Introducing Modular Signal System Components

You may have heard about the Modular Signal System – it’s been slowly gaining support in the Free-mo modular community for about a decade now.  If you haven’t, read on – it’s an exciting new (well, somewhat new) option to bring ABS signalling and more to your model railroad.

The initial Modular Signal System (MSS for short) proposal was put forth by Gregg Fuhriman in the February 2005 issue of RailModel Journal.  He’d developed the idea along with others to bring simple signalling capabilites to Free-mo modular meets.  Traditional solutions, using pieces such as C/MRI or Loconet-based systems, are impossibly cumbersome to deal with in an infinitely-reconfigurable modular setup with participants coming from all over.  What was needed was an acceptably realistic signalling system that was plug-and-play – no reconfiguration required for the myriad of ways their modules could be put together at each meet.

Modular Signal System 101

What Gregg and team have developed is nothing short of brilliant in its elegant simplicity.  MSS, at its core level, provides an approximation of a basic US prototype Automatic Block Signal system.  It’s fundamentally designed to be easy to implement.  Essentially, you plug the modules together with standard Cat5e crossover network cables, hook up the detectors and signals, provide a little accessory power, and you’re up and running.   There are no computers involved whatsoever in the basic setup.  MSS is not going to model your prototype’s strangest interlockings and aspect rules straight out of the box, though realistically with a bit of custom work you can likely get close.

Probably the best way to get started is a diagram, showing how the basic pieces go togther.  Here’s a simple diagram of an MSS setup, shamelessly stolen borrowed from the MSS website:

MSS Overview Diagram


MSS systems consist, at a minimum, of cascade nodes connected by crossover cables.  Cascade nodes are used at block signal boundaries.   Each is required to contain two current detectors for the blocks on each side of the signal and an optical detector to watch for trains right at the boundary, and then also needs signal decode and driver logic.  The dual optical-current detection provides one of the most clever features of MSS:  there’s no need to resistor-equip all your freight cars.   The engines will trip the current detectors for a block, but the optical sensors at boundaries will hold the signals to occupied until the train has passed completely.

The wiring between the modules is about as easy as it can get.  Using standard Cat5 ethernet cables wired in a “crossover” configuration, you just plug one cascade into the next, following the track.  That leads us to the first rule of MSS: you absolutely must have an odd number of crossovers between cascade modules.   The second rule of MSS club is….  er, nevermind, different movie.

You can purchase crossover cables from many places. Don’t worry about anything higher than Cat5e – MSS doesn’t transmit any high speed data, and would work just fine over coat hangers if they were wired up correctly.  (RJ45 ethernet jacks are just much more convenient…)  In fact, stay away from Cat6 cables as they’re generally wired for gigabit ethernet crossover, which reverses all four pairs rather than just two.   I personally recommend either making your own cables from a box of Cat5 and ends or purchasing them from a supplier like Monoprice.   The advantage of making your own is that you can build them to the exact length you want, whereas premade cables are only available in fixed lengths.

You’ll notice on the diagram above there’s a second type of fundamental module.  In a modular environment, there will be modules without a signal boundary, but we still need to get occupancy data from these.  Therefore, the MSS standard identifies another piece of hardware:  the crossover module.  Its purpose is to provide a current-based block detector connected to the MSS bus, and provide one “crossover” of the MSS signal wires that counts towards the rule of always having an odd number between cascades.

For further information, I’d direct you to the two user guides on the Modular Signal System official website:

Modular Signal System User Guide – Part 1: Fundamentals
Modular Signal System User Guide – Part 2: Planning

Meet the ISE Modular Signal System… Modules

Up until now, implementing the Modular Signal System standard has typically consisted of collecting the pieces – optical detectors, current detectors, signal logic, power supplies, RJ45 breakouts, etc. – from various suppliers and then spending tedious time wiring everything together.   Since we already offered many of the pieces needed to equip a module with MSS support, one of our long-time customers who’s heavily involved in Free-mo asked about a completely integrated module at the 2015 St. Louis RPM meet.  Basically, put all the pieces on one board where you plug in the wires, configure a few things specific to your module, and you’re good to go.

Before we got this request, I actually hadn’t heard of the MSS standard.  However, upon doing a little more research, I realized that not only does this have great potential in the modular community, but offers the sort of affordable, plug and play ABS signal solution that many of our home layout customers have been asking about.

Working hard in the UK with the Flying ScotsmanDesign work started immediately last fall, but has gone rather slowly because of other demands in my life – mostly my day job.  My day job has taken me back and forth across the Atlantic more than I’d really like, leaving little time for R&D for new ISE products.  So, after bringing the MSS-CROSSOVER to production relatively quickly, the MSS-CASCADE took quite a bit longer as time to work on it in dwindled.  MSS-CASCADE first testingInitial prototypes started working on the bench in about February, but it took until July to get the firmware working perfectly, and nearly September to complete the necessary documentation.

Finally, at the 2016 St. Louis RPM, we were finally ready to show the world and debuted our plug-and-play MSS modular system with six modules – five cascades and one crossover.

The 2016 STL RPM Modular Signal System Demo Layout #2The 2016 STL RPM Modular Signal System Demo Layout #3The 2016 STL RPM Modular Signal System Demo Layout


The MSS-CROSSOVERFirst to market is our MSS-CROSSOVER, which is exactly what it sounds like – a crossover module with one of our DCC block detectors combined with  a power regulator, a secondary occupancy input, rugged and reliable MSS bus interface circuitry that will withstand misconnections and static zaps, and two RJ45 connections for the bus.  It’s designed to run off 7-18 volts AC or DC, so it’s compatible with Free-Mo aux power or just about anything else commonly found on a model railroad.    In addition, it’s got an auxiliary occupancy input, so that you can trigger the module to send occupancy to the MSS bus from a toggle switch or switch point motor accessory contacts.   My intention was that it could be used for open siding switch detection and module testing, but I’m sure there are a dozen other uses I haven’t even considered.

If you’re curious to see how it’s connected up, download the manual and give it a read.  If you’re ready to add one to your module right now, the MSS-CROSSOVER can be ordered from our store.


Our second offering is the MSS-CASCADE, which integrates everything you need for a cascade node – two DCC current-based block detectors, an infrared detector, accessory occupancy inputs for both blocks, power regulator, and signal control logic.

The MSS-CASCADE detectors work just like you’d expect – they’re based on our tried and true CKT-IRSENSE and CKT-BD1 designs.   There’s an auxiliary occupancy input for both sides, so that you can wire in test switches or open siding switch indicators.

On the signal decode side of the cascade, we’ve tried to support a variety of common signals and indication combinations that we possibly can on the MSS-CASCADE.  The current version supports 3 LED signal heads wired as either common anode (common positive, which is most model railroad signals) or common cathode (common negative – all Atlas and some Tomars), 3-wire red/green LEDs wired common anode or cathode and 2-wire red/green LEDs for searchlight-style signals (GRS SA, US&S H-series).   The board contains integral resistors, so there’s no need to hand-wire them between the board and your LED signals.  Optionally, approach lighting of signals can be enabled as well if your prototype wasn’t always lit.

It has enough outputs to control double-headed signals in both directions, though it obviously can control single-headed signals as well.  By using dual-headed signals, the cascade can take advantage of the “diverging approach” signal present on MSS v2 systems and present modern aspects such as “Advance Diverging” (UP) or “Approach Medium” / “Advance Approach” (BNSF) one block ahead of a switch set to the diverging route.

If the second head isn’t being used, its outputs become a 3-LED common anode output mirroring the signal head aspect.  The intended use is for fascia signal repeaters, but there’s probably more things you can do with it.

The aspects displayed for each of the possible states are selectable for each direction from a number of common combinations.  In addition, it’s just mapped through software, so customizing the modules to different aspects will be fairly easy.

The outputs should be able to be connected to a semaphore driver as well as long as you don’t pick an indication mix with any flashing aspects, but for weirder stuff you’re going to need our input.  If anybody is wanting PRR position lights, B&O/C&O CPLs, or even stranger stuff, talk to us.  We can probably make the modules work, but we might need to tinker with the hardware a bit.

The MSS-CASCADE should be available shortly from our store.  It includes everything you’ll need except for the signals themselves and the cables to connect it to the blocks on either side.


MSS-DEBUG being used at the benchThe final new item in today’s MSS lineup is the MSS-DEBUG, a tool for monitoring the MSS bus and debugging issues.  Honestly I’ve debated for several months now if this was a tool or a product, but turns out I’ve found it so handy in tracking down issues that it seems that it might be useful to others as well.

I originally built the MSS-DEBUG so that I could exercise and test MSS nodes on the workbench, without needing to connect them to other MSS nodes.  It’s a simple handheld device that plugs into an MSS node and uses LEDs to show the state of all six signals as seen by the node, plus has switches so that any of the signals can be set manually.   The signals monitored and capable of being triggered are:

  • Local block occupancy (red)
  • Approach in (amber)
  • Advance Approach in (white)
  • Approach out (amber)
  • Advance Approach out (white)
  • Diverging Approach in/out (blue)

To allow the device to work without the bus connected to another signal node, it needs +12V to power the pull-ups.  I didn’t want it to be dependent upon external power sources, so it includes a small power supply that creates both the +5V for the LEDs and logic as well as +12V for the pull-ups from a trio of AAA batteries, installed on the back.

The MSS-DEBUG is available from our store for $25 if you’d like to add this handy tool to your toolbox.

Other Products That Support MSS

Our modules might not be for everybody – you may have a module with some sort of signals that we don’t yet support, such as PRR position lights, or you may already have your MSS wiring in place and just need a new detector.  Good news!   The detector components of our MSS modules are available as standalone products as well.

Our CKT-IRSENSE and CKT-IRSENSE-2PC products provide highly reliable optical detection that’s immune to ambient lighting of all sorts.  They provide exactly the sort of open drain / open collector output that you need for driving the MSS occupancy wire.  They’ve already been installed on a number of MSS-enabled modules.

If you’re also looking for new DCC current detectors, then look no further than our CKT-BD1.   The BD1 is a current detector designed for DCC that provides very sensitive isolated detection, and can be tuned to be as sensitive or insensitive as you’d like.

For any industry switch on your layout or module, be sure to protect the mainline with one of our CKT-TIMELOCK switch time lock simulators.  While compatible with a wide variety of signal systems, it works perfectly well with MSS and provides an extra layer of realism for crews switching industry spurs.

If you need to convert AC power from the Free-mo auxiliary power bus to the DC needed for various detectors and signal drivers, then there’s our CKT-DCCPWR.  It’s a simple board that converts AC to DC and provides a filter capacitor, and provides an easy way to get DC power at an affordable price.

I Can See The Future…

Okay, maybe I really can’t see the future.  I wish I could, then I’d have an idea how the next 9-month work-pocalypse that I’m facing is going to turn out.  Or I’d just go win the lottery and retire.  Or I’d know whether the Cubs are really going to win the World Series this year (and I could get my tickets to what will be final game early, while they’re only marginally insanely priced…)  What I can see are things that are in progress on my lab computer.   We’ve got a few more MSS goodies in the works at this point:

  • MSS-TURNOUT – A “complex cascade” module designed to signal the end of a siding or a junction.  It’ll have three current-based block detectors, the usual optical detector, three bus inputs, and a ton of signal outputs.
  • Arduino-Compatible MSS Building Blocks – Since we realize we can’t produce enough varieties of modules to fit every piece of trackwork, the obvious solution is to provide building blocks through which folks can build their own.  The Arduino provides a natural core for all of this, since it’s very easy to program and becoming more popular by the day.
  • MSS-CTRLPOINT – This one’s still a bit speculative, but basically the goal will be to build an MSS-TURNOUT module that acts like a proper control point, so that you can easily bring CTC to your layout.  You’ll have to remotely line routes through the CP.  I haven’t worked out the details yet, but my goal is to make it JMRI-compatible somehow.  That may require finally finishing the MRBus-to-JMRI code, or I may embrace either the C/MRI protocol or LCB to do it.


    1. Sorry, but the isolated transformer type detectors we’ve built into the MSS product line won’t really work reliably with DC. We’ve considered this before, but unfortunately a detector that works acceptably on both DC and DCC wouldn’t be optimal for either one. Since the Modular Signal System standard came from the Free-mo world, and they’ve standardized on DCC, we decided to target that first.

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