Context
The PRD ruled out "multi-axis synchronisation, EtherCAT-class determinism". Managed switches change the cost. IEEE 802.1AS (gPTP) gives sub-microsecond time across a switched network if the switches support it, and most single-pair automotive switches do. 10BASE-T1S PLCA already bounds medium-access latency on a multidrop segment.
MotorState carries position and velocity but no time base shared with other nodes. Fusing an IMU with wheel odometry, or correlating a current spike with a load-cell step, needs one clock.
Recommendation
Add a synchronised timestamp field to every SPACE message now, filled from the node's local clock until gPTP exists. Make gPTP a requirement on zone switches. Scheduled traffic (802.1Qbv) stays out of scope.
Consequences
- An API version bump in
rammp-rtps. - Zone switch selection (ADR-0012) requires 802.1AS.
- Nodes without hardware timestamping get millisecond-class accuracy from software PTP, which is fine for telemetry and too coarse for control.
Questions to settle
- Is there any control loop that crosses the network and needs a shared clock (both wheels in lockstep, for example), or is this telemetry only?