Hierarchy

Overview

On this page:
Colors | Lan | Usb | Fdx | Hdx | Edg | Example

Abstract: The bus is structured into 5 levels. The upper levels are optimized for distance whereas the lower levels for cost.

Color Coding

Every bus level is depicted in a distinct color for better readability. The five levels are,
  • LAN: IP network (≥ 1 Gbit/s, Ethernet/Wi-Fi)
  • USB: PC to RTB bridge (3 Mbit/s)
  • FDX: Full duplex bus (2 Mbit/s, RS-485)
  • HDX: Half duplex bus (2 Mbit/s, RS-485)
  • EDG: Edge bus (2 Mbit/s, TTL signal re-shaping)
The bus is always built from top (LAN) to bottom (EDG). Not all levels have to be deployed but no level can be skipped.

 LAN Level

The PC is the root of an RTB domain. If several RTB domains are used, the PCs are connected via LAN. A use case for multiple RTB domains can be an exhibition layout operated by different groups. Since RTB domains are only loosely coupled each group works largely independent.

LAN specs,

  • Ethernet should be preferred over WIFI
  • no distance limit
  • IP protocol
  • Speed should be >= 1Gbps
Real-time Bus LAN level

USB Level

The USB is not really a bus level because it only connects the RTB bus master, the top node in each RTB domain, with the PC. The bus master is identified by its serial number to the PC (RTB does not use virtual com ports). USB specs,
  • One USB bridge per RTB domain
  • Electrical isolation between PC and RTB
  • Max cable length according to USB specs
  • RTB runs 3 Mbps data rate over USB (full duplex)

FDX Level

The purpose of the FDX is to transmit critical real-time data across the layout for which the up-/downstream latency remains constant. DCC, Railcom and some sensors may be examples of real-time data sources. Physically the FDX bus runs between Powerboards (s. wiring).

FDX specs,

  • RS-485, 150m max distance end-to-end
  • Termination required on each end
  • FDX runs at 2 Mbps data rate (full duplex)
  • 32 real-time v-channels
  • 5 address channels
  • 1 bulk channel
  • Cable distributes bus signals only

HDX Level

The HDX bus sits below the FDX layer and distributes the DCC signal within the layout. For this reason, the HDX bus usually accompanies the tracks on the layout and allows feedback modules be placed very close to the tracks. This ensures short DCC cables to lowers wire cost and reduced built up time of the layout (s. wiring).

HDX specs,

  • RS-485, 10m max distance end-to-end
  • Termination required on each end
  • HDX runs at 2 Mbps data rate (half duplex)
  • Cable distributes bus signal as well as track signal and power

EDG Level

The EDG bus is highly cost-optimized for accessory interconnect. Unlike FDX and HDX, this bus uses standard 3.3V logic levels which makes the hardware inexpensive. No differential line drivers are required which are cost drivers. But this does not mean the EDG bus is less reliable, because the bus signal is actively reshaped by each module. EMI exposure only affects the cable length between two modules while the EDG bus end-to-end length becomes irrelevant (s. wiring).

EDG specs,

  • TTL level daisy chain /w signal re-shaping
  • EDG runs at 2 Mbps data rate (half duplex)
  • async event channel
  • Cable distributes bus signal and power

Example

A schematic drawing of a 4 segments layout. Three segments on the left physically belong together (e.g. main station), the one on the right may be an arbitrary extension segment.

  • The example deploys two power domains, each with its own booster and power supply.
  • The FDX bus runs end-to-end connecting the power domains.
  • An HDX bus always belongs to a booster. In this example, the entire ‘main station’ is powered by one booster so the HDX bus spans across all three segments. The ‘fiddle yard’ was decided to get its own dedicated booster.
  • The EDG bus for accessories runs within each segment.
The software can be distributed across one (or more) PCs. In the example,
  • the central object server runs on the “Server“
  • the layout proxy daemon runs on  the “PC“