ProtoLink 1.1.0
Troubleshooting
A practical checklist for launching the local web UI and diagnosing Modbus TCP/RTU connections
Local UI does not open
- Make sure the app is running.
- Open http://127.0.0.1:8000 manually.
- Check http://127.0.0.1:8000/api/healthz - expected status ok and version 1.1.0.
- If port 8000 is busy, close the other process and restart ProtoLink.
Modbus TCP does not respond
- Check IP/host and TCP port, usually 502 or the value from device documentation.
- Verify network, VPN, or VLAN reachability.
- Check Unit ID and register type: coils, discrete inputs, input registers, or holding registers.
- Lower count and increase timeout on unstable networks.
Modbus RTU is silent
- Check COM/serial port, baudrate, parity, stop bits, and byte size.
- Check Unit ID 1..247 and close other apps that may hold the port.
- Check A/B wiring, GND, adapter power, termination/biasing, and cable length.
- Increase timeout and lower count for slow devices.
COM/serial port is not visible
- Windows: check Device Manager and CH340/FTDI/CP210x or vendor driver.
- Linux: look for /dev/ttyUSB* or /dev/ttyACM*; add the user to dialout and log in again.
- macOS: for outgoing serial connections usually choose /dev/cu.* instead of /dev/tty.*.
Values look wrong
- Verify 0/1-based addressing in the device documentation.
- Select the correct register type and function code.
- For multi-register values, check byte order and word swap.
- Compare HEX and DEC to separate data format issues from wrong addresses.
Writes fail
- ProtoLink supports single and multiple writes for coils and holding registers.
- The device may block writes to a specific address or require another mode.
- Check value ranges, register type, and value lists for multiple writes.
Information to collect before asking for help
- OS and ProtoLink version
- screenshot of the TCP/RTU form
- host/port or COM parameters
- Unit ID, address, count, register type
- error text from activity log
- /api/healthz result
- device model and register map excerpt
Ready to download?
Open the download page and choose the version for your platform.