Editing Signalling control (section)

== Control apparatus ==

For more information, see also [[Interlocking]].

=== Lever frame ===
{{main|Lever frame}}
[[Image:Leverframe.jpg|thumb|right|A mechanical lever frame inside the signal box at [[Knockcroghery]] in [[Ireland]]]]
The earliest signal boxes housed mechanical lever frames. The frame was usually mounted on a beam beneath the operating floor. [[Interlocking]] was attached to the levers, which ensured that signals showed the correct indication concerning the points and were operated in the right order. Wires or rods, connected at one end to the signals and points and the other to levers in the signal box, ran alongside the railway.

In many countries, levers are painted according to their function, e.g. red for stop signals and black for points, and are usually numbered, from left to right, for identification. In most cases, a diagram of the track and signaling layout is mounted above the lever frame, showing the relevant lever numbers adjacent to the signals and points.

Hand-powered interlockings were referred to as 'Armstrongs' and hand throws in the United States.

'''Power frames''' have miniature levers and control the signals and points electrically. In some cases, the interlocking was still done mechanically, but in others, electric lever locks were used.

In a few cases, signals and points were operated pneumatically upon operation of the appropriate lever or slide.

=== Control panel ===

In a signal box with a control panel, the levers are replaced by buttons or switches, usually appropriately positioned directly onto the track diagram. These buttons or switches are interfaced with an electrical or electronic interlocking. In the UK, control panels are of the following types:

[[File:Trimley Jn IFS Panel.JPG|thumb|right|Trimley Junction IFS panel in the 1988 replacement signal box; built by [[York Carriage Works|BREL York]]]]

;Individual function switch (IFS)
:A separate button/switch is provided for each signal and each set of points. This type of panel is operated similarly to a lever frame. The signalman must move each set of points to the desired position before operating the switch or button of the signal reading over them.

:This type of panel needs the least complex circuitry but is not suited to controlling large or busy areas.

;One control switch (OCS)
:A separate switch/button is provided for every signaled route. There will be as many switches/buttons per signal as there are routes (i.e. signaled destinations) from that signal. To set the desired route, the relevant switch or button is operated. All points within the route are automatically set to the required position.

:Individual points switches are provided, but they are normally left in the central position, which allows the points to be automatically set by the action of setting a route.

;Entrance-exit (NX)
:This type of panel has one switch/button provided for every signal (except that some panels have separate 'entrance' and 'exit' devices). To set a route, the signalman operates the device for the 'entrance' signal, followed by the device for the 'exit' (destination) signal. All points within the route are automatically set to the required position and, provided all the points are detected by the interlocking in the correct position, the entrance signal will clear.

:Individual points switches are provided, but they are normally left in the central position, which allows the points to be automatically set to the normal or reverse position by the action of setting a route.

Similar principles of operation as described above are applicable throughout the world.

=== Video display unit ===

Modern signal boxes tend to be provided with [[Computer display|VDU]] based, or similar, control systems. These systems are less expensive to build and easier to alter than a traditional panel. In the UK, large modern signal boxes are typical of the [[Integrated Electronic Control Centre]] type, or, more recently, of the [[Rail operating centre|Rail Operating Centre]] variety. Variations of these control systems are used throughout the world.