Christopher Brimley updated August 5, 2011


Christopher Brimley's Tags


Browse Articles » How-To Text View Magazine View

  • Automated Block Signaling using DCC Signal Driver Decoders

    by David Butts

    Photos by the author

    CSX single-mast signal with x2 double-color light heads for controlling bi-directional rail traffic, Marietta, GA.
    Model Railroading - July 2006 - Page 28

    Functioning signals can add another dimension of realism to a model railroad. With the continuing popularity of DCC (Digital Command Control) on the rise it only made sense for manufacturers to couple train control with signal control products in order to offer a complete layout control package. Domestically in the United States, both Digitrax and North Coast Engineering offer a wide spectrum of DCC products, starter sets, mobile decoders, stationary (accessory) decoders, signal driver decoders (SE8C a nd SIG 4, respectively), etc. Moreover, I would be remiss if I did not mention that Lenz GmbH, which provided the foundation for the standardization of DCC through the National Model Railroad Association, also offers an extensive line of digital command control products and has some devices that c an be used to facilitate layout signaling (BM2 and BM3). The focus of this article is to share my experiences implementing ABS (Automated Block Signaling) on my 6' x 13' sectional, 1960s-era New York Central HO layout using Digitrax SE8C signal driver decoders and TrainController™ 5.0 by RailRoad & Co.®

    ABS is a method of rail traffic control whereby signals automatically control the movement of trains, passenger and/or freight, based on block detection. For example, when using searchlight-type signals that can emit green, yellow and red lights, trains can be controlled through the layout. Typically the way this might work is that a train will get a green light to proceed if several preceding blocks are clear. If preceding blocks are occupied the train will get a yellow light to proceed with caution. A red light indicates the block ahead is occupied and the following or oncoming train must stop and not enter the block. The signaling system will maintain a distance between the trains based on how the system is set up and the length of the blocks. The length of the blocks is subjective, but a good rule of thumb is that blocks should accommodate the longest train run on the layout. So if the longest trains average 8 ' to 9', then a 10' block would be practical...but that is not always possible on a model railroad.


    There are several elements needed to implement an ABS system onto a model railroad:

    • Block Detection System
    • Signal Driver Decoders
    •  Feedback Device
    • Train Control Software
    • Signal Heads

    CSX train passes beneath cantilever signal bridge in Kennesaw, GA.
    Model Railroading - July 2006 - Page 29

    Block detection is the foundation for ABS on a model railroad and on the prototype. Signal driver decoders operate the signals, and each signal head has addresses for lighting aspects. Lighting aspects are the colors of light emitted from a signal head (red, green, yellow). Feedback devices are those electronics that can transmit information back and forth to themselves, a DCC command station and or computer. Train control software (Digi-Toys/Winlock, KAM Industries, JMRI, RailRoad & Co.) and a computer are required to run the ABS unless the DCC signal driver decoder has standalone logic. The computer software has the logic to calculate signal colors based on the location of the train. Signal heads are positioned on the railroad to direct rail traffic and come in a variety of heads and mast types searchlight, position lights, tri-color, color lights, etc. Lest I forget, semaphores, in model form, can be used and were used by prototype railroads.

    My approach to this project of ABS implementation was to begin with research by reading up on signaling and reviewing the products available. Chapter 10.2 in the Digitrax Big Book of DCC by John Palmer provides a good overview on signals. Another contemporary resource is Railroad Signaling by Brian Solomon. His book is very detailed and provides prototype, historical and technical information on railroad signals. Of particular interest to me was chapter five, "Automatic Block." The Northeast Operating Rules Advisory Committees (7th edition) rules for color light, interlocking, and ABS signals can also be found in the book, rules 280 through 292.

    Choosing to go with DCC signal driver decoders and train control computer software to facilitate ABS on my railroad made the most sense to me. This is because it integrated nicely with the primary DCC control system that I use (Digitrax Chief) and allowed me to take advantage of possibilities that I could not get elsewhere. TrainController™ by RailRoad & Co.® is very flexible and has many features that I have yet to explore such as timetables and automated train running. After I had done my research I came up with a plan, then process, to make it all happen. It all revolved around several steps:

    • Layout analysis
    • Blocking the layout
    • Calculating the number of signals required (one head or two heads and mast type, bridge, etc.)
    • Installing the signals and signal driver decoders and subsequent wiring, and
    • Interfacing with the train control computer software and computer

    Layout Analysis

    Analysis of the layout means to review the trackplan and figure out how the rail traffic will flow and determine the current of traffic. Will the trains run in one direction or both directions on a given track? Where are the passing sidings for meets between trains? How many turnouts need to be treated as blocks/plants or security elements? The latter refers to a prototype term for using signals to protect sections of track and turnouts. Determination as to how the trains will run dictates the type of signaling to be employed and the number of signals utilized. A layout diagram or schematic helps to visualize the layout, map the rail traffic flow, position the signals, and provide the basis for dividing the layout into blocks/plants. Additionally, the layout schematic was an aid when it came to setting up the train control software.

    Digitrax Signal mast base (Part #SMBK) with integral IDC 10-pin plug showing the front of the mast with a pair of signal heads using LEDs. Digitrax Plug and Play SE8C Signal Driver Decoder with blue 44-pin edge card connector fully wired and attached. Loconet sockets and connections are made at the bottom of the card. The C at the end of SE8C means a computer is required to exploit all of the features of the decoder, especially for ABS.
    Model Railroading - July 2006 - Page 30

    My layout features a double-track mainline. In theory, trains run east and west. Since my layout is sectional, I decided to give each of the four sections (zones) a name, hence Town Line, West Side, Middle Yard and East Side. When an operator is facing the front of the layout (Town Line), the outermost track is the eastbound mainline track (right hand) and the innermost track is the westbound mainline track (left hand). Normal operation has the trains running in one direction on a given, non bi-directional mainline track. There is only one true passing siding on the layout, located off the westbound mainline track, and it serves multiple functions as a set-out/pick-up track and yard drill. The double-track mainline can fulfill the passing siding role by the use of the single crossovers in which a faster train can be routed around a slower train or train that has stopped for some reason. All turnouts on the double-track mainline are controlled by stationary decoders and switch machines and are treated as plants, meaning the signals are positioned to protect the turnout. Thus signals are used to indicate the alignment of the turnouts, either for a through or diverging route. The object is to prevent collisions and accidents by not running into an occupied block and not running through improperly aligned turnouts.

    Layout Blocking

    I made a layout diagram of my railroad, and this provided the basis for determining the number and locations of the blocks needed on the layout. It worked out that at a minimum I needed 16 blocks. Over time I may add more blocks to smooth out train operations in the Middle Yard and Town Line zones of the layout. Presently, eight of the blocks are treated as plants with turnouts, and the remaining eight blocks are plants without turnouts. The yard and industrial areas were not blocked. Block lengths range from under 2' to 6'. Easy Model Railroad Wiring by Andy Sperandeo provides some guidance for locating and determining the number of blocks on a layout. Though this book was written mostly with DC analog cab control in mind, the principles of blocking can be applied to a DCC layout for signaling.

    The purpose of a block is to provide separation between trains. Two short trains can be run on each main track, at moderate speed, in the same direction, and there is adequate separation (spacing) due to the arrangement of the blocks. Next, for ABS to function there has to be block detection (a means of indicating whether or not a block is occupied). A description of how I implemented block detection on my layout was described in the June 2006 issue of Model Railroading. To summarize, it is provided by Digitrax BD1s and BD4s wired to the sensor inputs on Digitrax DS54 and DS64 stationary decoders. The SE8C also has inputs for detection purposes, but I am not using them at this time. The stationary decoders are connected to the Loconet bus via the jacks on the units to provide feedback. Loconet is the Digitrax proprietary data communications bus. For data transmission Loconet uses flat six-wire telephone cable and six-pin RJ12 plugs and sockets (receptacles). This technology is derived from the computer industry and allows the hardware to be daisy chained or linked together.

    Digitrax SE8C showing the typical hookup of the IDC flat, 10-wire ribbon cable to the board. At the end of the ribbon cable a signal mast base kit has been attached. In actual use the signal mast base kit would be wired to a commercial signal on my railroad.
    Model Railroading - July 2006 - Page 31

    Signals Required

    I estimated that the 16 blocks on my layout required 32 signal heads. Over time I will add more signals as the operations dictate. To manage costs, I implemented signals over a period of time as my budget allowed. I began installing signals on the front of the layout (Town Line) and worked my way to the back of the layout (Middle Yard). My preferred signal head is the searchlight type though I have examples of other types on the railroad.

    The signal heads I used are mounted primarily to single masts. In certain locations I took advantage of other arrangements like cantilever, bracket or gantry bridges. For my railroad, blocks/plants with turnouts utilized a minimum of two signal heads and blocks/plants without turnouts used at least one signal head. I positioned two signal heads, (A1 main, A2 diverging) on a mast in front of the turnout to indicate to the train the condition of the track ahead and what it is to do (speed, diverge). The high signal head controls the mainline and the low signal head is for controlling the diverging route. Then in some cases one signal head (B main) is positioned in front of the turnout. On the East Side section of my layout I used a mast with a single head at both ends of the block/plant, A1 main and B main, on both sides of the double track for a total of four signal heads.

    Chapter 9 of How To Operate Your Model Railroad by Bruce Chubb has a thorough discussion on signals which I read to further my understanding of the different color combinations search light signals can emit (this book is out of print but may be found at swap meets). The colors emitted from the lighting aspects in the signal heads have a specific meaning, depending on the particular railroad and the rules in place. Based on what I gleaned from Bruce Chubbs book, green over red is clear, yellow over red is approach, red over red is stop, red over green is medium clear, and red over yellow is medium approach. Clear means that a train is free to go forward without obstructions. Approach is proceed and be prepared to stop at the next signal. When medium is the prefix it indicates that the train will diverge through the turnout and the colors emitted from the search light signals provide the indication.

    New York Central prototype, four-track signal bridge with searchlight-type signal heads from Model Memories.
    Model Railroading - July 2006 - Page 32

    After the purchase of the electronics, the purchase of the signals represents the next largest investment of dollars. Therefore I made the decision to signal the railroad for running in one direction on a given mainline track. This reduces the number of signals required and lowers the cost. In HO scale there are many choices for signal products. I purchased various signal heads and masts from Tomar, NJ International, Sunrise Enterprises, and Oregon Rail Supply. For my New York Central specific bridges I turned t o Integrated Signal Systems and Model Memories. Good news for NYC modelers is that Atlas Model Railroad company is scheduled to release Type G Target signals in N and HO scale.

    Signal Driver Decoder and Signal Installation

    I used the Digitrax SE8C signal driver decoder which can control up to 32 signal heads with popular LED signal types and also has the capability to control semaphores. It has 16 inputs (eight occupancy sensor inputs and eight control lines for local turnout control), Loconet sockets, and it can drive up to eight slow-motion switch machines for semaphores and/or turnouts. Yes, the Digitrax signal decoder can control 32 signal heads which really equates to eight blocks or plants, as it is designed on the principle of security elements (four signal heads per plant). Each signal head occupies a certain position in the block/plant - "A1 main," A2 diverging, "B main" or C siding. Care must be taken that the SE8C is controlling the signals in the desired direction, current of traffic, on a given track or route, (eastbound, westbound, etc). On my layout, signal head "A1 main" is positioned at the beginning of a block/plant. In the case of a block/plant with a turnout, a mast with two signal heads "A1 main" and "A2 diverging" is placed well before the throw bar, points and the frog leading into a turnout from the diverging side - see diagram example of plant signaling. For example, trains traveling eastbound encounter signal heads for "A1 main & A2 diverging" facing them. Signal head "B main" protects the opposite end of the block/plant (turnout) from rail traffic traveling in the reverse direction, against the normal rail traffic flow on a given track, a train headed west on an eastbound track. Therefore on my layout, turnouts are fully protected, and this is especially important for crossovers from main track EB to main track WB. On the eastbound track, the only spur serving an industry is located at Town Line and is protected for facing point or trailing point moves depending on the direction of the train. The example provided for plant signaling has the train coming toward the turnout and is a facing point. And vice versa, a train traveling in the opposite direction would have the turnout behind it as it passes and is a trailing point.

    Wiring from the SE8C to the signals is facilitated by IDC flat, ten-wire 30 AWG ribbon cable (see photo for example) pins and connectors (Part #SDCK). I used IDC crimping pliers to attach IDC connectors to the ribbon cable. There is a small arrow on the connector to identify the outside wire that is to be considered wire number one. Eight IDC ten-pin plugs, DRV1 to DRV8, are located on the circuit board of the SE8C t o allow IDC ten-pin connectors to be inserted. Each wire and pin plug has a purpose and relates to the four signal heads that are controlled by each of the eight DRVs - see the Digitrax SE8C manual for wire mapping. An edge card with 44 pins (22 pins per side) is used to facilitate wiring and connection to the SE8C circuit board for the external power, sensor inputs and slow-motion switch machines/motors. For my railroad with 16 blocks/plants, I needed two Digitrax SE8Cs. The first SE8C was used with the factory defaults for the signal head address range to control blocks/ plants B1 through B8. The second SE8C was programmed to the next range of signal head addresses to control blocks/plants B9 through B16 by changing the SE8C ID from one to two. Both SE8Cs are mounted together on a hinged panel connected to the benchwork. Prior to placing the SE8Cs beneath the layout I tested each DRV on the units, at my workbench, using a DCC throttle, signal base mast-equipped with LEDs and the short test IDC cabling with connectors that comes with each unit.

    I used a combination of Digitrax signal mast base (Part #SMBK) and terminal strip mount (Part #TSMK) kits to connect wiring from the LEDs in the signal heads to IDC flat, ten-wire ribbon cable, which was then connected back to the SE8C. To keep the wiring neat I cabled it together and attached it to the benchwork. The wires coming down from the signal heads through the mast to the signal mast base or the terminal strip mount need to be hooked up to the solder pad or terminal that controls the appropriate signal head and lighting aspect. To create a wiring harness for the signal base mast, I used four-wire telephone wire with black, red, green and yellow wires, and made solder connections to the solder pads on the Digitrax signal base masts for each of the signal lighting aspects, (green high, yellow high, red high) for the signal head to be used, A1, A2, etc. At the opposite end of the wiring harness I attached a European barrier strip. This allowed me to use the signal mast bases with smaller drill holes (3/32") and was more flexible than the prescribed method outlined in the Digitrax manual. The European barrier strips were hot glued into position underneath the layout on the benchwork, and the terminal strip mounts were attached with #4 wood screws to the benchwork under the layout.

    New York Central prototype, cantilever signal bridge with searchlight-type signal heads from Model Memories. LEDs from Miniatronics and signal hookup wire from Oregon Rail Supply.
    Model Railroading - July 2006 - Page 33

    The placement of the signals was determined by: 1) where the block/plant began, 2) location of turnouts to protect, and 3) the directional flow of the rail traffic (WB or EB) with the signal head facing oncoming traffic. Again, at the beginning of a block there is a signal facing the oncoming traffic, and it is classified as A1 main following the convention outlined in the Digitrax SE8C manual. On my layout, for uniformity, signal heads on a mast are located on the right-hand side of each mainline track, approximately a scale 9 6" from the center of the closest rail head to the center of the base of the signal mast, whereby they are to the right of the engineer in the locomotive coming toward the front of the signal. This treatment was based on the instructions that came with the signals I purchased from Sunrise Enterprises, which are based on Southern Pacific prototypes. However, in some situations I had to deviate from the righthand side placement in order to fit the signal mast where it needed to go (the left hand side). Signals on a bridge were placed to the front, overlooking the blocks/plants that they protected. I used a 3/32" diameter drill bit to make a hole in the sub-roadbed into which the shaft of the signal mast was inserted. The wires from the LED in the signal head are routed inside the tubular mast, through the hole in the sub-roadbed, and then to the terminal strip mounted underneath.

    Train Control Software

    I decided to use TrainController™ 5 .0 software by RailRoad & Co.®, a German firm, to provide computer-assisted control of the railroad and manage the ABS. There are several reasons why I chose this software. First, I had seen this software in action on Armin Bachers Alpine & Pacific and Norm Stenzels Brandywine & Benedictine model railroads during the Piedmont Pilgrimage, which is an annual program of layout tours sponsored by the Piedmont Division of the SER-NMRA. Both of these modelers are serious operators and members of JRAG, a round robin, prototype operations club in the greater Atlanta area. Second, Larry Puckett had written a review of the software in Model Railroading several years ago, and I found it very favorable. Third, I am not a computer science wizard, and the software is user friendly, Windows based, and point and click with a mouse.

    I am not going to fully rehash what is contained in the TrainController 5.0 software manual other than to touch on a few highlights as it pertains to the implementation of ABS on my railroad. My very first action was to set up the software to recognize Digitrax as my DCC system (Digitrax Loconet/ MS100). This is done by going to the Railroad dropdown menu and choosing the "Set Up Digital Systems" command. Then I had two main areas to focus on in order to get TrainController operational: creating fully functional switchboards and setting up the Dispatcher component. In using the Dis patcher component I chose to match my 16 physical blocks with detectors (BD1s and BD4s) to the blocks set up in Dispatcher, but the software allows the option of creating more or less blocks in the system. It pays to be systematic in installing and implementing the software. I found it helpful to use a checklist to document where I was in the process, keep track of the item names and addresses, and what needed to be done. The software manual is fairly comprehensive and should be read several times before commencing with the installation. The Help menu in the software is clear and concise and additional help is available online or by emailing the manufacturer at

    NYC Alco RS3 8304 diverges left on the Town Line single crossover as it hauls a commuter train to the passenger station located at East Side.
    Model Railroading - July 2006 - Page 34

    Since my layout is sectional and each section (zone) has a name (East Side, Town Line, West Side and Middle Yard), I decided to build a switchboard for each section. The TrainController software using Windows-based technology allowed this treatment. The switchboard is a track diagram (track schematic) control panel, and it is displayed on the computer monitor screen. Switchboards control and operate switches, signals, routes and other accessories on the layout. It helps to have drawn out a layout diagram or schematic on a sheet of paper to use as a guide in developing the trackplan in the software. Also, in my case, I chose to straight-line my track diagram in that it is linear; no curves are displayed. To draw the trackplan the software has a Tools dropdown menu where the track elements can be selected - turnouts, sections of track, etc. I must mention that the "Inspector" view, activated by clicking on the View dropdown menu and selecting the Inspector command is most helpful because it details what features have been chosen for the various elements.

    In regard to the handling of signals, the software allows the selection of three signal types two aspect, three aspect and four aspect. My signals use either two or three aspects per head and most of them, except the signals by NJ International, use three contact pins because the LEDs use three wires. Signal selections are made by clicking on the Tools dropdown menu and selecting the Accessories command to find the signal of your choice. In situations where I needed to have signals for the A1 Main and A2 Main (diverging) in front of a turnout, I used two software signals side by side differentiated by the image type. The software calculates the appropriate lighting aspect to be displayed by the signal head based on the position of the train(s) on the layout. Signals situated on the mainline are linked to contact indicators and have been set up for automatic reset using the Operations feature contained in the software.

    Once the track schematic has been drawn into the switchboard then, as appropriate, the turnouts, signals and occupancy detection sensors (BD1s and BD4s) for blocks/plants need to be identified with an address and name. Occupancy detection sensors are termed as contact indicators in the software. I had to create 16 contact indicators for all of the blocks/plants on the railroad. The contact indicators were assigned to a group of track elements using the highlight function and linked to the occupancy section. The contact indicator allows the software to monitor the state of the occupancy sensors in the switchboard window.

    The software has a "wizard" that provides prompts to guide the software user through the steps that need to be completed. I highlighted the turnout, signal or contact indicator and went to the Edit dropdown menu and chose the Properties command and followed the prompts. I matched the addresses programmed into the signal driver decoders for signal heads and stationary decoders used for turnouts to the same addresses referenced in the software for the signals and turnouts. For the contact indicators I used another set of numbers based on the quantity of plants on the layout for the addresses, 1 through 16. This is part of the connecting process contained in the software. I used descriptive names for each item. For example, a contact indicator name is "WB East Side Plant 10" and a signal name is "WB Signal East Side Plant 10 - A1." This tells me the track direction, location, block/plant number and for the latter, signal orientation (A1 Main, A2 diverging, B Main and C Siding).

    Next, I had to establish Routes. Route elements are used to operate and lock the track, switches and signals that belong to certain routes shown on the switchboard. What was required to do a Route was to highlight a track section and go to the Edit dropdown menu and choose the Properties command and select the tab labeled Route, then press the button labeled Record. Once again the software has prompts to guide the user through the steps to be performed.

    The final piece in dealing with the software, for my needs, was to handle the Dispatcher component. In order to gain access to this component the Easy Mode has to be turned off or unchecked. This is accomplished by going to the View dropdown menu and r emoving the check mark. Then go to the Window dropdown menu and choose the Dispatcher command. A series of prompts will then guide the user through activation of the Dispatcher component. The basic steps involved with the Dispatcher are to divide t he layout into blocks and enter the blocks into the software, arrange the lines and arrange the schedule. As I mentioned earlier, the blocks I set up in Dispatcher match the physical location of the blocks where I have detection sections - contact indicators - though there is flexibility to create more or less logical blocks governed by the software. Once all the components of the software have been completed (Switchboards and Dispatcher), I suggest the file be saved on CD or diskette as a backup just in case the computer hard drive fails and the work is lost.

    Tomar double-headed searchlight signal being installed on the Town Line zone of the layout. Wires are thread through a 3/32 diameter hole in the sub-roadbed plywood sheet.
    Model Railroading - July 2006 - Page 35

    Computer Interface And Feedback

    The computer interface is facilitated by a Digitrax MS-100. The MS-100 allows the computer (with an IBM-compatible COM or RS232 communications port) and software by RailRoad & Co. to communicate, monitor, receive and transmit data via Loconet to the DCC command station and other electronic devices on the layout and vice versa. The SE8C has Loconet sockets on the circuit board as does the DS54 and DS64, which provide sensor inputs for the detection devices. All of the electronics can communicate data back and forth to each other via Loconet computer, DCC command station, signal driver decoders and stationary decoders and this constitutes feedback. As an alternative to the Digitrax MS-100, the Loco Buffer USB can be used as a computer interface.


    Signaling can be complex. My purpose with this article was to boil it down, simplify the subject, point out some resources, and s hare how I implemented ABS signaling on my model railroad. It was not my intention to follow prototype practices exactly. For those modelers interested in following a particular prototype more closely I would suggest obtaining an operating-rules manual for that railroad for more specific guidance. In my case, I wanted to add another element of realism to my layout and this was accomplished. The message is that with DCC signal driver decoders and the appropriate computer software, ABS signaling is within reach of and attainable for the average modeler. The Digitrax DCC products I used are not limited to model railroads controlled by Digitrax DCC command stations and can be used with other DCC and non-DCC systems, especially the SE8C signal driver decoder ( Although I am using a computer and software to handle my ABS, Team Digital sells and manufactures a Loconet-ready device called the SIC24. This electronic device can control the Digitrax SE8C in the ABS mode. When using the Team Digital SIC24, a computer and software are not required, but it wont let you exploit all of the Digitrax SE8Cs rich feature set.


    I would like to recognize the following individuals for their support in my efforts with this article: Peter Youngblood for his editorial input, William Ello for his guidance in using digital camera equipment, Sandy Kersey for the inspiration to share what I have learned on ABS with the modeling community, and in memory of the late Bill Jewett, who was the former editor of The Dispatchers Office, journal of the Operations Special Interest Group.

    Article Album (1 photo)

    Share - Report