Addressing on the servo bus

The idea

On the SO-101 arm, the Feetech STS3215 servos share a half-duplex serial bus: a single data line, and the servos take turns to talk. Each servo has an ID stored in its EEPROM, which survives without power.

The packet

FF FF | ID | LEN | INST | parameters | CHK

• LEN = number of parameters + 2.
• CHK = bitwise NOT of the sum ID + LEN + INST + parameters (low byte).

Example: PING to servo 1 → FF FF 01 02 01 FB, because ~(1 + 2 + 1) = 0xFB.

Rules

• Every servo hears every packet; only the one with that ID replies.
• ID 254 = broadcast: all of them execute it and none replies.
• Valid IDs are 0 to 253; in practice the limit is set by the power supply and the bus speed.
• A new servo ships with ID 1. Two servos with the same ID reply at once and collide; that’s why you connect only one at a time when configuring them.

In the demo

Tick “two servos with ID 1” to see the collision, try the broadcast and an ID that nobody has (timeout).

Common questions

Why half-duplex? It saves wires: one line for everyone. The trade-off is that they can’t talk at the same time.