Rule 17 Unveiled...........
In the official locomotive Rulebooks there are sets of rules about standard operational procedures. Rule number 17 corresponds to the operation of headlights.
TCS decoders come equipped with many lighting options. They provide the capability to achieve any prototypical dimming features as well as some automated effects that have been built into the decoder and can be activated using CV61. Some of these effects apply to Rule 17 dimming and some do not.
One of the automated effects in CV61 applying to Rule 17 is “dimmed when stopped” which is a value of 16 in CV61. In real life the headlight would be dimmed when stopped if the locomotive is pulling into a siding to allow another locomotive to pass. In this instance the headlight will be dimmed so that the passing locomotive knows that the waiting locomotive is stopped. The dimmed light also is a courtesy to the passing locomotive so as not to blind the engineer. Headlights will also be dimmed when a locomotive stops at a passenger station so as not to blind the passengers.
The concept of Rule 17 can be quite confusing at the beginning, because it utilizes CV61 which is reserved by the NMRA for extra functions provided by the manufactures (us in this case). As a result CV 61 controls BEMF (Back Electromotive Force), Opposite dimming and Rule 17 dimming. These three features are combined in one CV. Let's start from the beginning - CV49 allows you to set the lighting effects that you desire for the white wire. CV50 allows you to set the lighting effects that you desire for the yellow wire. And see Table 11 in the supplied manual for more information.
Following the Lighting Control Table from the manual that comes with every decoder, place a value of 8 (for forward light), 24 (for reverse light) or 40 (if you want both directions) into CV49 to activate Rule 17 dimming. With those settings you will still have full brightness of the headlight in the selected direction until you hit button 4 (F4). When button 4 (F4) is activated the headlight will go to half brightness.
Just to clarify things:
If a value of 8 is placed into CV49, the forward headlight will be constant bright in the forward direction only. In the reverse direction the forward headlight will be off. While the locomotive is running in forward, with the forward headlight at full brightness, if function button 4 is pressed, the headlight will change to half brightness because Rule 17 is active in CV49. If the locomotive is running in reverse – the forward headlight will be turned off and therefore it will not respond to function button 4. The reverse light will operate as specified in CV 50. If Rule 17 is active, the reverse light will respond to function button 4. However, if rule 17 is not active in CV50 then the reverse light will not respond to function button 4.