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1.4.1. Setting Routes over Boundaries

Suppose that one wished to route a train from S7 to S5. On receiving the panel request for this route East first evaluates the availability conditions in its portion of the network: if these are not met the request simply fails, otherwise East must wait until it is certain the route is also available in the other Interlocking before locking it. To achieve this East issues a remote route request to West over the internal data link.

On receiving such an input West should handle the request just as it would handle route requests coming directly from the control panel---this simplifies the design of the control interpreter, and data preparation. Thus an incoming IDL request will be translated into a panel request and queued in the usual manner. When a remote route request is subsequently processed the difference is that West must communicate to East if, or when, the route is locked: West sends its acknowledgement via a reply telegram to East over the IDL.

In East the acknowledgement is also treated as a remote route request: on this occasion East proceeds to lock the route it had originally requested. The two Interlockings need to use a dedicated pair of IDL telegrams to communicate request codes and their acknowledgements. Normally, many routes over numerous lines link the two signalling areas, but a single pair of (eight bit) telegrams should suffice to carry all the necessary request codes and their acknowledgements. To summarise:

1 East receives a panel route request for a cross-boundary route. If the route is available in East, issue a remote route request to West.

2 West receives an IDL input conveying a remote route request. If the route is available, lock the route and reply to East with an acknowledge telegram.

3 East receives a reply telegram to the earlier remote route request: it can then lock the route and control the entrance signal as usual.
Once the route has been locked in East the aspect of the entrance signal can be changed if the prevailing conditions allow this. For example, only if the tracks down to the exit signal are clear, and if opposing signals are on, can East clear the signal---to green or yellow, depending on the aspect displayed by the exit signal. Thus, in addition to the telegrams used to convey request codes, another IDL telegram is needed to convey the status of tracks and signals in the fringe area. Such data are needed continuously.
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Matthew Morley, Edinburgh. Date: 29 November, 1998