Block Sections
A block section is the length of track between two signals on which only one train is allowed to move. The IRDASC and SA-BLOCK units can control the signal settings according to the movements and positions of trains in the block sections. The SA BLOCK and IRDASC-3 also allow several trains to run on the same track without colliding.
The SA5-SB, SA6-SB, SA7-SB and SA8-SB give block section control with gradual acceleration and braking of conventionally controlled locomotives. The other units will work with both conventionally and digitally controlled engines. Using the IRDASC 3 prevents collisions by isolating the track in front of signals when there is a train in the next block section. The stop will be abrupt however.
The IRDASC and SA-BLOCK units have two terminals for block operation. Terminal "TIS" (Train In Section) and terminal "OI" ("Override\Interlock").
Terminal OI is to send the message into the unit, "Don"t let the train leave your section". Terminal TIS is to send out the message, "my section is full".
When a train is in a block section the "TIS" terminal is at 0 volts. "TIS" sets to 0 volts when the "TES" (Train Entering Section) terminal is operated. If OI is not operated the signal will change to green for the approaching train, returning to red as the train leaves the block section.
When "OI" is connected to 0 volts the signal stays at red for approaching trains. Removing the 0 volts connection returns the signal to green if a train is in the section.
To provide block section working the "TIS" terminal is wired to the "OI" terminal of the previous unit. This wire "tells" the units when the next block section is occupied. Suppose a train is in section C. Section C"s "TIS" terminal will then be at 0 volts. As "TIS" is connected to "OI" of section B, section B"s signal will remain at red when a train enters its section. Once the train leaves Section C then the "TIS" terminal will switch off the 0 volts (it goes open circuit) and so the signal changes to green.
To provide block section working the "TIS" terminal is wired to the "OI" terminal of the previous unit. This wire "tells" the units when the next block section is occupied. Suppose a train is in section C. Section C"s "TIS" terminal will then be at 0 volts. As "TIS" is connected to "OI" of section B, section B"s signal will remain at red when a train enters its section. Once the train leaves Section C then the "TIS" terminal will switch off the 0 volts (it goes open circuit) and so the signal changes to green.
For the IRDASC-3 the track is isolated when the signal is at red. Hence the train will stop at section B until section C is empty. The SA BLOCK units will cause the train to slow gradually, halt at the signal and accelerate gradually once the next section is clear.