4. Uhlenbrock 63120 USB-LocoNet interface

The Uhlenbrock 63120 is a commercial USB ↔ LocoNet bridge with an internal microcontroller that handles LocoNet bit timing on the wire. The host (Pi 5) sees a USB CDC serial port carrying LocoNet frames — the same class of stream as a Digitrax LocoBuffer-USB when configured correctly.

• Uhlenbrock product information
• LocoNet-over-TCP interface comparison

4.1 Physical connection

Command station LocoNet ── RJ12 ── [optional 62280 Luisa] ── RJ12 ── Uhlenbrock 63120 ── USB 3 ── Pi 5

4.2 LNCV configuration (required for BigFred)

Factory defaults are often 115200 baud and filtered mode. BigFred needs 57600 8N1 and a transparent stream (LocoNet Direktmodus).

Program while the module is on a powered LocoNet bus (command station on), using one of:

Reference table from LocoNet-over-TCP (mode “LocoNet Direktmodus”).

4.2.1 rb lncv on Linux

The rb CLI implements the Uhlenbrock LNCV protocol (same message layout as JMRI LncvMessageContents). Use it when LocoNet-Tool is not available.

Prerequisites

• Command station on; 63120 on LocoNet-T (DR5000) or the correct LocoNet port for your central — not RailSync / booster (B) pins.
• LocoNet LED on the 63120 blinks on bus activity. A solid LocoNet LED means no valid bus traffic — fix wiring and power before LNCV programming.
• Host user can open the USB serial device (dialout group or udev rule).
• No other program holds the port (JMRI, xxd, minicom, dcc-bus).

Read module address (CV0)

rb lncv get --device /dev/ttyUSB0 --baud 115200 --article 63120 --addr 1 0

Expect 1 for a factory-default 63120. Article 63120 is normalised to 6312 internally.

Set Direktmodus and 57600 baud

Writes to the adapter's own configuration (CV2, CV4) reconfigure the USB link and are not acknowledged on the wire. Use --self-config:

# Still at factory 115200 — set Direktmodus first
rb lncv set --self-config --device /dev/ttyUSB0 --baud 115200 --article 63120 4 1

# Set baud to 57600 (LNCV 2 = 3); reconnect at 57600 for verification
rb lncv set --self-config --device /dev/ttyUSB0 --baud 115200 --article 63120 2 3

rb lncv get --device /dev/ttyUSB0 --baud 57600 --article 63120 2
rb lncv get --device /dev/ttyUSB0 --baud 57600 --article 63120 4

On success, --self-config prints that the value was sent and the adapter applies it without a LocoNet acknowledge — reconnect at the new baud to verify.

Useful flags

Diagnostic messages

The 63120 handbook (§5) requires echo-based flow control: each frame sent over USB is echoed back from the bus before the next send. rb lncv follows this for programming sessions and always sends prog-end so the adapter is not left in programming mode.

Limitation (JMRI / handbook): the 63120's own LNCVs are easiest to program from a LocoNet throttle on the bus. Programming via the adapter's USB port can work for reads and self-config writes, but the link may drop briefly while CV2/CV4 are applied — plan for a USB reconnect and baud change.

Mode comparison

Wrong baud or wrong mode produces garbage or missing frames in BigFred — not a Pi 5 defect.

4.3 Protocol contract with BigFred

BigFred loconet_serial expects:

1. 57600, 8 data bits, no parity, 1 stop bit.
2. Raw bytes per LocoNet message (opcode … checksum inclusive).
3. No ASCII SEND / RECEIVE lines (that is loconet_tcp / LbServer).

Checksum: XOR of all bytes including checksum byte = 0xFF (loconet_proto.go).

Example idle frame: 83 7C.

4.4 LocoNet frame loss — real or not?

Yes, it can happen, but with correct LNCV and a healthy bus it should be uncommon on Pi 5 + Uhlenbrock 63120 for normal throttle hub load.

Where frames are lost

LocoNet bus (collisions, weak signal)
        ↕ Uhlenbrock 63120 MCU
USB cdc_acm kernel buffer
        ↕
BigFred readLoop → rxCh (depth 64) → dispatch

Pi 5 USB 3 is not the weak link versus older SBCs. Misconfigured LNCV is the most common self-inflicted issue.

Symptoms

• loconet serial: dropping packet (bad checksum) in logs
• timeout waiting for slot on SetSpeed
• Throttle UI lagging while bus is busy

Mitigations

1. LNCV 57600 + Direktmodus (§4.2).
2. 62280 (Luisa) on long or heavily loaded LocoNet branches.
3. Short USB cable; performance governor / RT kernel (§3).
4. Avoid opening the serial port with other tools while dcc-bus runs.
5. On DR5000, compare once with USB LocoNet on the central — if stable there but not via Uhlenbrock 63120, suspect the T-bus segment to the Uhlenbrock 63120, not the Pi.

4.5 Optional: 62280 (Luisa) before Uhlenbrock 63120

If the Uhlenbrock 63120 sits on a long LocoNet run with many modules, put 62280 between the command station and the Uhlenbrock 63120 branch:

• Regenerates signals
• 12 V / 500 mA for the secondary segment
• Galvanic isolation — short on secondary does not kill primary T bus

BigFred behaviour is the same; Luisa improves power and signal, not the hub protocol.

4.6 Linux device path

After udev (§3.5):

Identify the device after plug-in:

ls -l /dev/ttyACM* /dev/ttyUSB* 2>/dev/null
dmesg | tail -20

permission denied → add user to dialout or fix udev GROUP/MODE.

Continue with §5 BigFred integration.