As with almost all aspects of the railways, signal box technology originated in the United Kingdom. Before signal boxes were invented and introduced, one or more signalmen were employed, depending on the size of the station, to switch each point manually for every individual train. No interrelationship existed between the points; signalmen had to make sure of the correct setting themselves. Where necessary, a flag or lamp was used to signal a train. Since the density of rail traffic is far higher today, however, the safety requirements and thus also the technology employed in signal boxes have naturally evolved greatly. Electronic signal box control systems have been employed since the mid-1980s, based on modules that themselves consist of multiple computers. The German railway network places very high requirements on the safety of the signal box system. Faults must not be allowed to affect the system reliability - they must therefore be detected, and the systems then be put into secure mode (switched off if necessary). This capability is achieved using redundant, so-called '2 out of 3' systems.

The supplier of the complete signal box system also places correspondingly high requirements on all components used to equip the signal boxes. Computer units manufactured by MEN in Nuremberg are among these components. MEN was founded in 1982 and develops and markets embedded solutions, including VMEbus and CompactPCI systems and also what are known as deeply-embedded solutions that function independently of any bus system. MEN supplies complete solutions, including various operating systems and associated input and output units for harsh industrial environments.

Half of the computers supplied by MEN are custom systems (Fig. 1). This is also the case for the computers used in the German railway signal box network. These are CompactPCI computer units in 6 U (double euroboard) format with Intel processors. These computers are a key element of the system, as they control all the interrelationships in a station, such as track and point switching, to one another. All relevant actions are communicated to actuator elements via suitable Ethernet interfaces and displayed to control centre personnel via output units. The entire track diagram is displayed on a number of monitors. The high-reliability computer modules supplied to German signal boxes always contain three such computer units, and a power supply unit specially adapted to the purpose is also built in. The same application software runs on all three computers of a module, resulting in a triply redundant system. During operation, at least two of the computers must produce the same result before this is used to control the external components such as points and signals.

In addition to the performance and functioning of the computer system itself, further requirements are determined largely by the operating location of the systems. Normally this is a fixed location in or close to the signal box, and always in a closed, non air-conditioned building. Here only a slightly higher temperature rating must be reckoned with, for example between -20 and +70°C. Greater stress is likely to be produced by the transporting of the system to its place of operation, and for this the relevant standards and regulations must be adhered to (e.g. drop test in packaging, etc). Normal requirements apply concerning EMC, and suitable tests are carried out to prove compliance. As for the packaging system for the computer units, MEN has chosen the electronics packaging specialist Schroff of Straubenhardt. The two companies have had many years of successful co-operation. Schroff can also respond very flexibly and quickly to implement customers' specific needs and offers a palette of products that exactly match the market supplied by MEN. The packaging consists of europacPRO subracks with appropriate modifications (Fig. 2).

These include for example an additional base plate that was added to provide extra reliability as a result of detailed EMC tests. Optimised perforation offers the best balance of good EMC performance and air flow. Heat dissipation issues were addressed from the design stage, for example through the use of components such as processor types that generate relatively little heat. In this application, the main producers of heat are the power supply units. These are therefore cooled by a fan sub-assembly mounted at the bottom of the electronics cabinet.

The Schroff europacPRO subrack offers high technical standards and can be modified easily and at low cost to suit a wide variety of requirements. By looking at the catalogue one quickly gains an idea of the flexibility and modularity of this subrack and its large range of accessories, such as guide rails that offer the keying/coding of 19" sub-assemblies and ESD clips for discharging electrostatic build-up. The subracks meet the specifications of the current 19" standard, the revision now being prepared and the US standard IEEE1101.10/.11, and are thus also capable of accommodating insertion/extractor handles for large insertion and withdrawal forces in CompactPCI and VME64x applications (e.g IEEE handles). The symmetrical design allows users to insert sub-assemblies both from in front (e.g. display elements) and from the rear (e.g. PSUs) without the need to remove the cover plates.

There are always special situations that clearly require the design of a customised subrack. The flexibility of the europacPRO subrack is so great that even in these cases it is sometimes possible to realise the client's requirements quickly and at low cost through a careful combination of standard and modified standard parts. Thus, for example, special sizes can be created easily by using suitably adapted standard parts. It is possible to create specific widths e.g. by altering the length of the horizontal rail. Heightwise, the side panels are adapted accordingly and the standard cover parts combined accordingly to achieve the desired height. The depth of the subrack can likewise be varied by altering the side panels. Despite these changes to the dimensions, all expansion and accessory components can still be used.

For the railway application considered here, the installation depth of the system must be modified since CompactPCI plug-in units are inserted both from in front and also from the rear (rear I/Os). Moreover, the rear I/O boards must be positioned so deeply into the case that the railway can also introduce cabling of its own during installation. This was achieved with ease thanks to a simple modification of standard components from the europacPRO components range. Schroff has also developed a special backplane for the signal box computer, which is supplied to MEN already fitted into the ready built subracks. MEN then fits the remaining plug-in units and performs a series of function tests before delivering the systems to the railway. The systems are installed by the railway into the appropriate switchgear cabinets and subjected to another temperature test in a climate bay.