Mark S

The USB converter cable I used has the PL2303HX chip built into it, and has the following signals:      Black wire -- GND      White wire -- RXD      Green wire -- TXD Note: the Red wire is +5 volts, but leave it unconnected.

Sorry for necroposting, but I am just beginning to dabble with using a Pico and came across your question.  Perhaps someone else might be in the same boat and needs a cable.  The programming port uses TTL transmit and receive signal levels, and a plain vanilla (no MAX232 device) USB converter for less than ten bucks works just fine.  If you want to get fancy, use a 1.27 mm pitch header plug for the three wires.