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The Computer Advantage

(Adapted from material in the V3.0 Railroader's C/MRI Applications Handbook)

The C/MRI can perform many tasks on your railroad, such as interfacing with Command Control (DCC being the most popular), signaling, computer cab control, computer clock control, lighting control, layout animation, automated display mode operation, and many other functions. However, in almost every C/MRI application, emulating prototypical signaling is a core ingredient.

C/MRI applications have proven many, many times over that the dream of an easy-to-install and simple-to-use model signaling system is readily attainable. With today's market flooded by the rapid advances of the computer industry, the power of the computer age is now available at bargain basement prices, frequently free for the asking, for anyone wishing to add computer capability to their model railroad.

As you might expect, prototype signal relay logic can easily be reproduced through basic computer programming. The result is a flexible signal operating system that is both easy and affordable. By adding an SMINI card to enable your computer to interface with the railroad, you have the makings of a fully functional signaling system that can be as simple as the most basic Automatic Block Signal (ABS) system, or as advanced as the most complex Centralized Traffic Control (CTC) system. If the size of your system needs more than one SMINI then you will need to add an RS485 card to support their interconnection.

Proper signaling for a layout is more complex than just looking at block occupancy or a simple turnout position and lighting a signal. To clear a signal properly you must ensure that all turnouts are correctly aligned for the desired track, that the trackage is clear, that no spurs are unlocked and that no other signal is clear for another train to enter the same trackage. As you can imagine, such crosschecking requirements quickly become increasingly complex as your trackage arrangement becomes more complex. Fortunately however, such arrangements are easily handled with software at zero cost. By contrast, trying to handle even simple interlocking plants via specialized dedicated logic circuitry becomes very complicated and expensive, as well as non-flexible.

The underlying principle to prototypical signaling is that it's very logic intensive. This makes signaling an ideal computer application. Here are some neat advantages you will receive by using the computerized approach to signaling.

Low Cost

A second-hand computer is essentially a zero cost investment. When used for signaling, the computer supplies all the necessary logic functions for free. Fundamentally, using a computer for signaling there is no added cost for implementing all the logic functions. No special signal logic cards are required as that comes free with the computer. All you need are the signals themselves and a straightforward interface circuit, such as the SMINI, to connect them to the computer.

As an additional saving, the oscillating voltage required to achieve the yellow aspect with the 2-lead 3-color LEDs is built into the SMINI's capability, so no additional signal driver circuitry is required. Also, the card is capable of driving a Tortoise switch motor directly, and reading an occupancy detector directly, so again there is no requirement for any other external circuit cards.

In addition to the low cost of the computer, the SMINI is an extremely economical way to drive signals and switchmotors. The low cost of this card places signaling within almost everyone's pocket book. For example, Do-It Yourselfers constructing a reasonable size system with quantity discounts can achieve costs of the order $110 per SMINI. This corresponds to approximately $1.50 per I/O line. This way we are talking a $4.50 total cost to drive a 3-color type D signal or $3 to drive a 2-lead or 3-lead 3-color LED searchlight signal.

Do-It Yourselfers building an even larger system with an assumed average quantity discount of 20 percent can get the cost down to as low as $72 per SMINI. This corresponds to $1.00 per I/O line or $3.00 for a 3-LED signal and $2.00 per searchlight signal. This makes the cost of all the electronics almost insignificant when compared to the cost of the signals themselves.

Going the complete kit route, we are talking $100 per SMINI or about $1.40 per I/O line. Fully assembled and tested SMINI cards cost $160 or about $2.22 per I/O line. If your need is to build up a small number of cards, then going the kit route, via Sliq Electronics, is the most economical approach. Doing so also saves time from not having to place orders for electronic parts plus it saves on shipping and handling charges.

However, if you are building up a significant number of cards, then going the Do-It Yourself route, where you purchase your cards directly from JLC Enterprises, at quantity discount, and then buy your own electronic parts, from the recommended sources called out in the parts lists, at quantity discount, then you can achieve substantial savings of the order of 40 percent lower than the kit prices.

Do-It Yourself cost data is presented as estimates only and do not include shipping and handling charges which can easily amount to $30 when one considers ordering parts from 4 different suppliers. Such added costs tend to make the prices charged for complete kits more reasonable then they might at first appear. However, when ordering large quantities of parts to create a sizeable system, the impact of shipping and handling charges becomes relatively insignificant.

Easy Expandability

Each SMINI card can handle 72 I/O lines (48 outputs and 24 inputs). Simply connect your signal devices directly to the nearest node and that's it! For example, a single SMINI can drive 48 signal LEDs and read 24 occupancy detectors or turnout and electrical switch position inputs. Depending upon the size of your layout, a single card may be all you need to add signaling to your railroad.

When more I/O lines are needed, simply distribute additional SMINI cards around your layout to build up to whatever capability you desire. This way each node is close to the signaling devices being interfaced, significantly reducing the need for a lot of wire.

For readers requiring large amounts of concentrated I/O, such as at a lever-type CTC machine, simply install an SUSIC based Maxi-node instead of an SMINI. Each SUSIC based node can support up to 64 of the new 32-bit I/O cards yielding a maximum capability of 64 x 32, or 2048 I/O lines per node. Because a distributed system can be expanded up to 128 nodes, the maximum interfacing capacity is 262,144 I/O lines, calculated as 64 x 32 x 128.


The only wiring required between nodes is a single 4-wire cable and this same cable is the only connection back to the computer. You do not need any signal logic wiring running from one signal logic card to another. There are no relays or multi-deck panel switches.

With the C/MRI, you simply connect each signal device, whether it is a pushbutton, a toggle switch, a switchmotor, or a panel LED or the signal itself, directly to the nearest node. All the complex signal logic is handled by software running on your computer. You just can't get wiring much simpler!

There are also numerous "canned" software packages available so that you really do not need to develop your own software. For those that wish to roll your own, there is an abundance of software examples to pick from, including those provided in the C/MRI User's Manual as well as in this handbook and the disks associated with each of these publications. Additional examples are available via the C/MRI User's Group.


Low cost and implementation simplicity are usually sufficient to justify going with the computerized approach to signaling. However another tremendous advantage of the C/MRI approach is system flexibility. Fixed hard-wired systems using groups of highly interconnected, specialized signal logic cards are difficult to change once they are installed. Alter your track arrangement a little, add in another signal or signal head, or select a different style signal and you encounter major rewiring.

By contrast, using computerized signaling, even extensive layout changes typically require minimal wiring changes. Frequently, the total change requirement is simply a few statement alterations in the software. The software is extremely easy to regenerate and update as needs change.

For example, suppose you decide you want to add approach lighting to all your signals - simply add a few statements to your program. Should you decide to change that junction with a foreign railroad to use semaphores - simply change the aspect constant in your software. Perhaps you desire to add flashing signal aspects - simply add a couple new statements to your signaling program.

Suppose you decide to add a protected grade crossing in the middle of one of your dispatcher controlled OS sections - no problem. Simply connect the grade crossing protection device to a spare output line on the nearest node and all the required logic changes are easily handled via minor software updates. The same is true if you decide to add a second turnout within an OS section to create a branch line or to incorporate a new major junction somewhere else on your railroad. Most modelers over time make alterations to their railroad. Having the signal changes imbedded in the software rather than in the hardware makes life easy.

I have been installing model railroad signaling for over half a century. At age eleven I built my first 10-lever mechanical interlocking plant to control track switches and signals at a junction on my 0-27 Sunset Valley. In the early 1970s I authored three articles in Model Railroader explaining the application of relay logic to control signals on the original HO scale Sunset Valley. Over the years I have gained extensive experience hard-wiring all types of specialized fixed-logic signaling cards. Every time you want to make a change - even simple layout modifications - the resulting wiring changes can become a nightmare.

The situation is exactly opposite when using a computer. The most wiring changes you will ever see are adding or deleting of a few simple I/O connections. All complex logic changes are handled by making easy software modifications. I can assure you from over 50 years of signaling experience that nothing comes close to the simplicity, affordability and ease of making changes provided by the computerized approach. It's a hundred times easier to make a software update than it is to change the wiring in a hard-wired signaling system.

Prototype Fidelity

The higher the level of prototype fidelity you seek the more optimum is the choice for computerization. Prototype signaling involves all types of specialized aspects, the incorporation of circuit delays and timeouts, specialized interlocking between all types of levers, buttons and toggles, and for modern signaling even keyboard/mouse inputs. Such operations are easily implemented by using the available-for-free inherent power of the computer.

For example, implementing an interlocking plant at a railroad junction, a terminal or at the entrance to a staging area is a snap with the computerized approach. The system works equally well whether the plant is an early era lever-type plant, an eNtrance/eXit (NX) style pushbutton plant, a modern keyboard/graphical plant or a fully automated plant. Also, nothing beats the computerized approach for implementing a dispatcher CTC panel, whether it is the older lever-style or a modern system using graphical display panels. Why use the computer? Using the computerized approach results in an easy to use system employing extreme flexibility coupled with prototype fidelity and packaged as a most cost effective solution to meet every signaling need.

The computerized advantage is independent of whether you desire to achieve a simple but totally effective system or a full-blown prototypical system using Automatic Block Signaling (ABS), Absolute Permissive Block (APB) signaling, junction and terminal interlocking plants, and/or a complete implementation of Centralized Traffic Control (CTC). For readers desiring greater insight into how these different signaling systems function, and how the techniques can be applied to your model railroad, I highly recommend studying Andy Sperandeo's article Understanding Railroad Signals in the December 2002 issue of Model Railroader .

As Andy points out several times, all signal logic is readily convertible to software and such software can be easily executed by a personal computer. To take advantage of this relationship, we need a simple way to interface the computer to our railroad's basic signaling elements. The C/MRI, and especially the SMINI card, provides us with that capability.