NT_Signals

• NT Signals Project
  • Quick Explanation
    • Data Format
  • Suggested BOM

NT Signals Project

This project is my interpretation of an idea by Leo Bicknell for a very simple, easy (and quick) to install signaling system for large modular layouts such as NTrak. The idea is to have something that is "plug and play", can be set up in minutes, and "just works". Many of the bells and whistles and complex signaling are intentionally thrown overboard, in favor of solid reliability, low cost, and quick setup.

I've put together a strawman design for the core logic and interconnect, leaving off for a moment the actual signal drivers.

Quick Explanation

Each module implements a block boundary controller. All the modules are connected in a chain (which can actually be a 2-D grid if necessary), and pass information about the state of the track ahead bidirectionally along a bus. The bus is physically implemented using RJ45 connectors and standard 8-wire Ethernet cabling (no particular CAT requirements).

The bus itself is SPI, implemented in a point-to-point fashion. Each module has four ports (N, S, E, W). Each module serves as the Master for its E and S links, and as the Slave for its N and W links.

Data Format

The state of each node is encoded in one of 8 values:

• 0 == Occupied (Red)
• 1 == Safe (Red)
• 2 == Approach (Yellow)
• 3 == Advance Approach (Yellow or Green)
• 4 == Clear (Green)
• 5-6 == Reserved
• 7 == "Dark Territory".

The nodes pass a simple message containing the node's own state, plus the last known state of the other three links connected. Thus, if two nodes are connected (A === B), the message sent from A to B would be:

| A State | A-West | A-North | A-South |

And the state clocked back from B to A would be:

| B State | B-East | B-North | B-South |

The message is 12 bits total per direction.

The actual signal aspects (colors, positions, color/positions) are not specified, and can be unique to each board (assuming the CPLD is appropriately programmed).

Suggested BOM

Item Description Manufacturer Mfr P/N Qty Notes

1
CPLD
Xilinx
XC2C64A-VQ44
1
Core logic

2
RJ45 Jack
Molex
95501-2881
4
Inter-module connect

3
6-pin header
Molex
22-03-2061
1
JTAG header

4
3-pin header
Molex
22-03-2031
6
Local inputs

5
2-pin screw terminal
Molex
399-70-0102
1
power in

6
Oscillator
Abracon
ASM-10.000MHZ-ET
1
Clock source

7
Power Regulator
ON Semi
MC7805BDTRKG
1
5V Regulator

8
Bridge Rectifier
Diodes Inc
DF01S
1
Diode Bridge

Of course, there are some Rs and Cs missing, as well as some isolation circuitry and the actual signal drivers themselves. But this gives a basic idea. The core stuff above fits on a 2.8" square board, which would cost about $2.25 in Qty/1000. With parts, the core comes to $15/board in Qty/1000.