The first revision of the brushless servo control board for SMMB was successful in getting a leg to jump.  I ended up doing a small-run second revision that addressed a few minor problems and added a couple more capabilities.

• RS422 Debug/Link Port: I had a 3.3V serial port exposed previously for debugging, however it caused my USB-serial converter to dislike itself due to common mode ground shifts and it wasn’t reliable at high baud rates (>3Mbps).  I also wanted to support “linked” modes, where two servos would perform control in the actuator space at full rate.
• Debug through holes: r1 had a number of debug connections, all of which were unpopulated SMD pads.  I decided that through holes were easier to connect debug wires to.
• Vertical SWD connector: I had initially thought I would hide the SWD connector within an enclosure.  However, the initial enclosure prototypes made that seem less desirable, so I switched it to vertical.
• More debugging points: When bringing up the first board, I ended up doing a lot of carefully balancing scope probes on various pins, when there was plenty of board room to just have through hole debug points.  Lesson learned.
• FET temperature sensing: r1 just had an external temperature sensor port, r2 additionally has a thermistor next to the FETS.

Macrofab’s current pricing scheme provides a great incentive to keep your BOM below 20 parts, as that is the only way to get quick turn service.  Otherwise you pay an extra 2 or 3 weeks of calendar time.  In r1, I went to some lengths to stay under 20, however, it just wasn’t going to work with r2, so I left a few easy-ish or non-critical parts unpopulated to do them myself: the connectors, LEDs, and one really big diode.