Information on ordering a commercial kit or assembled and tested unit of this circuit is available from the TracTronics Price List.
This is the fifth 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. In the first three articles we discussed SwitchWitch, SwitchLock, and SwitchMap, three electronic modules for switch machine control. Last time we began a set of four articles on signal control with a discussion of DetectTrain, a current sensing train detector circuit. This article will continue the articles on signal control with a discussion of BlockLock, a CTC panel block signal control circuit.
We are presenting these circuits both as circuit diagrams and as circuit layout patterns, allowing readers to breadboard or etch their own circuits. For those who do not wish to breadboard or etch these circuits, both kits and assembled & tested units are available from TracTronics, 1212 South Naper Boulevard, Suite 119, Naperville, IL 60540 (708-527-0000). As before, thanks to our friends Steve and Scott Ackerman of ACS in Sarasota, Florida (813-377-5775) for the excellent layout work on these circuit boards.
Centralized Train Control, or CTC, is a term applied to a wide variety of signal systems. What they all have in common is that train movements are controlled via trackside signals, which in turn are controlled by signal operators at one or more locations. The signals are always interlocked so that an operator error cannot result in two trains having permission to operate over the same track at the same time. A signal system on a prototype railroad typically has dozens of different signal indications for different circumstances. The variety of indications is confusing enough that professional train crews carry guides explaining all the different possible indications they might see. The distributed array of controlling electronics for the signal system itself is complicated beyond belief.
What we wanted for our layouts was something simple -- simple to build, simple to operate, and simple for non-professional train crews to understand without guidebooks and signal classes. We also wanted something that had the interlocking features of the prototype, so that the inevitable occasional operator errors of our dispatchers could be caught before they resulted in unfortunate meets. The minimum functionality we decided on was: red and green signal indications at each end of a signal block; interlocking signals so that both ends of the same signal block could not display green indications at the same time; signals at both ends of the signal block to drop to red and be held to red by a train occupancy control input from a detector; and operation from prototype controls, in particular, Union Switch & Signal type knobs and code buttons.
The circuit we designed, which we call a BlockLockTM, is shown in Figure 1. You do not need to understand the operation of this circuit in order to build and use it, but for those who want to know:
Consider first the two D-type latches contained in IC U2 (74LS74). These two latches 'remember' the desired states of the block signals, either red or green, for each end of the signal block. The upper latch in the diagram is for signals to leftbound trains, at the right end of the block, and the lower latch in the diagram is for signals to rightbound trains, at the left end of the block. (Contrary to typical model practice, most prototype CTC panels are marked 'L' and 'R' for leftbound and rightbound, not 'E' and 'W' for eastbound and westbound.)
The group of four 7406 inverters at the right hand side of the diagram will sink up to 40 mA each for driving the LEDs. The resistors R5 through R8 limit the current through the LEDs on the CTC panel, and determine their brightness. The resistors R9 through R12 limit the current through the LEDs on the signal masts on the layout, and determine their brightness.
The two 74LS08 'and' gates to the left side of the diagram's center generate the active-low control inputs which reset the latches to 'red' signal indications. A latch will be reset to a red signal indication: when the block becomes occupied, indicated by the active-low control input OCCUP going low; when the block is reset from the CTC panel, indicated by the active-low control input CENTER going low; or when the block is set from the CTC panel for a green signal in the other direction, indicated by one of the active-low control inputs LEFT or RIGHT going low. The active-low control inputs LEFT and RIGHT preset the appropriate latch to the green signal indication when they go low.
The two 74LS08 'and' gates to the right side of the diagram's center block a momentary green signal indication that would otherwise result if the CTC panel attempts to code a green signal when the block is held to red by the OCCUP control input. The two 7406 inverters in the lower left of the diagram generate an active-low signal UNOCC, indicating an unoccupied condition, for use by other circuitry. They also allow the sense of the OCCUP signal to be wire-ANDed with the CENTER signal without shorting the OCCUP signal to the CENTER signal, which would result in the occupancy indicators flashing when the panel operator locks the block by coding the signals with the control in the center position and knocks down the signals to red indications at both ends of the block.
The circuit etch pattern and component placement diagram 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. Note that the etch patterns are 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.
Note that this is a double-sided board, which makes alignment of the two sides important if you etch your own. Iron on one side and drill two of the holes at opposite ends of the board, then iron on the other side with the two hole pads lined up on the two holes. Also, if etching your own boards, components mounted on the board need to be soldered on both sides to form the connections from the front to the back sides of the boards; use long-tailed IC sockets so you can solder the IC connections on the top.
Please be very careful to install C1 to match the polarity indication in the component placement diagram; electrolytic capacitors will explode when power is applied if they are wired backwards.
This module, like most of the rest of the modules in the series, includes connectors instead of soldering the lead wires directly to the board. As we explained before, we put quite a bit of thought into what kind of connector we wanted, and settled on the Molex KK156 series. These connectors are durable, and will take quite a bit of abuse without bent pins and the like. We did not use screw terminals because we wanted to be able to remove and repair or modify units without having to disconnect and reconnect a dozen or more wires from the screw terminals. You will need a crimp tool for the KK156 connector pins; this tool is Digi-Key WM9903-ND. Dial 1-800-DIGIKEY.
The parts list gives two part numbers for header J1 and two part numbers for housing J1. This is because the 15 pin KK156 header and housing which we use are not commonly available in small quantities. Instead, you can use the 7 pin and 8 pin headers and housings specified. All of the control inputs and power supply connections will be on the 7 pin connector, and the LED connections will be on the 8 pin connector.
The circuit modules have been kept very small so that all of the circuits composing the CTC panel logic can be contained in the CTC panel itself. This results in the least cabling into the panel. The circuits can all be bolted together with four #6-32 threaded rods inserted through the mounting holes. Use plastic spacers between each pair of boards, and nylon washers at the ends of the threaded rods between the last board and #6-32 nuts. Plastic spacers and nylon washers are absolutely necessary to keep from shorting traces on these small and dense boards.
The stack of boards can be set on a shelf within the CTC panel, or clamped against a wall of the panel using nylon loop cable clamps such as Digi-Key 7624K-ND around the spacers.. We recommend that you mount one more circuit in your panel than you need. If a circuit fails for some reason during an operating session, you can remove the connectors from the failed unit and plug them into the spare unit and be operational again within seconds.
The circuit requires a regulated 5 Volt supply. Many of you who are installing electronics on your layouts will already have a 5 Volt supply to power the circuit. You can also build a separate power supply, such as the one we gave in the November 1993 Mainline Modeler for the SwitchLock circuit, which we repeat here in Figure 3. Please be very careful when wiring 110 Volt connections, and to fuse the power supply you build. If you do not know how to wire 110 Volt connections, get someone who knows what he is doing to help you, or buy a commercial supply!
If you are using the circuit on a modular layout, the power supply voltage can easily be provided by a lantern battery or batteries. The 6 Volt DC supplied by the battery will run the circuit a little 'hot', but we have done this often with no damage to the circuits.
The circuit should be wired as shown in Figure 4. The switches shown are a 2 pole 3 position rotary switch and a mini push-button, Radio Shack 275-1386 and 275-1547. Pin 1 of the connector is the leftmost pin when looking at the component side of the board with the connector at the top; this pin has a square solder pad. Either the DetectTrain current sensing train detector circuit from last time or another detector may be used; the requirement is that the train detector must have an open-collector output capable of sinking TTL signals and the 20 mA current of the occupancy LEDs.
The output signal names are denoted by three letters as follows:
(Leftbound or Rightbound) (Green or Red) (Layout or Panel).
The signal LGP for example is for the Leftbound Green LED on the Panel, while RRL is for the Rightbound Red LED on the signal mast on the Layout.
The three LEDs at the lower right of the diagram give occupancy indication on the CTC panel. The left LED indicates leftbound occupancy, the right LED indicates rightbound occupancy, and the center LED indicates occupancy without control operator permission. The left and right LEDs should be yellow and the center LED red; use Radio Shack 276-021 and 276-041. Three 150 Ohm resistors (Radio Shack 271-1312) are connected in series with these LEDs as shown in the diagram. You can also use a flashing red LED for the center occupancy signal, such as Radio Shack 276-036, in which case the 150 Ohm resistor for this center LED is not required.
The recommended CTC panel layout for one block is shown in Figure 5. The green and red LEDs at either end of the block indicate the state of the trackside LEDs. Operation is as for the prototype: the rotary switch is set leftbound or rightbound by the control operator, and the code button is depressed. The selected signal will turn green on the panel and on the layout, and will remain so until removed by the train occupancy signal, reset of the block, or set of an opposing green. A reset of the block is made by setting the rotary switch to the center position and depressing the code button. Coding a green signal on an occupied block is not possible; the CTC panel controls are locked out of the block until it is no longer occupied.
Note that the LEDs on the left end of the block are RGP and RRP, the rightbound LEDs, and those on the right end of the block are LGP and LRP, the leftbound LEDs. The connections for the trackside signal masts are similar, where LGL and LRL must be on the right end of the block, and RGL and RRL must be on the left end of the block. In our example, the leftbound signal for block 23, at the right end of the block, is called 23L, or 23 Left, while the rightbound signal for block 23, at the left end of the block, is called 23R, or 23 Right. When Signal 23 is set to the R position, and the code button is depressed, signal 23 Right will display a green indication for rightbound trains. This is called clearing the signal. Clearing 23 Right would tell the train crew and control operator that there is no broken rail or train occupancy in the cleared block.
The occupancy LEDs are across the center, in the track stripe between arrowheads. Here the left LED indicates occupancy by a leftbound train, while the right LED indicates occupancy by a rightbound train. The center red LED indicates a train has entered the block without control operator permission. This is permitted in some prototype situations, such as upgrade coal trains on mountain divisions, where a red signal may mean 'proceed at reduced speed prepared to stop short of train ahead'. The center red LED may also mean that a cut of cars is in the block, or that an electrically-locked switch on the main line is unlocked. We discussed implementing electrically-locked switches on the main line in the prototype manner using the SwitchLock circuit in the November 1993 Mainline Modeler. The SwitchLock circuit interfaces to the BlockLock circuit to knock signals down to red when a main line switch is unlocked.
In our model operations, the center red occupancy LED usually indicates that a model operator has violated an operating rule and entered a block which has been locked by the control operator. The most frequent violations are either running a red signal into a locked block, or making a reverse move into a locked block. For some reason, model operators who would never run a red signal going forward seem to consider running red signals in reverse okay! We use a flashing red LED for our center occupancy indication to ensure that the control operator is aware of this dangerous situation.
While the switch and code button below it are shown here as directly below the block diagram, on prototype panels the block signal controls are across the bottom of the panel. The top half of the panel is the block diagram for the entire span of control of the panel, and the lower half of the panel contains two rows of controls: the upper row of controls are for the switches, and the lower row of controls are for the signals. You should research the type of CTC panel used on the railroad you model and design your panel to follow the prototype. A full discussion of CTC panel layout and control is well beyond the scope of this article.
Increasing the brightness of the LEDs is not recommended. The 150 Ohm resistors recommended result in LED current of about 20 mA, which is a safe value for all the LEDs we are aware of. If a dimmer LED is desired, either on the panel or the layout, the resistor values can be increased. 200 Ohms results in a current of 15 mA, which will be 75% as bright, and 300 Ohms results in a current of 10 mA, which will be 50% as bright. Resistors R5 through R8 control the brightness of the LEDs on the CTC panel, while resistors R9 through R12 control the brightness of the LEDs on the layout signal masts. You can increase the resistance by using larger resistors on the board, or by adding the additional resistance in series with the LED leads. The occupancy LED resistors should be set to the same value as those used for resistors R5 through R8 so that all of the CTC panel LEDs are the same brightness.
One situation we faced when designing the circuit was a requirement to implement an interlocking plant at a grade crossing of two main lines, the Western Maryland and the Pennsylvania main lines out of Hagerstown on Bill and Wayne Reid's Cumberland Valley System. This grade crossing is shown in the photos. We designed the circuit so that you can cross-connect two circuits to implement the interlocking signal plant for such a grade crossing. The methods used here can also be used in other more complex situations, such as interlocking signals for double main lines crossing at grade.
One modification to the circuit board is required for use in an interlocking plant. On the circuit board, we have included current limiting resistors for the LEDs in series with the open-collector outputs of the circuit. This makes it easier to use in the standard installation. When building the circuit for use in interlocking grade crossings, however, we need access to the open-collector outputs for two of the green LEDs. When building the circuit for use in an interlocking grade crossing, you should assemble the circuit without the resistors R9 and R11, substituting a piece of wire in the resistor locations. These resistors will still be used to limit the current through the two green LEDs on the layout, but they must now be located off the circuit board.
The solution for two main lines meeting at an interlocking grade crossing is shown in Figure 6. There are two circuits, one for the interlocking block on each of the two main lines. The circuits are interconnected using eight 1N4001 1 Amp rectifier diodes, Radio Shack 276-1653. The 150 Ohm current limiting resistors (Radio Shack 271-1312) for the green LEDs, RGL and LGL, which must now be located off the circuit board, are also shown.
The diodes allow the open-collector outputs of the circuits and the train detectors to be combined in a wired-AND connection without shorting the outputs together. This is called a diode-AND connection. The concept is similar to that used in a diode-blocked matrix circuit. The signals generated by combining the outputs with the diodes are then used to drive the inputs to the circuits. The required pull-up resistors on the inputs are contained on the circuit boards.
The signal which needs to be created using the diodes is the OCCUP input to each of the circuit boards. Recall that the OCCUP signal drops the block signals to a red indication and locks out the CTC panel controls from coding in a green signal on that block. The OCCUP signal for an interlocking block needs to go low for any train occupancy in either of the two interlocking blocks, as well as for any green signal indication on the crossing main line. Four diodes are used to generate this signal for each of the two interlocking blocks.
We can write the signal we need for the first circuit as follows:
OCCUP (BlockLock 1) = TRAIN DETECT (BLOCK 1) AND
TRAIN DETECT (BLOCK 2) AND
LEFTBOUND GREEN (BLOCK 2) AND
RIGHTBOUND GREEN (BLOCK 2)
A similar equation can be written for the second circuit. The diodes are used to logically 'AND' these active-low output signals together and generate the required input signals for the circuits.
Then again, if the preceding discussion has left you gasping for air, just copy the figure. You don't need to know how it works for it to do so!
The operation of the interlocking is as on the prototype: either main line CTC control can code a green as long as no other green signal is in the interlocking, and as long as no train occupies the interlocking. Any train entering the interlocking will knock down any green signal to a red signal indication for all four signals, and will hold those signals red and lock out both CTC controls as long as it occupies the interlocking. Once the train has left, either CTC control can once again code a green signal in either direction.
If a green signal is left up on either main line, the other main line CTC control is disabled from coding a green signal for either direction. The interlocking can be unlocked by resetting the green signal to a red signal indication by setting the rotary switch to the center position and pushing the code button. This will drop the green signal to a red signal indication and unlock the interlocking.
As an example of a more complex situation for interlocking grade crossing signals, a little thought and some scratch paper will give you the solution for two double main lines crossing at an interlocking. For tracks 1 and 2 crossing tracks 3 and 4, the equation for the circuit on track 1 is:
OCCUP (BLOCK 1) = TRAIN DETECT (BLOCK 1) AND
TRAIN DETECT (BLOCK 3) AND
TRAIN DETECT (BLOCK 4) AND
LEFTBOUND GREEN (BLOCK 3) AND
RIGHTBOUND GREEN (BLOCK 3) AND
LEFTBOUND GREEN (BLOCK 4) AND
RIGHTBOUND GREEN (BLOCK 4)
Similar equations can be written for the other three circuits in the interlocking. This solution will require seven diodes for each of the four circuits involved in the interlocking, or 28 diodes total. The SwitchMap diode matrix control circuit board we discussed in January 1994 Mainline Modeler can be used to build the matrix. The diode matrix solution for the double main line diamond interlocking plant is given in Figure 7.
One problem with electronic controls on any layout is electrical noise. A model railroad typically has a great deal of electrical noise. Long wires on electronic control inputs act as antennas, picking up the electrical noise from motors and switch machines on the layout. The only noise-sensitive inputs on the BlockLock circuits are the RIGHT, CENTER, LEFT, and OCCUP inputs. Locate the Circuit boards in or near the signal panel to keep the leads on the RIGHT, CENTER, and LEFT control inputs short. Route the OCCUP signals from the detectors in different cable runs than those used for track power and switch machine leads to avoid noise pickup in these leads.
If you still have problems with noise on BlockLock inputs falsely setting or clearing signals, solder 0.1 uF ceramic disk capacitors from the control inputs to ground for each of the circuits which are falsely changing. You will probably need to do this only for the OCCUP input. These capacitors should be soldered directly on the back of the circuit boards for the best results.
The BlockLock circuit board, an assembled unit, and the CTC panel of Bill and Wayne Reid's Cumberland Valley System are all shown in the photos, as well as a close-up of the portion of the CTC panel which controls the interlocking plant at the WM/PRR diamond outside Hagerstown and the diamond itself. The CTC electronics has been in use for about a year now with one failure; one of the 1N4001 diodes in the interlocking plant failed to a shorted condition and locked up the plant.
Operating sessions on the Cumberland Valley System have been much more fun since the CTC electronics went in (not that they weren't fun before, Bill!), with no need to call in to the dispatcher every time a train crew needs or takes a signal, and no frantic calls from the dispatcher wanting to know just where the Sam Hill train number XYZ is. The combination of the train detectors and the CTC logic relieves the dispatcher's job of a lot of tedious work and frees up time for the thinking and coordinating part of the job.
Next time we'll discuss SeeTrain, an optical infrared train detector circuit. We use this circuit on our layouts for staging yard and engine house tracks, to tell when trains are properly positioned to clear switch fouling points, block boundaries and the like. We'll see you then.
