Relay Board Controller

A cross-platform controller for managing an 8-channel relay power strip over serial/USB. Implements a hand-rolled binary protocol at 57600 baud with CRC8 checksums for reliable communication. The repository ships two parallel front-ends: a small Linux CLI built in C with CMake, and a feature-rich Windows GUI built in C++ Builder 6.0 with VCL.

The Linux side is intentionally minimal but production-ready: native termios for serial configuration (no shell-out to stty, no command-injection surface), reply validation on every received frame, and a state file + systemd unit so the board's last known configuration is automatically restored on boot or after a power loss. The Windows side adds a tray-resident GUI with per-relay scheduling, a TCP server for remote control, and Windows autostart via the registry.

Serial Protocol

All communication uses a fixed 5-byte frame at 57600 baud, 8N1:

• Byte 0: 0x55 — sync byte (start of frame)
• Byte 1: 0x01 — device address (USB/RS-232)
• Byte 2: command — 'S' set, 'O' on, 'F' off, 'G' get status
• Byte 3: data — relay bitmask (Set/Get) or relay index 0–7 (On/Off)
• Byte 4: CRC8 over bytes 1–3 using a 256-byte lookup table

The board responds to a Get command with a frame whose command byte is 'R' and whose data byte is the relay states as a bitmask (bit 0 = relay 1, bit 7 = relay 8). Every received frame is validated for sync byte, device address, command byte, and CRC8 before its contents are trusted.

Linux CLI

• Native termios serial configuration — no shell-out, no command-injection surface
• State persistence: every change is written to power_strip.state as a single hex byte; for single-relay operations the program first reads the board's own state with a G request and then sends the updated mask, keeping the file consistent with hardware even if it was edited or deleted
• Restore command: ./power_strip restore reads the saved bitmask and reapplies it in a single S frame — perfect for systemd boot restore
• Configurable relay names via plain-text power_strip.conf (line 1 = serial device, lines 2–9 = human-readable labels)
• Strict reply validation: sync, address, command, and CRC8 all checked before a reply is accepted
• Exit code 1 on any error — bad arguments, missing config, serial I/O failure, malformed reply, CRC mismatch — with details on stderr

./power_strip status        # query and refresh power_strip.state
./power_strip allon         # turn all relays on
./power_strip alloff        # turn all relays off
./power_strip 3 on          # turn relay 3 on
./power_strip 3 off         # turn relay 3 off
./power_strip restore       # reapply last saved state

systemd Boot Restore

A small oneshot unit reapplies the last-known relay state on every boot — invaluable after a power loss, since the board itself does not preserve state across reboots.

[Unit]
Description=Restore RelayBoard relay state on boot
After=local-fs.target

[Service]
Type=oneshot
WorkingDirectory=/opt/power_strip
ExecStart=/opt/power_strip/power_strip restore

[Install]
WantedBy=multi-user.target

Windows GUI

• Per-relay toggle buttons with state indication and configurable labels
• Scheduled actions: per-relay day-of-week + time scheduling (turn relays on/off automatically)
• TCP server for remote network control of the relay board, with obfuscated-password authentication
• Standalone TCP client (separate executable) for controlling the relay board over the network from another machine
• System tray support — minimize to tray, popup menu for quick relay toggling without opening the main window
• COM port auto-detection with a configurable selector
• Windows autostart option via registry integration

Technical Details

• Linux: C99, CMake 3.10+, POSIX termios, -Wall -Wextra -pedantic warning-clean
• Windows GUI: C++ Builder 6.0 with VCL; sockets for TCP server; custom hash for password obfuscation
• Repository layout: linux/ (CLI), caly program/ (Windows GUI), klient sterowanie/ (Windows network client), and Specyfikacja protokolu.txt (Polish protocol spec)
• CRC8 implementation: 256-byte lookup table, applied to bytes 1–3 (sync byte excluded)
• Configuration: Linux uses a plain-text config; Windows GUI persists settings via the registry and INI files

Build Instructions

Linux

cd linux
mkdir build && cd build
cmake ..
make
./power_strip status

Windows

Open the .bpr project file in C++ Builder 6.0 and build (Project → Build All). The standalone network client lives in klient sterowanie/ as a separate .bpr.

Check out the project on GitHub: Relay Board Controller on GitHub

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