Fix device pairing failures by transport

The E-BON app supports four transports — Bluetooth, USB (via a USB-to-Serial bridge), Serial and TCP — and a pairing failure almost always sits at the transport layer rather than in the fiscal protocol itself. This recipe covers the symptom you see per transport.

Identify the likely cause

• Bluetooth — printer not bonded in Android's system Bluetooth settings, Nearby devices runtime permission denied, or the printer is still connected to a different host.
• USB — USB-OTG cable is charge-only (very common with phone cables), Android per-device USB permission prompt was dismissed, or the printer presents itself as CDC-ACM but no driver-level UART node was created.
• Serial — wrong baud rate (Datecs/Tremol default to 115200, Incotex and the generic Custom adapter to 9600), wrong device node (/dev/ttyUSB0 vs /dev/ttyACM0), or the cable is straight-through where the printer expects null-modem.
• TCP — printer's IP is on a different subnet, port is not the one the firmware listens on (most fiscal families: 9100 or 4999), or a router/firewall ACL is dropping the connection.

Verify the transport works

Before retrying from the app, confirm the transport works at the OS level. Examples below — pick the one that matches your transport.

# TCP — confirm the printer accepts a TCP connection on its declared port
nc -zv 192.168.1.42 9100

# Bluetooth — list bonded devices on the Android device via adb
adb shell dumpsys bluetooth_manager | grep -A2 "Bonded devices"

# Serial — list serial nodes the kernel exposes (USB-to-Serial adapters appear as ttyUSB* / ttyACM*)
adb shell ls -l /dev/ttyUSB* /dev/ttyACM*

If you have already paired the device once and want to re-check the cloud-side record, use the API:

curl https://api.e-bon.ro/api/v1/devices/{deviceId} \
  -H "Authorization: Bearer <jwt>"

A 404 NOT_FOUND here means the device exists locally on the phone but was never registered with e-bon — re-run Pair Device on the Pairing screen so the cloud claim succeeds.

Apply the fix

Get help if you're still stuck

Open a support case at support@e-bon.ro or e-bon.ro/contact and include the printer model, the transport you tried, the exact OS-level message and a screenshot of the Device Discovery screen.