Extension register layouts

Block reference for protocol v9.0. Offsets start at the beginning of each block; they are not separate Legacy register addresses. Read each complete block at its exact length. See packet format and startup order separately.

An offset is the starting byte of a field, counting from zero within its block. Offsets below are decimal; selectors use hexadecimal notation (0x...). All writes use instruction 0x91; reads use 0x90 with the exact byte count.

Selector	Block	Access	Bytes
0x90	Capabilities	Read	8
0xA0	Configuration	Read/write	24
0xB8	Controller selection and session setup	Write	3
0xC1	Operational feedback	Read	32
0xC2	Tuning feedback	Read	54

Capabilities — 0x90

Offset	Type	Value / meaning
0–3	4 bytes	ASCII ARMC
4	u8	Major version: 9
5	u8	Minor version: 0
6	u8	Reserved: 0
7	u8	Normal controller update interval: 1 ms

The protocol version defines the available blocks and their exact lengths. Selection and session setup share one write-only block, documented below.

Configuration — 0xA0

Offset	Type	Field	Units / valid range
0	u16	kp_q8	Gain × 256; 0–65535
2	u16	kd_q8	Gain × 256; 0–65535
4	u16	ki_q8	Gain × 256; 0–65535
6	u16	integral_limit	Effort; 0–effort_limit
8	u16	effort_limit	Effort; 1–1000
10	u16	velocity_limit	Counts/s; 1–65535
12	u16	acceleration_limit	Counts/s²; 1–30000
14	u8	velocity_filter_q8	Speed-filter response; 1–255, higher responds faster
15	u8	ramp_time_ms	Acceleration ramp; 1–255 ms
16	u16	velocity_ff.positive_gain_q8	Output/(count/s) × 256; 0–65535
18	u16	velocity_ff.negative_gain_q8	Output/(count/s) × 256; 0–65535
20	u16	velocity_ff.effort_limit	Effort; 0–main effort_limit
22	u16	gravity_slew	Compensation change, effort units/s; 1–10000

Gains are sent as the desired value multiplied by 256. Their units are output/count for P, output/(count/s) for D, and output/(count·s) for I. For example, P = 4.5 produces 4.5 output units per count of error and is sent as 1152.

The servo has one active configuration. Writing it requires motor power off and a disarmed session. Every write replaces the entire configuration, including feedforward settings. Invalid settings reject the entire write without changing anything. Read 0xA0 to check the settings that were accepted.

Velocity-feedforward gains are nonnegative. A zero feedforward output limit requires both gains to be zero. The same feedforward output limit applies in both directions and must not exceed the main output limit.

After a reset, configure the servo before enabling motor power. Keep saved settings and alternative tuning values on the host; change gains between runs with the arm supported and motor power off.

Selection and session setup — 0xB8

Offset	Type	Meaning
0	u8	Mode: 0 legacy, 1 extension
1	u16	Session: 0 for legacy, 1–65535 for extension

The exact payload length is 3 bytes, with the session in little-endian order. For example, extension mode with session 0x1234 is 01 34 12; legacy mode is 00 00 00. Invalid combinations reject the whole write without changing state. Both modes require motor power off and replace any earlier session setup. Selection alone does not start control or capture a holding position.

Legacy torque-enable register 40 starts the prepared extension session when written to 1. It requires available motor output, a valid configuration, initialized sensors with readings no more than 2 ms old, no fault requiring shutdown, and usable saved legacy position limits. The measured position must be within those limits when they are enabled. These checks happen at enable time, so movement or faults between setup and enable are taken into account.

Torque-off cancels both running control and unused setup. Sensor failures, protection shutdown, position-limit or encoder-offset changes, calibration and saving permanent settings also invalidate session setup. Read feedback to check whether control has actually started; there is no separate ARM command. The former 0xBA selector and former one-byte selection payload are rejected.

Operational feedback — 0xC1

Offset	Type	Field / units
0	u32	Measurement time, ms since startup; returns to zero after 4294967295
4	u16	Measurement number; returns to zero after 65535
6	u16	Session
8	u16	Accepted target sequence
10	u16	Accepted gravity sequence
12	u16	Flags, below
14	i32	Measured position, Q8 counts
18	i32	Filtered measured velocity, Q8 counts/s
22	i16	Applied output; positive follows increasing encoder counts
24	i16	Current-sensor reading relative to its baseline; not amperes
26	u8	Supply voltage, 0.1 V units
27	u8	Validated temperature, °C
28	u8	Legacy servo fault/status flags
29	u8	Reserved: zero
30	u16	Measurement age at read time, ms; maximum reported value 65535

Flag bit	Meaning when set
0	Sample is fresh and usable for current arm control state
1	Controller active
2	Reserved: zero
3	Motor output limited
4	Arm drive enabled
5	Session started by torque enable
6	Reserved: zero
7	Output ready
8	A gravity command has been received
9	Reserved: zero
10	Validated temperature available
11	Reserved: zero
12	Motion target active
13	Reserved: zero
14	Arm controller selected
15	Reserved: zero

Each flag is one bit: bit 0 has value 1, bit 1 has value 2, bit 2 has value 4, and so on. Several flags can be set at once. An active controller with bit 12 clear is holding its destination. A clear motion bit alone does not mean the motor is powered or ready.

Feedback describes a recorded measurement. If the servo’s state changes before the host reads it, some fields may describe the earlier state. Bit 0 is cleared when the measurement is over 2 ms old, the controller is no longer selected, armed, driving, or ready, or its session/sequence numbers no longer match. Check bit 0, age, session, and accepted sequence together.

Position and health readings are also available in legacy mode and with motor power off. Unavailable control fields are zero. The sample sequence advances when measurements are acquired, not each time the host reads them. It distinguishes samples that share the same millisecond timestamp. Unexpected large temperature changes are checked across several readings before appearing here; see the protection limitation below.

Tuning feedback — 0xC2

Bytes 0–31 have exactly the 0xC1 layout. The remaining bytes are:

Offset	Type	Field / units
32	i32	Reference position, Q8 counts
36	i32	Reference velocity, Q8 counts/s
40	i16	Requested output after its configured limit, before motor protection
42	i16	P contribution, effort
44	i16	D contribution, effort
46	i16	Integral contribution, effort
48	i16	Gravity compensation currently applied, effort
50	i16	Local velocity feedforward, effort
52	u16	Accumulated extra controller delay, ms; maximum 65535

The delay counter adds the extra time whenever a controller update takes more than the normal 1 ms (elapsed_ms − 1). It does not count lost messages. Reported P and D contributions are limited to ±32767, so adding the displayed values may not reproduce the full internal calculation.

The servo sends feedback only when asked. Use 0xC1 for normal monitoring and 0xC2 when tuning needs the additional values. Reading 0xC2 already includes all of 0xC1, so reading both unnecessarily adds traffic. Separate reads can refer to different moments; compare their timestamps and sample numbers.

Interpretation rules

effort is a signed PWM output command, not amperes or measured torque. currentAdcDelta is a relative current-sensor reading, not physical current without a separate conversion. Q8 position and velocity have different units: dividing by 256 removes the fixed-point scale but does not convert counts into degrees. Joint angles require a physical mapping.

Extension temperature feedback checks suppress isolated jumps. Motor temperature protection still uses raw readings: absence of a jump in C1/C2 does not prove that it could not affect protection.

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Documentation home · Source revision: a995f77. Extension 9.0.