Hardware

What each block on the board does, and how a DCC signal becomes four driven outputs.

FemtoLCC is a layout accessory node. It listens on the LCC bus, watches an isolated feed of the DCC track signal, and drives four independent block outputs. Everything below traces that path, from the bus in to the terminal blocks out.

Signal path

   LCC bus  ──▶  CAN transceiver  ──▶┐
   (RJ45)        MCP2562             │
                                     ├──▶  ESP32-C6  ──▶  4:1 mux  ──▶  H-bridge  ──▶  Block
   DCC track ─▶  optocoupler      ──▶┘     controller      ×4          driver ×4       outputs
   (terminals)   HCPL-0630                                                             J5–J8

Controller

An ESP32-C6-WROOM-1 module runs the node. It is a RISC-V part with Wi-Fi, Bluetooth LE and 802.15.4, so the node can join the layout over the wired LCC bus and still be reachable wirelessly for configuration or firmware updates.

Programming and console access come out on a 6-pin UART header, with a USB-C port alongside it. A push button pulls the module’s EN line to ground for a manual reset.

LCC bus interface

LCC — Layout Command Control, the NMRA’s OpenLCB standard — runs over CAN. An MCP2562 transceiver drives the bus, and it lands on a dual-port RJ45 jack. Two ports rather than one is the important detail: the node daisy-chains the bus rather than terminating it, so you run the cable in one side and straight out the other to the next node. A 120 Ω termination resistor is fitted for use at the end of a run.

DCC isolation

The DCC signal on the track carries the full track voltage and is electrically nothing like the node’s logic supply, so it is never connected directly. An HCPL-0630 dual optocoupler sits between the two, fed through 2 kΩ 1 W wirewound-class resistors sized for track voltage. The optocoupler passes the signal across as light, leaving no conductive path between track and logic.

This is what lets the node share a common ground with the LCC bus while the track floats at its own potential.

Block driver channels

Four identical channels, one per output. Each pairs an SN74HC253 dual 4:1 multiplexer with a DRV8874 H-bridge driver:

• the multiplexer selects which signal reaches the driver — the isolated DCC feed, a static level, or a control signal from the controller
• the H-bridge does the driving, capable of well over what a block section needs, with current sensing on each channel so the controller can tell how much is being drawn

Each channel gets its own local decoupling — a 10 µF bulk capacitor plus 100 nF and 22 nF close to the driver’s supply pins — because an H-bridge switching a block draws current in sharp steps.

Outputs land on five-millimetre-pitch screw terminals, J5 through J8, one pair per block.

I/O expansion

An MCP23018 port expander adds sixteen general-purpose I/O lines over I²C, for whatever the controller’s own pins do not cover. A PMOD socket and a 4-pin JST header break out further connections for daughterboards and sensors.

Power

Input arrives on a 3.5 mm pluggable terminal header. A P-channel MOSFET in the input path acts as an ideal diode, protecting the board against reversed supply leads without the voltage drop a plain diode would cost.

Two linear regulators follow: 5 V for the driver logic and 3.3 V for the controller and the digital side. The USB-C port can supply the board as an alternative to the terminal input.

Connectors at a glance

Board outline, mounting pattern and drill details are on the mechanical page.