SA BLOCK SECTIONS
The following SA units have block section control. This allows a number of trains to run on the same oval. As a faster train catches up with a slower train it will slow, halt and wait until the slower train has left the next block section. There must be at least one more block section than trains on the oval. For example to run two trains you need at least 3 block sections. Trains occupy two of the sections. The third is empty allowing a train to move into it. Four block sections would be better as this would leave two empty sections allowing the two trains to run at the same time.
SA block units and IRDASC units can be combined together. An economical method of operating two trains on an oval is to use one SA block unit followed by a long section controlled by an IRDASC-3. The "TIS" terminal of the IRDASC-3 is wired back to the "OI" terminal of the SA BLOCK unit. Whilst the first train is in the IRDASC section the second train will be halted at the SA BLOCK unit. As the first train passes the IRDASC-3, the second train will leave the SA BLOCK unit. The disadvantage to this method is that if a train becomes "stuck" outside the IRDASC block section a collision will occur.
SA5-SB BLOCK SECTION WITH PASSING LOOPS
This controls a loop allowing one train to arrive, the points and signals to change, then the second train to depart in the same way as with the SA5 and SA5-S. In addition there is block section control provided by the "OI" and "TIS" terminals. When the "OI" terminal is connected to 0 volts (or a "TIS" terminal of another unit) both trains are held in the loops with the signals set at danger until the "OI" terminal is no longer connected to 0 volts. Whilst both loops have trains in them the "TIS" terminal is set to 0 volts.
SA6-B BLOCK SECTION WITH STORAGE SIDINGS
This is the SA-6 with block section control. Whilst all the storage loops are full "TIS" is set at 0 volts. If the "OI" terminal is at 0 volts, no trains can leave the storage loops. The IRDOT-1 and IRDOT-Ps are arranged in the same way as the SA6.
SA7-SB BLOCK SECTION WITH ONE SIGNAL
This unit forms a block section which also operates a signal.. When the next section is empty, the signal will change to clear as the train passes detector 1 and return to danger shortly after the train passes the signal. When the "OI" terminal is at 0 volts (due to a train in the next section or a point wrongly set) the train will slow and stop at the signal. The signal will remain at danger until "OI" is released. Whilst a train is in the SA7s block section the "TIS" terminal will set to 0 volts. Detector 1 is positioned at the start of the section. Detector 2 is positioned just in front of the signal. Its purpose is to detect the front of trains stopping at the signal. Detector 3 is positioned at the end of the section. This detector is wired into the SA7-SB to tell it when the train is leaving the section it is also wired into the next section to tell the SA BLOCK unit that the train is entering that section. Hence two detectors are needed for every SA7-SB.
SA8-SB : BLOCK SECTION WITH STATION STOP
The SA8-SB forms a station stop with block section control. Every train entering the SA8 block section will slow and wait at the station for an adjustable time. At the end of the adjustable time, the signal will change to clear and the train accelerate out of the station provided the "OI" terminal is not at 0 volts. However the train will not leave the station whilst "OI" is at 0 volts. The SA8-SB holds the "TIS" terminal at 0 volts whilst there is a train in the station's block section.
PREVENTING COLLISIONS AT JUNCTIONS:
The SA units can be used to prevent collisions at junctions. The unit is connected to control the track in front of the junction. The "OI" terminal is wired to a micro switch or contact operated by a point. When the point is incorrectly set the train will wait at the junction. Where there is a diamond crossing junction the opposing line can have a block section across the diamond. Wiring the "TIS" terminal from this section to "OI" will cause trains to wait at a red signal until the diamond is clear.
Units can either be part of a block section system or used on their own at junctions.
MANUAL OVERRIDE:
Wiring the "OI" terminal of a block section to a manually controlled switch gives some interesting possibilities. The switch can be in addition to the block section wiring giving a method of gradually stopping the train. Once stopped a controller can be switched in for shunting. With the SA6-B the switch can be used to manually control when trains leave the storage sidings.