PgTgPiSequent Daemon for PgTgBridge

TCP/JSON I2C relay control service targeting Raspberry Pi 4 (Debian Trixie) with a Sequent Microsystems 16-relay HAT, Sequent Microsystems 8-relay HAT, or Sequent Microsystems 4-relay HAT. Board type is selected via the board config key; a single binary supports all three. Specifically for use with PgTgBridge built-in plugin PgTg PiGpio. Can be used as a smart wrapper to the relay board that is accessible using JSON strings such as from Node-Red.

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JSON Protocol

Commands are newline-delimited JSON sent over TCP. The connection remains open between commands.

Direction	Example
Set ON	{"cmd":"set","output":"output1","value":1}
Set OFF	{"cmd":"set","output":"output2","value":0}
Timed ON (30s)	{"cmd":"set","output":"output1","value":30}
Read all outputs	{"cmd":"read"}
Read all inputs	{"cmd":"read_inputs"}
OK (set)	{"status":"ok","output":"output1","value":1}
OK (read)	{"status":"ok","outputs":{"output1":1,"output2":0}}
OK (read_inputs)	{"status":"ok","inputs":{"input1":0,"input2":1,"input3":0,"input4":1}}
Error	{"status":"error","message":"unknown output: output5"}
Error (no inputs)	{"status":"error","message":"board does not support inputs"}

Timer Behaviour

Value	Behaviour
0	Static OFF — cancels any running timer
1	Static ON — cancels any running timer
2–999	Drives output ON; auto-forces OFF after that many seconds. A new value ≥ 2 before expiry resets the timer.

On expiry the server sends {"status":"ok","output":"<name>","value":0} on the open connection if a client is connected.

PgTg Bridge Watchdog Integration

When the PgTgBridge GPIO plugin is configured with an Amp PTT output, it sends value: 15 (not 1) whenever the amplifier PTT is keyed. This starts a 15-second channel watchdog timer on the service side. PgTgBridge then re-sends value: 15 every 10 seconds to keep the watchdog alive for the duration of the transmission. When PTT is released the PgTgBridge sends value: 0.

If the PgTgBridge loses connectivity, the watchdog expires after 15 seconds and the PTT output is automatically deactivated — preventing a stuck transmitter if the connectivity to the host machine is lost.

If the PgTgBridge service is stopped, PgTg sends value: 0 to all configured channels before halting.

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Hardware Prerequisites

Enable I2C on the Raspberry Pi (required once):

sudo raspi-config
# Interface Options → I2C → Enable

Or add to /boot/firmware/config.txt:

dtparam=i2c_arm=on

Verify the relay board is detected (stack 0 appears at address 0x27):

i2cdetect -y 1

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Configuration File

Location: /etc/PgTgPiSequent/PgTgPiSequent.conf

# PgTgPiSequent configuration file

# TCP port for the JSON command interface
port = 5556

# Board type: 16relay (default), 8relay, or 4relay
board = 16relay

# Sequent Microsystems relay board stack level (0-7, default 0)
# Stack 0 = I2C address 0x27  (formula: (stack + 0x20) ^ 0x07)
# Verify with: i2cdetect -y 1
stack = 0

# Relay output mappings: output<N> = relay channel number
#   16relay board: channels 1-16
#   8relay board:  channels 1-8
#   4relay board:  channels 1-4
output1 = 1
output2 = 2
output3 = 3
output4 = 4

Key	Description
port	TCP port for the JSON command interface
board	16relay (default), 8relay, or 4relay — selects the Sequent HAT
stack	Board stack level 0–7 (physical jumper); each stacked board gets a unique level
output<N>	Maps logical output N to a relay channel on the board (1–16 for 16relay, 1–8 for 8relay, 1–4 for 4relay)
input<N>	(4relay only) Maps logical input N to a digital input channel (1–4)

The live conf file is created from the package template on first install and is preserved on upgrade and removal. Edit it directly on the Pi; it is never overwritten by apt.

8-relay board example

port    = 5556
board   = 8relay
stack   = 0
output1 = 1
output2 = 2
output3 = 3
output4 = 4
output5 = 5
output6 = 6
output7 = 7
output8 = 8

4-relay board example

port    = 5556
board   = 4relay
stack   = 0
output1 = 1
output2 = 2
output3 = 3
output4 = 4
input1  = 1
input2  = 2
input3  = 3
input4  = 4

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Systemd Service

The service runs as the dedicated system user PgTgPiSequent (member of the i2c group) — no root required.

[Unit]
Description=PgTgPiSequent TCP/JSON I2C relay control service
After=network.target
Wants=network.target

[Service]
Type=simple
ExecStart=/usr/bin/PgTgPiSequent /etc/PgTgPiSequent/PgTgPiSequent.conf
Restart=on-failure
RestartSec=5
StandardOutput=journal
StandardError=journal
SyslogIdentifier=PgTgPiSequent
User=PgTgPiSequent
Group=i2c

[Install]
WantedBy=multi-user.target

The service file is installed and enabled automatically by the Debian package.

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Build & Deploy

Two build workflows are available — cross-compile from a WSL2 host, or build natively on the Pi. If building under WSL2, a deb package is created.

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Option A — Native build on the Pi

Prerequisites (install once on the Pi):

sudo apt install cmake ninja-build

./build
# Produces: build-native/PgTgPiSequent

You will be responsible for copying the PgTgPiSequent binary to /usr/bin/PgTgPiSequent:

sudo cp build-native/PgTgPiSequent /usr/bin/PgTgPiSequent
sudo chmod +x /usr/bin/PgTgPiSequent

And creating the configuration file at /etc/PgTgPiSequent/PgTgPiSequent.conf.

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Option B — Cross-compile (in WSL2)

(you need to setup the cross-compiler yourself)

Prerequisites (install once in WSL2):

sudo apt install gcc-aarch64-linux-gnu binutils-aarch64-linux-gnu cmake ninja-build

rm -rf build-aarch64 && mkdir build-aarch64
./build-with-cross-compiler
# Produces: build-aarch64/PgTgPiSequent (ELF aarch64)

1. Package (after cross-compile in WSL2)

./package
# Produces: PgTgPiSequent_1.0.2_arm64.deb
# Sources binary from build-aarch64/PgTgPiSequent

2. Deploy to Pi (use after cross-compile and deb package creation)

./deploy
# SCPs PgTgPiSequent_1.0.2_arm64.deb to pi@<yourPiIpaddress>:/home/pi/

3. Install on Pi

sudo apt install /home/pi/PgTgPiSequent_1.0.2_arm64.deb

The package installer:

• Creates the PgTgPiSequent system user and adds it to the i2c group
• Copies the factory-default config to /etc/PgTgPiSequent/PgTgPiSequent.conf (first install only)
• Copies the daemon binary to /usr/bin
• Installs and starts the systemd service

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Upgrade & Removal

# Upgrade (live config is preserved)
sudo apt install /home/pi/PgTgPiSequent_1.0.2_arm64.deb

# Remove (binary and service removed; config kept)
sudo apt remove PgTgPiSequent

# Full purge (config and system user also removed)
sudo apt purge PgTgPiSequent

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Testing on the Pi

# Set output1 ON
echo '{"cmd":"set","output":"output1","value":1}' | nc localhost 5556

# Read all output states
echo '{"cmd":"read"}' | nc localhost 5556

# Set output1 OFF
echo '{"cmd":"set","output":"output1","value":0}' | nc localhost 5556

# Timed ON — output1 goes ON, forces OFF after 30 seconds
echo '{"cmd":"set","output":"output1","value":30}' | nc localhost 5556

# Monitor in journald
journalctl -f -u PgTgPiSequent

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Service Management

# Start and follow logs
sudo systemctl start PgTgPiSequent && journalctl -u PgTgPiSequent -f

# Stop (triggers clean I2C release — verify in journal)
sudo systemctl stop PgTgPiSequent

# Confirm service is running as PgTgPiSequent (not root)
ps aux | grep PgTgPiSequent

# See user and group
ps -eo user,group,cmd | grep PgTgPiSequent