The V1.21 firmware is a minor change from the V1.19 firmware so that the speedometer is controlled correctly for a 30-1220 EMS. If you do NOT have a 30-1220 EMS, there is no advantage of running the V1.21 firmware so we recommend sticking with the V1.19 firmware to keep things simple.
In short- DO NOT USE V1.21 unless you have a 30-1220 EMS
