SLSignal_Prototype

• Searchlight Signal Prototype
  • Features
  • Track Diagram
  • Use Cases
    • Entering Trains
    • Exiting Trains
    • Crossovers
  • Logic Table
  • Logic Equations
    • Naming Conventions
  • Hardware Implementation
    • Signal Cable Pinout
    • Estimated Bill of Materials

Searchlight Signal Prototype

The prototype version of Searchlight Signal is a simple logic controller that uses DIP switches for turnout indication and 7400-series logic elements to determine the signal outputs. The system's logic is specifically designed around the staging yard portion of my model railroad layout.

Because the logic implemented is specific to the particular track arrangement and use case, this design as-is may not be applicable to other layouts. However, the basic design can be adapted by reworking the logic portion to suit the user's layout.

Track Occupancy is not implemented in the prototype, but could be added. I may yet decide to allow provision for track occupancy.

Why do this? What value add does the logic give vs. directly controlling the signals with individual switches? The logic links the turnout position to the signal indication such that operation is simpler for the user. One sets the (logic input) switch exactly the same way as the track switch. The only thinking required of the user is to correlate the track switch to its matching DIP. Plus, when the remote track switch control is implemented, the signal logic is automated by simply tying the track switch control directly to the signal logic, removing human intervention entirely.

Features

• 8 turnout indication inputs (DIP switches)
• 12 2-light Green/Red signal outputs
• Simple logic implementation (can be breadboarded)
• Dispatcher override turns any output red.
• Self-contained design

Track Diagram

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In the above drawing, the lines represent tracks. Turnouts are numbered 1-8, and signals are designated with letters A-M (omitting "I"). The tracks marked (EM) and (WM) are the main lines entering the yard, and the tracks marked (ES) and (WS) lead to off-layout staging.

For the prototype, each signal location (A-M) will have a 2-color-light (green/red) dwarf signal installed so as to be visible from a train approaching the associated turnout. Signal aspects will be derived from logic based on the selected turnout positions.

Because at the current stage of layout development all turnouts are under manual control - and to simplify the design of the controller, turnout positions will be indicated by a set of DIP switches on the logic board. This requires that the operator throw TWO switches - the actual turnout switch on the layout and the corresponding DIP switch on the control board - in order to properly set the signals. This is admittedly error-prone. Future versions of the controller will incorporate automatic turnout position sensing, eliminating this dependency.

Use Cases

The following rules describe the usage of the tracks.

Entering Trains

• Trains may enter Tracks 1-3 Eastbound from the West Main (WM) via Turnouts 8, 7, and 6.
• Trains may enter Tracks 1-3 Westbound from the East Main (EM) via Turnouts 1, 2, and 3.
• Trains may enter Track 1 Eastbound from the Mine Branch via Turnouts 4/5.
• Trains may enter Track 3 Eastbound/Westbound from External Staging (ES or WS) via Turnouts 3 and 6.

Exiting Trains

• Trains may exit Westbound to West Main (WM) via Turnouts 6, 7, and 8
• Trains may exit Eastbound to East Main (EM) via Turnouts 1, 2, and 3
• Trains may exit Westbound to Mine Branch via Turnouts 4/5
• Trains may exit Eastbound/Westbound to External Staging (ES or WS) via Turnouts 3 and 6.

Crossovers

Turnouts 2/3, 6/7, and 4/5 are crossover pairs, and should be switched in tandem. Because all switches are manual at this time, it is possible to switch them independently, and the signal logic must anticipate this possibility. When remote / power switch control is installed, these crossover pairs will be mated so that they cannot be operated independently.

Logic Table

The following logic table shows the signal indications based upon the turnout positions. Values are defined as follows:

Turnout Positions:

• '0' : Lined for main (through)
• '1' : Lined for diverging
• 'X' : Do not care

Signal indications:

• 'G' : Signal is green
• 'R' : Signal is red

Gray cells are not considered in the logic for the corresponding signal.

Note that due to the physical arrangement of the turnouts, "lined for main" is actually the physically diverging path for Turnouts 3 and 6. This altered convention allows for the "lined for main" positions of TO3 and TO6 to provide a continuous path from (EM) to (WM) mainlines, while the "diverging" positions provide access to the external staging (ES and WS) tracks.

[-img src= PrototypeSignalLogic.png: missing =-]

Logic Equations

From the above logic table, a set of equations can be derived for each signal. I choose to standardize on NAND/NOR logic, and come up with the following. I do not claim it to be optimal.

Naming Conventions

Each turnout is labled T1 through T8. T1=TRUE ('1') indicates diverging, just as in the table above. Signal outputs are indicated by the signal letter followed by 'G' or 'R' (e.g. AR is the Red LED on signal A). NOT() function is indicated by an exclamation point ('!'). Logic functions are indicated by the function name with its inputs in parentheses, separated by commas -- example: NAND(T1, T2).

• AR == NOR(T1, T2, T3)
• AG == !AR
• BG == T1
• BR == !T1
• CG == NOR(!T2, T1)
• CR == !CG
• DR == T3
• DG == !T3
• ER == NOR(!T3, NAND(T3, NOR(T1, T2)))
• EG == !ER
• FR == NOR(!T6, NAND(T6, NOR(T7, T8)))
• FG == !FR
• GR == T6
• GG == !T6
• HG == NAND(!T7, T8)
• HR == !HG
• JG == T8
• JR == !T8
• KR == NOR(T6, T7, T8)
• KG == !KR
• LG == NAND(T4, T5)
• LR == !LG
• MR == T4
• MG == !MR

An astute observer will note that many of these equations are symmetric, as is the track diagram...

Hardware Implementation

The controller will be implemented as a row of 8 DIP switches as inputs to a set of 7400 logic chips that implement the above equations. Separate 5V drive outputs for each LED (12x G+R or 24 total) will be generated by the logic and fed to a set of 12 3-wire headers for the signal cables.

Current limit and LED drive will be provided on the control board for all 12 signals.

Signal Cable Pinout

• Pin 1 : Common Anode
• Pin 2 : Green Cathode
• Pin 3 : Red Cathode

Estimated Bill of Materials

Note, costs shown at exactly $1.00 are placeholders for now.

Item Description Qty Unit Price Cost Notes

1
7400 (Quad 2-input NAND)
1
$0.27
$0.27

2
7402 (Quad 2-input NOR)
1
$0.27
$0.27

3
7404 (Hex Inverter)
3
$0.24
$0.72
4 spare gates

4
7427 (Triple 3-input NOR)
1
$0.62
$0.62
3rd gate used as 2-NOR

5
8-input DIP
1
$2.00
$2.00
Est. Price

6
3-pin header
12
$0.45
$5.40
Est. Price

7
Dual LED Driver CKT
12
$1.00
$12.00
Design TBD

8
5V power connector
1
$1.00
$1.00

9
8-input header
1
$1.00
$1.00

10
Breadboard or PCB
1
$1.00
$1.00