197503 – Niehoff – Signalling Developments on the Victorian Railway at Oakleigh: Electronic Route Setting Controls & Indications

Railway systems who are known to have used solid state logic for route setting and indication circuits, which may be classified in the non-vital area, are New Zealand who used a system based on low level D.T.L. (diode – Transistor – Logic) integrated circuits and London Transport, who used a simple diode matrix system for route setting controls only.

After evaluating the techniques used by N .Z. railways, it was realised hat the system would need to be extended to provide single push button operation, storage of routes for through route seeting and automatic overlap selection.

In the first attempt it was decided to convert the NX circuitry to O.C.S. by one button energizing the entrance and exit circuits simultaneously. However some modifications were subsequently necessary.

Simple circuits were designed around 2 signals and set of free facing points. The circuitry was tested on a computer at Computer Sciences of Australia, using their program Dilos, the language used being Basic. The computer confirmed that the logic presented to it was satisfactory.

A full system was designed from this original concept. Firstly T.T.L. (Transistor – Transistor – Logic) seemed the obvious choice, but with advice from people in the electronics field C.M.O.S. (Complimentary Metal Oxide Silicon) was chosen due to simplicity of design and high immunity to noise. Printed circuit cards where designed using the red, blue and black pressure sensitive tape method.

A model layout was chosen to test the concept and geographical circuit modules, printed circuit cards and a control panel were constructed. When the model was operational, the limitations of simple circuit design were discovered and so a more comprehensive design was prepared for actual route settings. This system is currently being installed at Oakleigh and Flinders Street “E” Box.

Modifications necessary in the model:-

he first and most obvious short coming of the original design was in the push button indication lights.

If two buttons from one signal were pressed simultaneously both route set indication lights in the buttons would light even though only one route was set. This was because the push button card for that signal could not distinguish which route was set.

The first attempt to solve this problem was to inform the push button card which exist was being energized.

Another problem was found when trying to set two routes that cross one another. The result was to set two parallel routes and light the wrong push button indications. AI l of these wrong route and indication problems were overcome by causing each route setting button to preset every point in its route. This was achieved by taking away availability to the other lie from the points in the route.

It was realized with this system that it was no longer necessary to energize the exit circuitry from the push button nor to inform the push button card which exit was selected, since pushing two buttons simultaneously for the same signal would prevent either route from setting.

From the model it was also found that a locked set of points should not be driven as this would interfere with any “catch handle locking” on other sets of points preventing them from operating.

Due to the model circuits being simple passive circuits, it was noticed hat for some routes where points had to be operated in sequence, the push button had to be held for a considerable time and this would be much longer with actual point machines rather than the simulated model circuits. This problem was overcome by providing memories on the push button cards so that as soon as a route was determined to be available, the button could be let go, and the route setting would continue to set the route.

When these modifications had been made, a routiner was designed and constructed to sequentially set each route and then cancel it by de-energizing the track repeat relay of the track ahead of the signal. This was to check the reliability of the system.