Lenz and Zimo DCC chips (* below) support an automated stopping method called ABC (assymetric braking control). This allows a very simple trackside circuit to cause a train to slow to a stop. The train can be reversed away from the stop signal. There are differences in how each company has implemented ABC.
I've got interested in ABC for two uses. Firstly, an automatic shuttling railbus which can be set to run from fiddleyard to platform, and back again on a timed sequence using features within the Lenz Gold chip. This automation requires nothing more than two ABC stopping modules. Secondly, using ABC to protect the end of the running area for a small demonstration track. The ABC module will make it impossible for a visitor to drive a locomotive into the buffers or off the end of the track.
Both decoder makers offer an option of constant braking distance. This means a train will always stop in the same place regardless of approach speed, which means it will stop ahead of a signal.
Lenz's version of ABC has two modes, stopping and slowing. They sell modules (BM1 for stopping and BM2 for slowing) to trigger the effect, though they seem quite expensive. The stop module (BM1) is well documented as an array of diodes, and a DIY one is trivial to construct. The exact number of diodes seems to vary between different websites. My experiments have suggested six (diagram on left) are required for a reliable stop with a Lenz Gold.
I have been unable to find details of the slowing module. An email sent to me indicated that the BM2 works by sending the assymmetric signal on alternate cycles of the DCC signal. This would make a home-brew BM2 somewhat more complicated than the home-brew BM1. (I have tried a four diode matrix, and that seemed to cause my solitary Lenz decoder to slow correctly, but it might be an artifact I'm seeing, rather than correct operation given the comments of my email correspondant).
A "slow approach" could be deployed at a caution (distant) signal, and a "stop" ahead of a stop signal. A simple on/off switch is used to disable the stopping or slowing module.
Lenz' implementation of constant braking distance is a bit limiting.
- Firstly, its an all or nothing setting, so it can be quite awkward to manually drive a locomotive with constant braking distance enabled as stops will always take the same (long) distance from the place the throttle moves to zero (!). Judicious use of F4 to turn on the shunting mode (disables acceleration and constant distance braking) can give some manual control. One can use Ops-mode programming to turn constant distance braking on and off, but is a pain in the neck to keep doing that. The net result is a loco has to be set for constant-distance stopping combined with automatic running.
- Secondly, the manner of stopping seems to be an initial decelleration at the programmed decelleration rate (CV4) followed by a creeping crawl up to the stop line. If the speed is too high for the stopping distance, the loco decellerates following CV4 until it reaches the distance and then stops dead from whatever speed it was doing. Not nice in either case.
Zimo doesn't offer a "slow approach" mode for ABC (they do have their own HLU system, which I think is considerably more powerful, but I think only supported on Zimo command stations), but in all other respects has far more user-settings which allow detailed customisation. [ 2018 update - Zimo do now offer "slow approach" support, if the decoder is updated to the appropriate firmware].
Zimo allows the voltage assymetry to be set in a CV, this allows a grid of only four diodes to trigger a "stop", which means the track voltage drop is lower. This does mean that if Lenz and Zimo chips are mixed, there can be a problem in Zimo chips slowing noticeably on crossing the braking zone margins.
Zimo also have a much better implementation of "constant braking distance".
- Firstly, the user can set "constant braking" for only automatic stops (ie. ABC stops) leaving throttle stops at the rates specified in CV4. (There is an option for all stops to be constant distance).
- Secondly, the deceleration is done far more effectively; the train drives at its initial speed until the decoder decides it is time to slow to reach the stopping point. The visual effect is far better, it looks as if the train driver has chosen the right time to apply the brakes depending on approach speed. Alternatively, a Zimo can be set to decellerate smoothly using a rate calculated from the speed when the loco enters the stopping zone. This mimics a driver applying gentle braking from a low speed or harder braking from higher speeds.
So, that's my quick summary of ABC Braking experiments. I'd like some more details of the inner workings of the BM2 if they are available.
(*) 21st September 2009. For another project, I've been experimenting with a CT Elektronik SL75 sound chip. This tiny chip has Lenz ABC braking support, and it works on my test rig (6 diodes, diode sensitivity set to 15 in the CT CV's). I am editing the JMRI file for this chip, so this detail should appear along with a large number of other changes. I haven't found ABC to work in other small CT chips, such as the DCX74 or DCX75.