NR SSI8003-51B ISSUE 6 PDF

Introduction

In any signalling scheme there may be a requirement to split the railway into a number of areas, each controlled by a separate Interlocking. In a relay scheme, these areas are determined by the boundaries of the areas allocated to each Control Centre and to each relay room. In an SSI scheme, there also needs to be a subdivision into areas which are within the capacity of a single SSI Central Interlocking; this is determined primarily by the limit on the number of Trackside Functional Modules (TFMs) which can be addressed (63), but there are other factors, see SSI8003-52.

Methods of Communication

In order to control its operations in the fringe area, each SSI Central Interlocking needs to be able to exchange information with the adjacent Interlocking areas. Two mechanisms are provided for this:

(i) Communication between SSI Central Interlockings in a single Control Centre takes place over a dedicated pair of data links known as the Internal Data Link (IDL). This is entirely independent of the Trackside Data Links. Up to 30 Central Interlockings can be connected to the IDL in a Control Centre. Each Central Interlocking transmits a message containing 15 eight bit data telegrams, and can decode up to 15 of the telegrams transmitted by the other Central Interlockings.

(ii) Communication with other types of Interlocking, or with remote SSI Central Interlockings, is achieved by using the inputs and outputs of standard SSI TFMs. Each Signal Module can provide 8 outputs to drive relays and 6 inputs to detect relay contacts.

The data to process these inputs and outputs is written in the appropriate telegrams in the Input Telegram Data and Output Telegram Data. The Input Telegram Data sets up the Interlocking memory with the information received from the fringe Interlocking, and the Output Telegram Data generates the outputs to the fringe area based on the current state of the Interlocking memory.

As part of writing this data, the appropriate forms listing the information transferred by these telegrams has to be updated accordingly. Guidance on completing the forms for the IDL is given in GS/SI8300 Part 3 and in SSI8003-40B Section 3.

When sending and receiving information to and from other Interlockings, single bits in IDL telegrams or TFM/relay interfaces should only be used to convey information across interlocking boundaries where the consequence of transmission failure is tolerable i.e. fail-safe. Intermittent transmission failures are possible and the 'nature' of the data being transmitted by a single bit should be considered - could failure of the transmission result in a wrong-side failure?

Types of Boundary

The task of data preparation is greatly simplified if the division of the railway into Interlocking areas is designed so as to minimise the information to be transferred - a boundary on plain track is much easier to deal with than one which runs through a complex station area. Boundaries between Control Centre areas, and between separate Signalman's Console areas within a large Control Centre are usually chosen on this basis in any case, but the limited capacity of each SSI Central Interlocking means that each Signalman's area will usually have to be split between three or four SSIs. In a complex area this means that some awkward boundaries are inevitable. In these circumstances it is also important to ensure that the group of Central Interlockings operate together so that the Signalman does not need to be aware of the sub-division of his area. For the purposes of this document, we have categorised 3 types of boundary of increasing complexity:

(i) Where there is no route locking across the boundary, and the information to be transferred relates to signal controls and approach locking only. This occurs when the boundary can be drawn "through the signal post" on unidirectional running lines with no points in the overlap beyond the signal.

(ii) Where the boundary is on a bidirectional line, and route locking is provided by means of "slots" on the signals controlling the entrances to the single line section. This will usually occur on a boundary between Control Centre areas.

(iii) Where the area controlled by a single Signalman has to be split so that routes cross the boundary between Interlocking areas, and the Central Interlockings have to operate together so that the Signalman is unaware of the split.

Items peculiar to boundaries between SSI Central Interlockings and relay controlled areas are covered by Chapter D Section 2.

Certain special or more complex features are covered by Chapter E. These include routes that involve three Central Interlockings or require flank points which are in another Central Interlocking.