, you'll need to plug your programmer's ISP into the 3pi as well as the serial jumper wires. Now you're all set up for wired serial communication.