The new PX4 Pixhawk module is an evolvement of the existing FMU and IO modules and completely compatible. The main difference is the target audience: While the FMU and IO stack is super small (the size of an average 8 ch RC receiver) but in some ways almost too densely packed, Pixhawk has more space, more serial ports and more PWM outputs.
As the above picture shows, there are two groups of servo connectors, one main group of 8 outputs which are wired through the backup processor, and an auxiliary group of 6 outputs directly wired to the main processor. The port labeled "RC" can take normal PPM sum or Futaba S.Bus inputs, the port labeled "SB" can read RSSI our output S.Bus to servos. A Spektrum satellite compatible port is on top (labeled SPKT/DSM).
The basic operation is the same, and the software is shared. Inside Pixhawk a FMUv2 and an IOv2 do their duties on a single board (and developers will find that the software will refer to FMUv2 and IOv2)
The main differences between old and new are:
- 14 PWM outputs vs. 12 PWM (old)
- All PWM outputs on servo connectors (old: 8 on servo, 4 on DF13)
- 5 serial ports vs. 4 (with some double functionality, so only 3 in some configurations on old version)
- 256 KB RAM and 2 MB flash vs 192 KB RAM and 1 MB flash (old)
- Modernized sensor suite (latest generation)
- High-power buzzer driver (old: VBAT driven, not as loud)
- High-power multicolor led (old: only external BlinkM support)
- Support for panel-mounted USB extension (old: not present)
- Revised, improved power architecture
- Better protection on all input / output pins against shorts and over voltage
- Better sensing of power rails (internal and external, e.g. servo voltage)
- Support for Spektrum Satellite pairing (needed some manual wiring work in v1, but also software-supported)
- No more solid state relays on v2 (was not really used)
- Connectors easier to disconnect in case, as the surrounding plastic helps to place the fingers correctly (more on this in a separate post)
- Case prevents one-off failure operation of servo connectors
- The new unit is consirably larger, has the same height, but offers in general more handling convenience.
- External power supply similar to existing 3DR power brick (every unit comes with a free module)
Both generations offer the same backup / override processor that allows failover to manual if the autopilot fails in fixed wing setups. For software developers the differences are nicely abstracted in the PX4 middleware, and can be sensed / configured at runtime.