Absolute-Permissive Block Signaling

by Rich Weyand, with Bill Pistello & Bill Reid

This model railroad electronics article originally appeared in Mainline Modeler magazine in the November 1994 issue. Excerpts of it will also be published in N-Scale magazine sometime in 1995 or 1996. We would like to thank Bob Hundman of Hundman Publishing, publishers of Mainline Modeler and N-Scale, for being so supportive of our projects and encouraging us to make these articles available on-line.

Information on ordering a commercial kit or assembled and tested unit of this circuit is available from the TracTronics Price List.

A Modular Electronic Control System

This is the tenth in our series of articles describing a set of electronic building blocks we have designed to control our model railroad layouts: Rich Weyand's N&W Pocahontas Division, Bill Pistello's Union Pacific, and the Reid brothers' Cumberland Valley System. This month we will continue the discussion of signaling. We began last month with a pair of signal driver circuits which provide a standard connection to the many different types of signals we can use, and showed how these circuits can be used to implement Automatic Block Signaling. This month we will describe a new circuit which will allow us to implement Absolute-Permissive Block signaling.

We are presenting these circuits both as circuit diagrams and as circuit layout patterns, allowing readers to breadboard or etch their own circuits. This month's circuit is not yet available as a commercial product, but TracTronics will be bringing it out as a commercial product in the future. As we mentioned last month, the series of articles has caught up with our product introductions, which has both good and bad repercussions. The good part is that the prototype board layouts are a little easier to etch, usually being single-sided and with wider traces than our commercial product. The bad part is that the boards are bigger than they need to be, and they have not been tested as thoroughly as our commercial products.

Absolute-Permissive Block Signaling

APB is a common signaling method used on prototype single track lines. It consists of two absolute signals which control the entrance of trains from either direction onto each single track portion of the line. Between the absolute signals, intermediate permissive signals maintain spacing between trains heading in the same direction. These intermediate signals are called permissive, because the most restrictive signal, red, means "stop and proceed at restricted speed." In contrast a red absolute signal means "stop."

In operation, the system works in this way. When no trains are present, all signals are green, in both directions. When a train leaves a double track portion and proceeds onto the switch in front of the green absolute signal, all opposing signals turn red all the way back to the next double track portion. Each signal circuit knocks the next one down, and so the opposing signals are said to tumble down, like dominoes. The absolute signal and the intermediate block signals in the train's direction now operate like ABS signals, so that the signal immediately behind the end of the train will always display red, and the signal behind that will always display yellow, keeping following trains in the same direction spaced properly.

The opposing signals will remain red until all trains are out of the single track portion. All signals will then go back to green, permitting a train from either direction to proceed onto the switch and through the absolute signal into the single track portion and start the process over.

This is a very simple description of how an APB signaling system works. It is not the goal of this article to instruct in how APB systems work, but in how to build one. John Armstrong's book "All About Signals" from Kalmbach is a good source for more information on APB systems. Be assured, however, that we know how they work, and the system we describe here is really very close to the prototype in operation.

Direction Precedence Circuit

Last time we described two circuits which allow us to implement ABS signaling. If we now add a direction precedence circuit to our bag of tricks, to determine and remember which way the signals should tumble down, we can implement APB signaling with very little additional work. The circuit we designed for determining direction precedence, which we call WhichWayTM, is shown in Figure 1. This circuit is not just forward logic. There is some logic feedback in this circuit which makes it difficult to follow, but I'll try to describe it.

The inputs to this circuit, EOS, IB1, IB2, IB3, IB4, and WOS are from the track detectors of the single track portion and the OS sections. The small section of track containing the switch, which is from the fouling point of either branch of the switch to the absolute signal, is called an OS section, where OS means "over switch." The OS section at each end of a single track portion has its own detector. The circuit input EOS is from the east OS section detector, the input WOS is from the west OS section detector. The inputs IB1 through IB4 are from the intermediate block detectors on the single track main line. The outputs EAST and WEST connect to the D direction inputs of the AutoBlock signal driver circuits for the eastbound and westbound signals.

The key to the circuit is two feedback loops, each made of two 7438 open-collector AND gates, which are used to remember which direction takes precedence, and to hold that precedence until all of the detector inputs to the circuit are released. These feedback loops act like one bit memories to remember the direction required.

If none of the inputs are pulled down by the detectors, the output of the center 7406 inverter will be low, and the outputs of the second 7438 of each feedback loop will be high. This means the outputs of the circuit, EAST and WEST, will be high, allowing green signals to be displayed in both directions.

If one of the OS section detector inputs is pulled low, when a train moves onto one of the switches at either end of the single track portion, the output of the first 7438 gate for that direction will go up, as will the output of the center 7406 gate. This will cause the output of the second 7438 gate for that direction to go low, which will cause the circuit output to go low, forcing red signals to be displayed for opposing trains. As the train moves through the intermediate blocks, the feedback from the output of the second 7438 for that direction to the input of the first 7438 gate for that direction holds the direction precedence, and maintains the output, keeping the opposing signals red.

When the entire single track portion is clear, and all of the inputs are released, the output of the center 7406 gate goes low, breaking the feedback loop and allowing the output to rise, clearing all signals to green. The other two 7406 gates in the center of the diagram are wire-ANDed between the two feedback loops such that, once one feedback loop has been triggered, the other will be disabled from being set up. Only once all of the inputs have been released will both feedback loops be available to be triggered once more.

You will have to think about this for a while if you really want to understand how it works. The circuit is not an easy one. Then again, it will work whether you understand it or not, so you can just copy the circuit, call it magic, and not worry about it.

Building the Circuit

The circuit etch patterns and component placement diagram for the WhichWay circuit are given in Figure 2 for those who want to etch boards, rather than perfboard the circuit. The component placement diagram includes the hole locations to aid in drilling your board. The component listing is given in Table 1.

Note that the etch pattern is always printed as seen from the component side of the board, per electronic industry standards. The solder side image must be reversed on the board you build, so that the text and the image are correct. The convoluted logic of the circuit made a single sided board very difficult if not impossible, so this is a two-sided layout, which is a little tougher to etch. Iron the pattern onto one side of a double sided blank first. Then center punch and pilot drill the four mounting holes. Now iron the other pattern on the other side, using the mounting pilot holes as registration guides. Etch the board and drill out all the holes. When you mount components, you must solder the component lead on both sides of the board whenever there is a trace from the pad on both sides of the board. Use long-tail sockets so you can mount the sockets slightly above the board to solder the top side connections.

Please be very careful to install C5 to match the polarity indication in the component placement diagram; electrolytic capacitors will explode when power is applied if they are wired backwards.

Powering the Circuit

This circuit must be run on a filtered and regulated 5 volt DC supply. You can go to a radio hamfest or flea market and buy an old PC or computer supply cheap. These supplies usually provide 10 to 20 or more amps of 5 volts, as this is the most needed voltage in a computer. You can also use the power supply we have discussed for the other circuits we've done, which is repeated here as Figure 3.

While this circuit requires a 5 volt regulated supply, it can be used with DetectTrain and AutoBlock circuits which are running on different supply voltages. The circuits have all been designed to work together even when run on different supply voltages, as long as all the power supply grounds are connected together. This allows you to use 12 or 24 volt bulbs with the AutoBlock circuit while maintaining the 5 volt regulated supply on the WhichWay circuit.

A Quick Look at the APB Circuit

Figure 4 shows the wiring for APB signaling for a stretch of single track with three intermediate track blocks, and Figure 5 shows it for two intermediate track blocks. The direction precedence circuit is actually set up to handle as many as four intermediate track blocks, but this is an unlikely model situation. Bear in mind that on the prototype, the blocks are usually about two train lengths long, or about two miles per block. In HO scale four intermediate blocks would be about 500 feet long! Even for short train lengths, four intermediate blocks of two train lengths each is an unusual operating bottleneck on a model layout. Figure 4 and Figure 5 should handle most model situations.

Figure 4 and Figure 5 have been set up to overlap with each other to form a signal wiring diagram for your layout. Make multiple copies of Figure 4 and Figure 5, and cut them off along the dotted lines at either side of the figures. If you now butt the figures together, using Figure 4 where you have three intermediate blocks, and Figure 5 where you have two intermediate blocks, you can build up the diagram for the system you need.

You should note that Figure 4 and Figure 5 are very similar to the diagram we used last month for ABS signaling on a single track with traffic in both directions. The left to right, or eastbound, signals and signal driver circuits are on the bottom, and the right to left, or westbound, signals and signal driver circuits are on the top. Only one set of detectors is required. The positive supply voltages provided to the detectors, the signal driver circuits, and the direction precedence circuit need not be the same, or even from the same power supply, but the grounds of the supplies must be connected together. Of course one supply can also be used. Again, the RLED limiting resistors are probably not necessary, but are shown in case they are required for your application.

As before, the OCCUP signals from the track block detectors are connected to the two westbound signal driver circuits located east of the track block, and to the two eastbound signal driver circuits located west of the track block, but in this diagram all of the OCCUP signals from the detectors in the single track portion are also connected to the direction precedence circuit.

Also as before, the DIRECTION inputs of all the eastbound signal driver circuits are all bussed together, as are the DIRECTION inputs of all the westbound signal driver circuits. But now the direction precedence circuit replaces the direction toggle switch of last month's diagram, and we have added OS section detectors to the switches at either end of the single track portion. The DIRECTION inputs of the signal driver circuits are now connected to the outputs of the direction precedence circuit. If the direction precedence has been set for westbound, the EAST signal goes low, forcing all eastbound signals to red, and if the direction precedence has been set for eastbound, the WEST signal goes low, forcing all westbound signals to red. One direction precedence circuit is required for each section of single track in which APB signaling is to be implemented.

Wiring the OS Section Detectors

The OS section detectors are wired differently than the others, because they are not really separate blocks. The OS sections are part of the main line track through the double track portion. We use 1N4001 diodes (Radio Shack 276-1653) to connect the OS section detectors at either end of a double track section to the main line track detector of the double track section. This allows the OS sections to participate in the ABS signaling of the permissive signals along the main line. The OS sections also connect to the direction precedence circuit to begin the tumble down when a train leaves the double track portion and moves onto the switch.

The two SPST switches on the OS section OCCUP signals at the bottom of the diagram are the switch circuits. If the switch at the end of a double track portion is in reverse, the OS section is occupied. On the prototype this is accomplished by a set of contacts attached to the switch points, which shunts the OS section track circuit. On the model we can accomplish this by using a set of contacts on the switch machine to ground the OS section OCCUP signal when the switch is in reverse. If the switch is being controlled by a SwitchLock circuit, and the AutoBlock circuits are running on a 5 volt supply, we can instead use the Y output of the SwitchLock to ground the OS section OCCUP signal, as shown in Figure 6.

Note that there is no detection of the auxiliary main line through the double track portion, as the OS section provides all the train control required. If the switch is in reverse, the signals will protect the auxiliary main, and if the switch is in normal, the main line detector will operate the signals in the ABS mode as required. Note also that putting the switch into reverse will occupy the OS section, and tumble down the opposing signals in the single track section. Finally, if a train crew leaves the switch in reverse, or the train extends beyond the fouling point of the switch into the OS section, the signals will not clear, and the direction precedence will remain locked into the circuit. This operation is as on the prototype.

Wiring the Last Permissive Signal

The last permissive signal of a single track section needs some special handling. The absolute signal for the next section of single track can be red under two conditions, either because of a train in the same direction in the first block of the next single track section, or a train in the opposing direction anywhere in the next single track section. These two conditions are carried by the S stop and D direction inputs to the signal driver circuit of the absolute signal. Both of these inputs need to generate a yellow signal on the last permissive signal of the previous single track section. The A1 and A2 approach inputs of the AutoBlock circuit are used for this purpose as shown in Figure 4 and Figure 5.

Wiring the Second Last Permissive Signal

The prototype wires the second last permissive signal on a single track portion in each direction slightly differently also. While the last permissive signal of a single track portion is wired to display a yellow signal if the next absolute signal is red, there is a problem if two trains are approaching a double track section from opposite sides. There is a chance that one or the other or both will get to a red signal without having seen a yellow signal first. The possibility of a collision under these circumstances is very real. To get around this problem, the second last permissive signal of a single track portion is also wired to display a yellow signal if the next single track section has opposing traffic.

We can also do this using the second approach input, A2, on the AutoBlock board. Connect the A2 input of the second last permissive signal to the direction precedence signal of the next single track section. This wiring is also shown in Figure 4 and Figure 5.

APB With Other Signal Types

For other signal types, such as signals using bulbs, searchlight signals, and position light signals, use the modifications from last month's article on the signal driver circuits to modify Figure 4 and Figure 5.

For searchlight signals the AutoSearch signal driver circuit from last month is used. The current version of the AutoSearch circuit does not have two approach inputs, however. Figure 7 shows the modification to the APB wiring diagrams required to connect two approach inputs to the AutoSearch circuit for the last two permissive signals. The two diodes shown are 1N4001 rectifier diodes, Radio Shack 276-1653.

Electric Lock Switches Off the Main Line

We talked about electrically locked switches off the main line back in the November 1993 Mainline Modeler. The basic idea is that a switch off the main line into a siding or spur must interface with the signal system to ensure that through trains don't end up with green signals into a lumber yard or a chemical plant. The controls for these switches are locked, and unlocking the controls shunts the track circuit for that main line block.

In the APB system of Figure 4 and Figure 5, the electric lock switch circuit from the November 1993 Mainline Modeler article is connected to the detector output of the main line block in which the siding or spur switch is located. This will result in proper and prototype operation of the signals for all switching activities along the main line.


The APB system described here is an elegant and simple implementation of a common prototype signaling system. The modules are small and inexpensive, the wiring is simple, and the operation is very, very close to that of the prototype. This is the simplest and most complete model APB system implementation we have seen.

The direction precedence circuit is the last of the electronics modules that we need for signaling. There is one more very common prototype signaling system which we have not discussed, however. Next time we'll use the modular circuits we've already presented to take on the toughest one of them all: Centralized Traffic Control with intermediate Automatic Block Signals and fleet mode for express tracks. We're most of the way there now; just a little more work and we've got it all. Don't miss it.

Copyright 1992 - 2012 by TracTronics, Incorporated.
Last updated: December 28, 2011
These pages maintained by: Marc Haas