APC Modbus word-order probe test - tests EchterAgo's CANCEL|ON|OFF detection method

bug4-probe-test.py

#!/usr/bin/env python3
"""Test EchterAgo's probe idea: write CANCEL|OUTPUT_ON|OUTPUT_OFF to OutletCommand_BF.

On spec-compliant firmware: EMBXILVAL (invalid command combo detected)  
On LE-buggy firmware: different error or no error (combo lands in reserved bits)

Must run with NUT driver stopped: systemctl stop nut-driver@apc
"""
import sys

try:
    from pymodbus.client import ModbusSerialClient
except ImportError:
    from pymodbus.client.sync import ModbusSerialClient

SERIAL_PORT = "/dev/ttyS0"
BAUDRATE = 9600
SLAVE_ID = 1

REG_OUTLET_CMD = 1538  # OutletCommand_BF (2 regs)

# Bit definitions from apc_modbus.h
CMD_CANCEL     = 1 << 0   # 0x0001
CMD_OUTPUT_ON  = 1 << 1   # 0x0002
CMD_OUTPUT_OFF = 1 << 2   # 0x0004

# Invalid combo - multiple commands at once
INVALID_COMBO = CMD_CANCEL | CMD_OUTPUT_ON | CMD_OUTPUT_OFF  # 0x0007

print(f"Probe value: 0x{INVALID_COMBO:04x}")
print()

client = ModbusSerialClient(
    port=SERIAL_PORT,
    baudrate=BAUDRATE,
    bytesize=8,
    parity='N',
    stopbits=1,
    timeout=3
)

if not client.connect():
    print("ERROR: Could not connect to serial port")
    sys.exit(1)

print("Connected to", SERIAL_PORT)
print()

# Test 1: BE order (NUT's current behavior)
# reg 1538 = 0x0000 (empty), reg 1539 = 0x0007 (combo)
print("=== Test 1: BE order [0x0000, 0x0007] (NUT's current code) ===")
result = client.write_registers(REG_OUTLET_CMD, [0x0000, 0x0007], slave=SLAVE_ID)
if result.isError():
    print(f"  Result: ERROR - {result}")
else:
    print(f"  Result: SUCCESS (no error)")
print()

# Test 2: LE order (the "fix" order) 
# reg 1538 = 0x0007 (combo), reg 1539 = 0x0000 (empty)
print("=== Test 2: LE order [0x0007, 0x0000] (reversed) ===")
result = client.write_registers(REG_OUTLET_CMD, [0x0007, 0x0000], slave=SLAVE_ID)
if result.isError():
    print(f"  Result: ERROR - {result}")
else:
    print(f"  Result: SUCCESS (no error)")
print()

client.close()
print("Done. Restart driver: systemctl start nut-driver@apc")