Programming 101–Service Mode
There are two ways to program decoders, (1) service mode, and (2) ops mode. In the past I have talked a lot about ops mode programming so let’s focus a little on service mode. With service mode you can program all CVs without regard to the decoder address. Accordingly, all decoders in locomotives on the service mode programming track will be reprogrammed. This was an important feature for many years because with some decoders it was the only way to change the address–you couldn’t do it with ops mode. However because it indiscriminately programs any decoder on the programming track you need to be careful with some DCC systems. Some entry level systems such as the Digitrax DB150 and NCE PowerCab only have a single pair of track connectors that are used for both powering the track and for service mode programming. Because of this it is necessary to have an electrically isolated service mode programming track to prevent programming all your locomotives.
To make this possible you need to have some way to switch your track output from the main track to the service mode programming track. This is easily done using a double pole double throw switch set up with the single pair of wires from the command station connected to the two center switch contacts. Another pair of wires connected to two of the contacts at one end of the switch are run to the track power bus. Finally, a third pair of wires are connected to the two contacts at the other end of the switch and run directly to the programming track. When you want to run trains flip the switch to route power to the main tracks and when you want to use service mode programming switch it so power goes to the isolated programming track.
The problem is remembering to flip the switch—one forgettful moment could result in reprogramming all your locomotives. Because this is enough of a problem for users, NCE now offers a circuit, the Auto-Sw, which senses service mode programming commands and switches power to the programming track for you. This device is not system dependent so you can use it with the Digitrax DB150 or any other entry level system that does not have separate track power and programming outlets.
One useful feature of service mode programming is the ability to read the CV settings in a decoder–you can’t do that with ops mode. (To my knowledge only Lenz has offered a way to read CVs in ops mode and that requires installation of special feedback modules). Using this feature you can check the settings of particular CVs so you are not programming blind. For example it is helpful to know your current momentum settings when making changes just to give you an idea of how much of a change is needed. The same is true for sound levels and speed curves.
One downside to service mode is the current to the programming track is limited to about 250 milliamps. This is done to prevent damage to a newly installed decoder if it is not correctly wired. The downside is that with many sound decoders and when keep alive capacitors are installed, there may not be enough current to program the decoder. This is why Soundtraxx and DCC Specialties introduced the PTB-100 and PowerPax programming track boosters. Ops mode offers a convenient work around for this which I will discuss next time.