Server resetter from Waveshare Ethernet Relay module

Originally published on 2025-12-11, 1 min read, ~230 words.

Just something quick here. I had a need to create an emergency reset for servers in a rack cabinet. The same rack I made a long-lasting UPS setup for. Needing it done promptly, I simply bought a Waveshare POE ETH Relay adapter. It is an 8-channel relay module one can control via Modbus TCP. The relays’ outputs were connected to the RESET headers on the motherboards, in parallel with the main reset buttons.

Cabling

I bought a box of so-called DuPont or Berg connectors. I didn’t have a crimping tool, so I simply soldered them. In my setup, I needed four cables, one for each RESET header. The relays are SPDT; connect the cables to normally open pins. There are four relays left unused; I might connect them to the POWER header if the need arises.

Waveshare POE ETH Relay with wires attached.

In parallel with the motherboard's main reset button.

Python program

The relay module was configured in accordance with its documentation. In my setup, it works in a separate management VLAN. Warning! This module doesn’t have any authentication on Modbus TCP.

The sample code provided by Waveshare doesn’t work correctly. For some reason, the module requires a CRC16 checksum to be sent with each command, despite Modbus over TCP not requiring it.

I created a simple Python script to reset the desired machine:

import argparse
import socket
import sys
from time import sleep

host = "192.168.1.10"  # change this to your relay controller's IP address
port = 4196

# map server identifier to channel
machine_to_relay_channel = {
    'your-server-1': 1,
    'your-server2': 2,
    'test': 8,
}

COM_ON = 0xFF
COM_OFF = 0


def calculate_crc(data):
    crc = 0xFFFF
    for byte in data:
        crc ^= byte
        for _ in range(8):
            if crc & 0x0001:
                crc = (crc >> 1) ^ 0xA001
            else:
                crc >>= 1
    return crc


def send_cmd(s: socket.socket, n, command):
    cmd = [0] * 8
    cmd[0] = 0x01  # Device address
    cmd[1] = 0x05  # Modbus command
    cmd[2] = 0
    cmd[3] = n - 1
    cmd[4] = command
    cmd[5] = 0

    crc = calculate_crc(cmd[:6])
    cmd[6] = crc & 0xFF
    cmd[7] = (crc >> 8) & 0xFF

    s.sendall(bytearray(cmd))
    response = s.recv(1024)
    return response


def reset_machine(machine_name: str):
    """Reset a machine by toggling its relay channel"""
    if machine_name not in machine_to_relay_channel:
        print(f"Error: Unknown machine '{machine_name}'")
        print(f"Available machines: {', '.join(machine_to_relay_channel.keys())}")
        return False

    channel = machine_to_relay_channel[machine_name]

    try:
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            s.connect((host, port))
            print(f"Resetting machine '{machine_name}' (channel {channel})...")

            send_cmd(s, channel, COM_ON)
            sleep(1)
            send_cmd(s, channel, COM_OFF)

            print(f"Machine '{machine_name}' has been reset successfully.")
            return True

    except socket.error as e:
        print(f"Error connecting to relay controller: {e}")
        return False
    except Exception as e:
        print(f"Error during reset operation: {e}")
        return False


def main():
    example_reset_calls = "".join([f"  ./resetter.py reset {n}\n" for n in machine_to_relay_channel.keys()][:2])

    parser = argparse.ArgumentParser(
        description='Machine resetter - Controls relay channels to reset machines',
        formatter_class=argparse.RawDescriptionHelpFormatter,
        epilog=f'Examples:\n{example_reset_calls}  ./resetter.py list'
    )

    subparsers = parser.add_subparsers(dest='command', help='Available commands')

    reset_parser = subparsers.add_parser('reset', help='Reset a machine')
    reset_parser.add_argument('machine_name',
                              help='Name of the machine to reset')

    subparsers.add_parser('list', help='List available machines')

    args = parser.parse_args()

    if args.command == 'reset':
        success = reset_machine(args.machine_name)
        sys.exit(0 if success else 1)

    elif args.command == 'list':
        print("Available machines:")
        for machine, channel in machine_to_relay_channel.items():
            print(f"  {machine:<10} (channel {channel})")
    else:
        parser.print_help()
        sys.exit(1)


if __name__ == "__main__":
    main()