The diodes in the supply wire to the section or station block provide a small asymmetry of the digital supply voltage as long as the contact is open (signal closed). As soon as an engine runs into such a track, the module decodes this asymmetry and starts to count down the internal speed reference counter, until the engine stops. Of course the track must be long enough so that each train will stop before the end of it. You can program the brake distance for each engine individually. The engines may be reached for telecommands also in front of closed signals, in contrary to solutions with special brake modules (e.g. DIGITALplus) or DC voltage (e.g. Digitrax). Another big advantage: Trains passing the track isolations cannot short circuit different supply voltages as with those other solutions. Of course all this will happen without turning off the lights as with some PC-solutions!
To start again you just close the signal contact. So the diodes are short circuited making the supply again symmetrical. This makes the module accelerate the train to its programmed signal operation speed. Should the signal open during a brake, the train will accelerate again, without any jump, of course! After a powerup trains on open track will accelerate, meanwhile those in front of closed signals will wait for green light.
Another impressive advantage is the symmetry of stop position. You can enter to each block from both sides without any switchings. The engine coming from the opposite direction will therefore not stop at the begin but at the other side of the signal controlled range. This offers very flexible layout concepts, where all the tracks may be used in both directions. Just put signals with contacts on both sides of a signal block applying just one diode combination in the supply. So changing your layout to ATL system is quite simple and cheap.
ATLplus modules allow even braking to stop in two steps automatically. In the engine module the two-step braking is activated and a reduced speed programmed. A locomotive entering the brake area of a station will slow down to the reduced speed and then proceed through the station to the signal. There it will brake down to stop by a simple change to the opposite polarity of the diodes asymmetry. To do this you need a bistable relay triggered by a reed contact in the rail just before the signal (magnet under the loco needed). The relay is then set back from the green light of the signal which makes the engine go back to low speed cruise. As soon as the engine leaves the brake area it will resume the normal speed. By the way, you can use this function also for working areas on your layout, where trains should slow down automatically. Nice function, isn't it? And all without PC!
New: With the new ATL2066 the stop in front of the signal may also be triggered by a reed contact that is on the engine and connected to the module. Then you need only a magnet in the rail in front of the signal, but no relay control. The polarity of the diodes asymmetry has no meaning any more. For stops in both directions the magnet must be put in the middle of the station.
Another new feature is the exponential instead of the linear brake curve. This
is for engine drivers, who start with strong braking in sight of a signal and
then loosen the brake at lower speeds successively just to roll out towards the
signal. All this is programmable in the new ATL2066.