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.