External Hardware
Trackside equipment such as signals and points are connected to nearby 'trackside functional modules' (TFMs). Each module has a number of outputs and inputs. Each output drives an individual function, such as a signal lamp or an AWS inductor. Certain outputs are capable of driving a flashing lamp directly. The inputs are used to send information back to the interlocking, such as indications determined by track circuit relays or points detection circuits, for example.
There are two kinds of TFM; the signal module (identified by a red label) and the points module (black label). A maximum of 63 TFMs may be addressed by one SSI interlocking; in practice the number will be limited by timing issues and the need to allow for future expansion.
Read more about this topic: Solid State Interlocking
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