sync_read does not detect partial motor failures (missing isAvailable check)

[Escenda]

Problem

sync_read does not check isAvailable() before calling getData(), making it impossible to detect partial motor failures in a sync read operation.

Current Behavior

In motors_bus.py line 1118:

values = {id_: self.sync_reader.getData(id_, addr, length) for id_ in motor_ids}

When a motor fails to respond during sync_read:

• The overall comm result may still be COMM_SUCCESS if other motors responded
• getData() returns 0 for non-responding motors
• This 0 is indistinguishable from a valid position value of 0

Expected Behavior

The implementation should check isAvailable() before calling getData():

values = {}
for id_ in motor_ids:
    if self.sync_reader.isAvailable(id_, addr, length):
        values[id_] = self.sync_reader.getData(id_, addr, length)
    else:
        values[id_] = None  # or raise, or handle differently

Use Case

When building real-time motor position streaming (e.g., for calibration UI), we need to:

1. Detect which motors are responding
2. Show errors for disconnected/failed motors
3. Continue operating with the motors that are still working

Currently, we cannot reliably detect partial failures.

Proposed Solution

1. Add isAvailable() check in _sync_read()
2. Return None or a sentinel value for motors that didn't respond
3. Or add a new parameter like partial_failure_mode to control behavior

Related Code

• src/lerobot/motors/motors_bus.py - _sync_read() method (line 1095-1119)
• scservo_sdk GroupSyncRead.isAvailable() - available but not used