STS3215 servo control table

A general reference for the Feetech STS3215 serial bus servo's memory map — every SO-101 joint uses one of these. This is the generic register set (same for any STS3215), not a live reading from a specific arm; addresses and field meanings below are the ones soarm_sdk actually reads and writes (soarm_sdk.protocol.registers), cross-referenced against Feetech's published memory table.

The memory is split into two regions with very different write semantics:

• EEPROM — persists across power cycles. Writable only while Lock (addr 55, in SRAM) is set to 0; writing through EEPROM wear-levels far fewer times than SRAM, so these are meant to be set once at bench-configuration time, not every control tick.
• SRAM — volatile, resets to firmware defaults on power-cycle. This is where soarm_sdk's control loop writes goal position every tick and reads present position/speed/load back.

EEPROM — read-only

Addr	Size	Name	Meaning
3–4	2	Model Number	Servo model ID, set at the factory

EEPROM — read/write

Addr	Size	Name	Range / units	Meaning
5	1	ID	0–253 (254 = broadcast)	Servo bus address. soarm_sdk.bus.discovery scans this range to find connected servos
6	1	Baud Rate	code 0–7	See baud rate codes below
9–10	2	Min Angle Limit	ticks, 0–4095	Software-enforced lower position bound, firmware-side
11–12	2	Max Angle Limit	ticks, 0–4095	Software-enforced upper position bound, firmware-side. Both limits are read by soarm_sdk.ServoHardwareInterface.read_angle_limits() and are the ultimate authority on how far a joint can move — a planner or calibration file can believe a wider range, but the servo will silently refuse to go past this one (see Troubleshooting § A joint silently stops moving)
21	1	P Coefficient	0–254, default ~32	Position-loop proportional gain
22	1	D Coefficient	0–254, default ~32	Position-loop derivative gain
23	1	I Coefficient	0–254, default 0	Position-loop integral gain
26	1	CW Dead Band	ticks	Minimum error (clockwise) before the servo reacts — a deadzone against jitter
27	1	CCW Dead Band	ticks	Same, counter-clockwise
31–32	2	Homing Offset (OFS)	signed, sign-magnitude (bit 11 = sign)	Applied by firmware before any host ever sees a tick: reported = raw − OFS. This is what "zeroing" a joint actually writes
33	1	Mode	0 or 1	0 = position/servo mode (closed-loop to Goal Position), 1 = wheel mode (continuous rotation, used during ROM sweeps to find hard stops)

SRAM — read/write

Addr	Size	Name	Range / units	Meaning
40	1	Torque Enable	0 / 1	0 = free-spinning (backdrivable), 1 = holding/driving position. Defaults off on every soarm_sdk connection (torque_on_start=False) — nothing enables it without an explicit command
41	1	Acceleration	0–254	Motion-profile acceleration for the next Goal Position move
42–43	2	Goal Position	ticks, 0–4095	Target position in position mode
44–45	2	Goal Time	ms	Optional move duration (alternative to speed-based profiles)
46–47	2	Goal Speed	ticks/s, signed	Target speed — in wheel mode, this is the command; in position mode, it caps the move's velocity
55	1	Lock	0 / 1	0 = EEPROM unlocked (writable), 1 = locked. soarm_sdk unlocks immediately before an EEPROM write and re-locks after, so a servo spends almost all its time locked

SRAM — read-only (live telemetry)

Addr	Size	Name	Range / units	Meaning
56–57	2	Present Position	ticks, sign-magnitude (bit 15 = sign)	Current measured position
58–59	2	Present Speed	ticks/s, sign-magnitude (bit 15 = sign)	Current measured speed
60–61	2	Present Load	sign-magnitude (bit 10 = sign), magnitude 0–1000	PWM duty cycle applied, in units of 0.1% — a proxy for how hard the servo is pushing, not a true torque/current sensor reading
62	1	Present Voltage	0.1 V / LSB	Bus voltage at the servo
63	1	Present Temperature	°C	Internal temperature
65	1	Status	bitfield	Active-alarm bitfield (per Feetech's status-byte convention: voltage, sensor/angle, temperature, current, overload flags)
66	1	Moving	0 / 1	Whether the servo is currently executing a move
69–70	2	Present Current	sign-magnitude (bit 15 = sign), 6.5 mA / LSB	Measured bus current draw

Addresses 56–70 form one contiguous block (soarm_sdk's STS_TELEMETRY_START/STS_TELEMETRY_LENGTH, with addresses 64/67/68 as unused gaps read-and-discarded) — this lets a single GroupSyncRead pull all of a joint's live telemetry in one bus transaction instead of six separate reads. See soarm_sdk usage § GroupSyncRead for bulk state reads for the pattern.

Baud rate codes

Code	Baud
0	1,000,000
1	500,000
2	250,000
3	128,000
4	115,200
5	76,800
6	57,600
7	38,400

soarm_sdk defaults to 1 Mbps (code 0) — confirm both the controller board and every servo on the bus agree, since some USB-serial adapters top out below 1 Mbps (see Troubleshooting § Servo bus / hardware issues).

Sign-magnitude registers

Position, speed, and current are sign-magnitude, not two's-complement: the sign lives in a single high bit (bit 15) and the rest is a plain magnitude. Present Load uses the same convention but with the sign at bit 10, since its magnitude only needs 10 bits (0–1000, in 0.1% steps). Decoding these requires masking out the sign bit and negating separately — you cannot just cast the raw register to a signed integer.

Where this is read/written in soarm_sdk

• soarm_sdk.protocol.registers — the address constants themselves (source of every address above)
• soarm_sdk.protocol.sts — the typed read/write methods (ReadPosition, ReadStatus, IsMoving, …)
• soarm_sdk.calibration.recentre — writes Homing Offset to re-zero a joint
• soarm_sdk.calibration.rom_sweep — switches Mode to wheel mode to find hard stops
• soarm_sdk.calibration.widen_limit — writes Min/Max Angle Limit after empirically finding the true mechanical stop (see Troubleshooting)
• The dashboard's Monitor → Servo Inspector panel reads the full telemetry block per servo on demand — see Bringup & Dashboard