Signs of Signals

November 11, 2018

SE8C with SMBK.

I’ve been helping Mark install signals on his layout, essentially working as the assist to the Chief Signal Maintainer, Rick. The system we’re using on Mark’s layout can be summarized as:

  • JMRI - the brains
  • Digitrax BDL168s - block detection
  • Digitrax SE8Cs - LED signal drivers
  • Digitrax SMBKs - the signals, painted silver and black
  • Tam Valley Signlets - switch control

I started out knowing very little about any of this ended up understanding the system pretty well, thanks to Rick. Turns out you can learn a fair amount simply by running wires and what is essentially data entry in JMRI’s tables.

I’ve been wanting signals on my layout(s) for years now. Thankfully I haven’t considered the technical aspects to be much of a barrier, but it always seemed so expensive. That all changed with the introduction of Digitrax’ “Plug’n’Play” signals for HO scale, the SHABC and others, for about $20-25 per plant! They’re nothing fancy – essentially a black PCB with surface mount LEDs – but they’ll get the job done and over time I can either replace them with more realistic signals, or mod them to look more realistic (silver paint, hoods for the LEDs, ladders, that sort of thing).

The combination of affordable, simple entry level signals and relatively affordable control hardware have finally made it possible to implement signaling on the Murphy Branch (never mind that the Murphy Branch was probably never signaled, as far as I know). It’s certainly not cheap, mind you, but it’s within reach. Going into the particulars of my setup is best left for a dedicated future entry (once it’s done or half working, at least), but suffice to say that a single BDL168 and an SE8C are sufficient to run the signals for my entire layout.

The photo above shows my SE8C driving an SMBK, the N scale equivalent of the SHABC signals. The SE8C includes one of these N scale double-headed test signals, which Mark is using on his layout (after a careful application of black and silver paint). I’ve built a basic “signal board”, shown below. Ribbon cables will come in from each “plant” to the SE8C on the left, and current-sensing feeders to the BDL168 on the right.

Signal board.

A little while later I set up one of the SHABC signals and made a simple video. In the video the signal aspect is being controlled manually using JMRI.

I’m presently working on wiring up blocks and putting in a few signals – among several [dozen] other projects. More to come!

Briefly: switch commands and track power

The instructions for the SE8C walk you through sending switch commands to change the signal (which is how I made it show yellow over red here). When I frist tried it, however, it didn’t work very well: the signal LEDs flickered, and the status LEDs on the boards lit up red for a few seconds. It turns out this is sort of a known issue with Digitrax command stations when the track power is off. From the JMRI docs on LocoNet:

Turnout command rejection when track power is off

Some more recent Digitrax command stations will refuse to accept switch commands when track power is turned off. This can result in a “storm” of repeated switch messages on LocoNet if track power is off when switch messages are sent. This problem can be avoided by ensuring that track power is on when switch messages are to be sent.

Groan. I actually figured this out by checking the LocoNet Monitor in JMRI, which allowed me to see that my switch commands were not getting acknowledged and were being rapidly retried. Searching on that led me to those JMRI LocoNet notes – thanks to the JMRI folks for documenting it. At least I know my equipment is working “as expected”.

Technical bumps aside, exciting times for the railroad!