SPPro/docs/PROTOCOL.md

SP PRO serial protocol

This documents the Selectronic SP PRO serial "memory" protocol as implemented in src/sppro.c. It is a port of the open-source Python project neerolyte/selpi (memory/*.py), cross-checked against the Go project angus-g/splink-influx.

Physical layer

Setting	Value
Interface	RS-232 (not RS-485), DB9 or RJ45 jack on the SP PRO
Baud	57600 (Port 1; Port 2 defaults to 9600)
Framing	8 data bits, no parity, 1 stop bit (8N1)
Flow control	none

See HARDWARE.md for pinout and ESP32 wiring.

Frames

All multi-byte fields are little-endian. A frame is built from an 8-byte header:

byte 0      message type: 'Q' (0x51) query/read, or 'W' (0x57) write
byte 1      word count minus one (0x00 = 1 word, 0xff = 256 words)
bytes 2..5  memory address (uint32, little-endian)
bytes 6..7  CRC-16 over bytes 0..5  (little-endian)

Query (read)

Request is just the 8-byte header. The response echoes those 8 bytes, appends the requested data (words * 2 bytes), then a final CRC-16 over the whole response:

request :  Q | len-1 | addr[4] | crc[2]
response:  Q | len-1 | addr[4] | crc[2] | data[words*2] | crc[2]

Worked example (documented in selpi) — read 1 word at 0xa000:

request :  51 00 00 a0 00 00 9d 4b
response:  51 00 00 a0 00 00 9d 4b 01 00 d8 19   -> data word = 0x0001

Write

Header (with its own CRC over bytes 0..5), then the data words, then a CRC-16 over the whole message. The device echoes the entire request back to confirm:

request :  W | len-1 | addr[4] | crc[2] | data[words*2] | crc[2]
response:  <exact echo of the request>

CRC-16

Reflected CRC-CCITT ("Kermit"): 256-entry table, init 0x0000, crc = (crc >> 8) ^ table[(crc ^ byte) & 0xff], emitted little-endian. A received frame is valid when the CRC computed over the entire message (data + trailing CRC) equals 0. Implemented as sppro_crc16().

Login (MD5 challenge / response)

Reading data registers requires authenticating first:

1. Query 8 words (16 bytes) at 0x1F0000 → challenge seed.
2. Build md5( seed[16] + password_padded_to_32_bytes ). The password is the SP PRO's serial-port password (blank if none); it is padded/truncated to 32 bytes with spaces.
3. Swap each adjacent byte pair of the 16-byte digest (a Selectronic endianness quirk) → 16-byte response.
4. Write the 16-byte response back to 0x1F0000.
5. Query 1 word at 0x1F0010; value 1 means authenticated.

Pure helper: sppro_login_response(). Full handshake: sppro_session_login().

Scaling

Raw register words are integers; engineering units come from per-device scale factors read once at startup from six consecutive words at 41000:

Address	Name
41000	CommonScaleForAcVolts
41001	CommonScaleForAcCurrent
41002	CommonScaleForDcVolts
41003	CommonScaleForDcCurrent
41004	CommonScaleForTemperature
41005	CommonScaleForInternalVoltages

With MAGIC = 32768.0 (from selpi memory/converter.py):

Conversion	Formula
ac_w	raw * AcVolts * AcCurrent / (MAGIC * 800)
ac_w_signed	raw * AcVolts * AcCurrent / (MAGIC * 100)
ac_wh	raw * 24 * AcVolts * AcCurrent / (MAGIC * 100)
dc_w	raw * DcVolts * DcCurrent / (MAGIC * 100)
dc_wh	raw * 24 * DcVolts * DcCurrent / (MAGIC * 100)
dc_v	raw * DcVolts / (MAGIC * 10)
temperature	raw * Temperature / MAGIC
percent	raw / 256
shunt_name	enum → string (sppro_shunt_name)

Register map

The curated subset shipped in SPPRO_REGISTERS (transcribed from selpi memory/variable.py):

Name	Address	Type	Conversion	Units
BatteryVolts	0xa05c	u16	dc_v	V
BatteryTemperature	0xa03c	u16	temperature	C
DCBatteryPower	0xa02f	s32	dc_w	W
BattSocPercent	41089	u16	percent	%
LoadAcPower	41107	u32	ac_w	W
CombinedKacoAcPowerHiRes	0xa3a8	u32	ac_w	W (AC solar)
ACGeneratorPower	41098	s16	ac_w_signed	W
Shunt1Power / Shunt2Power	0xa088 / 0xa089	s16	dc_w	W
Shunt1Name / Shunt2Name	0xc109 / 0xc10a	s16	shunt_name	—
DCkWhInToday / DCkWhOutToday	41135 / 41137	u32	dc_wh	Wh
BattInkWhTotalAcc / BattOutkWhTotalAcc	41354 / 41381	u32	dc_wh	Wh
ACLoadkWhTotalAcc	41438	u32	ac_wh	Wh
TotalKacokWhTotalAcc	41519	u32	ac_wh	Wh

selpi's memory/variable.py lists many more accumulators (7/30/365-day, yearly, resettable); add rows to SPPRO_REGISTERS as needed — the table is the single source of truth and decoding is data-driven.

Credits

• neerolyte/selpi — primary reference (MIT-style, see repo).
• angus-g/splink-influx — independent confirmation.