Fullscreen HTML5Med Res (??MB)HTML4

By Tim Blythman Decoder Base Station Using DCC Remote Controller DCC Booster Getting Started with DCC Digital Command Control (DCC) is a versatile standard for model railways that continues to evolve. Our recent DCC project articles have included some basic background information; this article provides an in-depth guide to using the Decoder and Base Station, plus more details on DCC. W e have covered the background and workings of DCC in a few different articles over the years. We have also produced several related projects, including the recent DCC Decoder and DCC Base Station. The Decoder article included a glossary, which you might find useful if you are new to DCC. Even if you aren’t using our hardware, you might find it to be a handy guide. There is a vast range of model railway gear available that is DCC-­ compatible, or can be modified to work in a DCC system. We can’t provide enough detail to address every scenario or manufacturer, so we’ll assume you have some basic knowledge of the operation of model railways under DC or analog control, and the related electrical principles. The DCC Decoder project showed a simple example of fitting our Decoder to a small N-scale mechanism, with some general advice. If you aren’t sure about your locomotives and rolling stock, a web search for the manufacturer and model appended with “convert to DCC” can be a good start. You should verify that your locomotives will run well on DC before undertaking the conversion. While DCC has many talents, it won’t help if a locomotive is not in good mechanical shape. This includes making sure that the motor and gearbox are lubricated and running smoothly. You should also check that the wheels and the track pickups are clean. Power supply We used a regulated 12V 2.5A power supply for most of our testing, and found it to be perfectly adequate. The driver ICs have a low on-resistance, so the track voltage will be very close to the supply. 12V is the rated voltage for the motors in many locomotives around N & HO scales. So a 12V supply should be suitable if you are starting out with our Decoder and Base Station. siliconchip.com.au The over-current sensing of the Base Station is intended to be fast, since short circuits are very possible. A metallic item dropped on the track, or a derailed vehicle, can create a direct connection between the rails. Our tests showed reaction times of around 200μs to shut off power in such a condition. A discharged capacitor can appear like a short circuit, which is why the keep-alive capacitor on the Decoder is charged through a resistor. It’s not advisable to connect large capacitors directly to the Decoder supply rails. Sound-equipped decoders can be troublesome in this regard, since they usually include large capacitors to ensure that sounds are played without interruption. Our DCC Decoder We noted in the Decoder article that our Decoder design offers a few handy connections that are not seen in many commercial decoders. We wouldn’t be surprised if our readers use DCC to add ‘bells and whistles’ to their rolling stock, but there are a few things to check before doing so. Normally, the 12V BLUE connection works as the supply for the function outputs, which are switched on by having their negative terminals pulled to circuit ground by the Mosfet drains. It’s also possible to power a fixed output by connecting it between the BLUE pad and the ground pad, both shown in Fig.1 overleaf. A high-value capacitor directly connected here would appear like a short circuit to the Base Station at switch-on. Incandescent globes, which have a low resistance when cold, might behave similarly. So these things should be approached with care or avoided entirely. A continuous load can also interfere with programming, since it will draw current in a similar fashion to Australia's electronics magazine an acknowledgement signal. Our Base Station measures the quiescent current during programming to help differentiate the acknowledgement, but it’s possible that a heavy load will cause excessive drop across the 33W resistors and not leave enough voltage to power the Decoder. One way to avoid this is to provide a switch of some sort to disable such a load when needed. Since space is often tight in a scale locomotive, a pair of header pins closed by a jumper shunt could be an option. A couple of small LEDs (with their ballast resistors) should be fine and will help give an indication of when power is present at the Decoder. Drawing power from the 3.3V and ground connections (also shown in Fig.1) will present much the same concerns. Be aware that the 3.3V regulator must be able to handle any extra dissipation caused by an external load current. Thus, we suggest that no more than 10mA load be applied to the 3.3V connection. A separate regulator could be fed from the 12V BLUE connection if you need a lower-voltage and/or higher-current regulated supply. The track Trackwork is the other aspect that may need attention. For example, you should already know about running feeder wires and how power is routed through things like points and crossings. Some points (or turnouts/ switches, as they might be known) can make or break certain connections depending on how they are set. Many sets of points are designed to isolate unused tracks, making it easier to operate multiple locomotives on a DC power supply, since the isolated tracks can be used to change which trains respond to an analog controller. Peco’s InsulTrack system is an example of this. A good rule of thumb with DCC is to January 2026  49 Fig.1: These are the connections to the Decoder as presented last month. The 12V and 3.3V connections can be handy, but there are a couple of provisos that must be observed. Fig.2: the problem caused by a so-called balloon loop is the potential for a short circuit at the place where the loop closes on itself. It is not limited to DCC operation, although it might not be apparent on some DC layouts where the points are used to switch track power. If you can run your finger along one rail and end up at a point on the other track opposite to where you started (such as following the outer track in this diagram), you might have such a loop. 50 Silicon Chip Australia's electronics magazine power all tracks at all times, since we can rely on DCC to ensure that each locomotive operates independently. One option is to divide the layout into electrically isolated sections with separate feeds, often called blocks. Many manufacturers provide insulated rail joiners for this purpose. This can allow a block to be isolated if there is a fault, such as a derailed train causing a short circuit. A separate switch, breaker or fuse can be used to control power to each block. Separate blocks also allow the locations of trains to be sensed electronically, by monitoring the current draw of the rolling stock within each of those blocks. We presented a design in Circuit Notebook of June 2023 to do just that (siliconchip.au/Article/15828). Being able to sense trains can allow for some clever operations, such as automatic operation of signals or level crossing lights. Some keen modellers have even used this as part of an automatic train control system. Another proviso is that some track arrangements that loop back on themselves (such as triangles and balloon loops) can cause problems, as you can see in Fig.2. These concerns are much the same for layouts that operate with DC. Some strategically placed insulated track joiners can also help with this. The DCC Reverse Loop Controller from October 2012 (siliconchip.au/ Article/494) explains the concern in more detail and provides a circuit that can be used to solve it from a different angle. The Loop Controller uses a DPDT relay to reverse the polarity of the DCC signal to avoid a short circuit; a manually operated DPDT switch can be used to test if this approach would work. If you are starting out with model railways for the first time, you don’t need much track to test the DCC Decoder and DCC Base Station. You might prefer to set up a length of standalone track to see what is possible, and to get an idea of how DCC behaves. Photo 1 shows the short test track we used during development and testing of the Decoder and Base Station. It will be a good idea to have a safe place at each end of the track in case you get a runaway. A circular track loop can help to lessen the damage that might occur if something goes wrong. One option is to put some tape on one rail to break siliconchip.com.au Fig.3: using a DPDT switch like this can make it easier to use the programming track. The locomotive can be driven onto the track while the switch is in the MAIN position (to the left). The switch is changed to the right (PROG) position so that the locomotive’s decoder can be programmed. Then the switch is returned to MAIN so that the locomotive can be driven away. Photo 1: we used 1m of ‘flexi-track’ as our initial test track. The track can be easily connected to the main or programming outputs on the Base Station using the pluggable terminal blocks. the circuit to the wheels if the locomotive gets too close to the end of the track. This can at least ensure that it isn’t able to launch itself off the workbench! You’ll need to move the locomotive between the main and programming tracks. We have seen some modellers use a DPDT switch to effect this, as shown in Fig.3. This allows locomotives to be driven onto the programming track, programmed, then driven away, instead of needing to be lifted from one to the other. Make sure that the switch is never in the programming position while a locomotive is sitting over the gap between the rails, since this may cause a short circuit between the programming track and mainline track circuits. For the arrangement in Photo 1, we can simply unplug the track and move the plug over to the other socket to connect our locomotive to the programming output. There is a negligible chance of a short circuit occurring with this technique. Programming CVs Configuration variables (CVs) are an aspect of DCC that does not have a parallel in DC or analog operation. CVs can be incredibly powerful, and at the same time, can be confusing and may cause unpredictable side effects if they are not understood. siliconchip.com.au If you have just fitted your first locomotive with a Decoder and want to simply test it out, you don’t need to worry about CV programming at all. The Decoder should respond to address 3 without any changes, and this will be sufficient to see that the Decoder installation has worked. If you have a handful of locomotives, we recommend sticking to using short (two-digit) addresses, since it is one less factor to worry about if things aren’t working. Even if your locomotive carries a three- or four-digit fleet number, the last two digits are usually unique enough to identify it, so they can be used as the short address. How CV programming works The details of CV programming are laid out in full in Section 9.2.3 of the DCC standards. Still, we’d like to offer a brief, practical overview for those who are interested in simply having something that works and how to fix it if it doesn’t. As we mentioned in the Base Station article, the Base Station sends out specific packets to the programming track to perform programming. Apart from the actual programming packets, there are ‘reset’ packets that form part of the sequence to ensure that programming only occurs when intended. When the Base Station sends out a packet, the Decoder may choose to Australia's electronics magazine acknowledge the packet by placing a 60mA (or higher) load on the programming track. This is the only means of the Decoder communicating back to the Base Station. The acknowledge is typically achieved by the Decoder briefly driving the locomotive motor for around 5ms; this can be seen by the locomotive appearing to twitch sporadically during programming. A handy side-effect of the 33W resistors on our Base Station is that this load will cause LED2 to briefly dim during an acknowledgement. Some modern motors we tested are so efficient that they would not even sink 60mA, which can hamper programming. On our Base Station, this condition is shown with the message “Low acks”. If you are sure that acknowledgements are occurring, the I2x multiplier can be increased to trick the Base Station into thinking that the correct amount of current is being sunk. Table 1 lists some of the messages that might be seen on the Base Station during programming. These appear in the top-right corner of the LCD. The table includes possible reasons for errors and potential solutions. We’ll concentrate on direct-mode programming, since this is generally the best mode to use; it is supported by both our Decoder and Base Station. January 2026  51 Table 1: DCC Base Station programming error messages Message Notes OK, done A successful read has occurred and the value shown for the CV contents is correct. Read OK A successful read of a long address has occurred and the value shown is correct. OK, verified A successful write has occurred and the data has been verified. Sent Since there is no acknowledgement possible in operations mode, this indicates that the programming packets were sent correctly. Out of range The CV number or value is out of the valid range. CV values are only eight bits (values between 0-255). Check the value before entering it again. Select mode No programming mode is selected. Cancelled The operation was cancelled by the user. Not supported Physical programming modes only support a limited range of CVs (1, 2, 3, 4, 7, 8 & 29). Check the CV or choose a different programming mode. Read error The Base Station did not receive the expected acknowledgement and the read did complete successfully. This is typically caused by poor track contact corrupting communication, but it may occur if the Decoder does not support the requested CV. Read error #1, Read error #2, Read error #3 These only occur in paged mode, since multiple packets must be sent to configure the Decoder’s page register before programming. Higher numbers indicate that the failure occurs at a later stage. Write error #1, Write error #2, Write error #3 Writing (in all modes) involves performing a write followed by a verify, so higher numbers suggest that the verify might have failed. In this case, the CV might contain the correct value, but it could not be confirmed. Not allowed In operations mode programming, writes to CV1, CV17, CV18 or CV29 are not permitted. Power off Operations mode programming cannot occur if the track power is off, so try switching it on, if safe to do so. No address There isn’t an address selected for the current L1-L5 tab, so there is no address to use for operations mode programming. Timeout Operations mode programming has not completed within the expected time. It may be that a fault has shut off the track power so that packets cannot be sent. Low acks Direct mode programming has not seen any acknowledgement activity. Check track contact and if you are sure that the Decoder is sending acknowledgements, or try adjusting I2x to increase sensitivity. Data error The two high bits of CV17 are not set as required for a valid long address. The decoder may or may not respond correctly. Value error The value of CV17 and CV18 is not in the range for a valid long address. The decoder may or may not respond correctly. Some of the CVs are also supported by operations mode programming, meaning that they can be edited on the main track. Unfortunately, there is no acknowledgement or read-back on the main track. The direct-mode programming packets fall into four categories: byte write, byte verify, bit write and bit verify; the Base Station uses all but the bit write method. Each CV is effectively an 8-bit value in an EEPROM location on the Decoder, so CV programming is little more than reading and writing these memory locations. A byte write updates an entire 8-bit 52 Silicon Chip value. The byte-write packet includes a 10-bit CV address and the new 8-bit value. If the Decoder receives the packets (two consecutive, identical packets must be received for security), and successfully performs the write to EEPROM, it responds with an acknowledgement. We can then send a byte verify command containing the 10-bit CV address and the 8-bit value, effectively asking, “Does the 10-bit address contain the 8-bit value?” An acknowledgement means “yes”. So performing and confirming a write to a CV is straightforward. Australia's electronics magazine Reading a CV is a bit more complex. We use the bit verify command instead; this includes a 10-bit CV address, a three-bit value (allowing one of eight bits to be selected) and one data bit. The question becomes, “Does the 10-bit address contain this data bit at the selected bit position?” Thus, 16 bit-verify commands are sent, both of two values (0 and 1) for each of the eight bit positions. If all is well, the Decoder will reply with eight acknowledgements out of 16. If we receive a different number of acknowledgements (or none), we know the data is incorrect. This is the advantage of direct mode, since the physical and paged modes can only perform a byte verify command. Without knowing what the value might be beforehand, the Base Station must cycle through all 256 byte values and receive exactly one acknowledgement to be sure of correctly reading the CV. You’ll come to recognise whether a CV read is occurring correctly. Since you can typically see or hear the locomotive twitching, you can count the eight acknowledgements as they happen. An unfortunate side-effect of the twitching is that the locomotive can move to a dead spot on the track, which can cause programming to fail. We find that simply holding the locomotive gently in place and applying gentle downward pressure (to enhance track contact) can help with programming. Patience is often the key. An important question is which CVs to program; we’ll cover these roughly in order of importance. CV29 CV29 is unique in that it contains several important but unrelated option bits. Our Decoder implements only three bits in CV29. If used with our Base Station (which only produces 128-step speed packets), bit 1 should be set (a value of 2) for compatibility. Bit 0 can be used to reverse the direction of the motor, while bit 5 selects between short and long addressing, which we will cover shortly. In case you aren’t familiar with binary arithmetic, the following offers specific CV values for our Decoder working with our Base Station. For our Decoder, CV29 can only have a value of 0-3 or 32-35. If the value is 0-3, the short address is used; otherwise, the long address is used. If CV29 siliconchip.com.au is odd, then the motor will operate in reverse compared to if it is even. If the value is outside this range, something may not be right. In summary, set CV29 to 2 if you want to use the short address or 34 if you want to use the long address. If the locomotive operates in the opposite direction to that expected, add 1, giving a value of either 3 or 35. One handy feature is that, once fitted with a decoder, the direction becomes intrinsic to the locomotive. A DC or analog locomotive will move in the same direction (along the track) after being picked up and rotated 180°, since both the track and motor direction have been reversed. DCC does not care about track polarity, so its ‘front’ is always the same end. Addressing The Decoder address is paramount. For this, you might find the glossary in the Decoder article to be a handy reference because there are three addresses that can be associated with a Decoder. The short address (CV1) is the first, and is set to 3 by default. You might hear this called the twodigit address, since all values from 1 to 99 are valid. Address 0 is never valid for any address type. For the very first DCC decoders, the short address was the only CV. The most significant bit (uppermost) of CV1 is always ignored. Values from 100 to 127 may work, but might be ignored by some systems, since packets to some of these addresses have the same format as service mode programming packets. It is best to avoid them. There is a long address that can be used instead; this can be from 1 to 10239 (40 × 256 – 1), so two CVs are needed to store the necessary 14 bits. CV17 holds the top six bits (in its six lower bits); it must also have its upper two bits set. Therefore, values of 192 to 231 are valid for CV17. CV18 simply holds the lower eight bits, and all values are possible. The long address might sometimes be called a four-digit address. Note that long addresses and short addresses can both take on values from 1 to 99, but they are not the same. For example, short address 42 and long address 0042 (written as four digits to show it is a long address) can both be used without conflict at the same time by separate decoders. Finally, there is a consist address siliconchip.com.au (CV19), which can be considered more dynamic. While the short and long addresses would probably be set once when the decoder is installed, the consist address allows a Decoder to be allocated an address on a more short-term basis. In DCC, a consist typically refers to two or more locomotives that are coupled together and thus should be operated in synchrony. Temporarily assigning the same consist address to all the locomotives in a consist allows this to happen transparently. The consist address, like a short address, is seven bits in length and responds to the same packet addressing scheme as other short addresses. The most significant bit is used to operate the locomotive in reverse to its normal direction, which is useful if it is coupled back-to-back with another locomotive. Briefly, if the consist address is set (ie, the lower seven bits are non-zero), the Decoder will respond to speed and function packets to this address. Otherwise, bit 5 of CV29 will decide whether long addressing (bit 5 set) or short addressing (bit 5 clear) is active. So there are five CVs that affect what address a Decoder responds to. It’s a good idea to check all these CVs if there is an apparent failure of the Decoder to respond to the selected address. Table 2 shows some example combinations and the resulting behaviour. We also found a handy online tool to calculate values for CV17, CV18 and CV29 at siliconchip.au/link/ac7x Speed and acceleration CV2, CV3, CV4, CV5 and CV6 control the speed and acceleration behaviour. It’s not necessary to change these, but we find that setting at least CV2 (start voltage) makes for more intuitive operation. Fig.4 shows in graphical fashion how CV2, CV5 and CV6 work. Their setting can vary depending on the motor and the condition of the Table 2: configuration variables related to addresses CV1 CV17 CV18 CV19 CV29 Behaviour 3 0 N/A N/A Bit 5 clear, eg, 2 Typical factory default; the Decoder will respond to short address of 3. 0 0 0 0 Bit 5 clear, eg, 2 Not valid for DCC; the Decoder will not respond to any packets. 3 0 0 21 N/A Since the consist address is set, the Decoder will respond to short address 21. 3 0 0 149 N/A 149 − 128 = 21. Since the consist address is set, the Decoder will respond to short address 21; the locomotive will operate with forwards and reverse swapped. N/A 209 120 0 Bit 5 set, eg, 34 (209 − 192) × 256 + 120 = 4472, and bit 5 in CV29 is set. The Decoder will respond to long address 4472. Photo 2: guides like this YouTube video can be helpful in finding tips and tricks for installing a DCC decoder. The 8-pin socket (above the right-hand brass flywheel) is common on locomotives labelled as ‘DCC-ready’, and conforms to the NEM652 standard. Matching plugs can also be found by searching online stores for NEM652. Source: https://youtu.be/h8YT16ZAKKY Australia's electronics magazine January 2026  53 locomotive. The general idea behind these CVs is to adjust the locomotive operation so its performance is similar to others on the layout. CV2 sets the voltage that is applied at the lowest speed step, so a good principle is that CV2 is set at a level that just causes the locomotive to start moving, eliminating the dead spot that would otherwise occur at the lower speed steps. The easiest way to do this is to simply run the locomotive a bit and determine the lowest speed step (as shown in the top line of the Base Station display) at which the locomotive moves. Note that it might require a higher step to get started than to continue moving. For example, our test subject chassis from the Decoder project starts moving at around step 17, but will continue if the speed is dropped to 12. This is due to the extra voltage needed to overcome static friction while stopped. We double this value to 24, since there are 127 speed steps, but CV2, CV5 & CV6 work on a scale up to 255. If you find that the top speed is too high, CV5 can be lowered to reduce this; the default value of 0 for CV5 means the same as 255 (ie, full voltage). Set this in a similar fashion, by finding a comfortable ‘fastest’ speed step value; double it, and program it into CV5. CV3 and CV4 control acceleration and deceleration. These should be treated with care, since high values (which mean slow acceleration) can make it appear that the locomotive is not responding to controls. Experiment with CV3 first, since keeping CV4 at 0 will allow prompt deceleration in an emergency. Values around 5 should allow you to get a feel for what is a useful value for CV3; you can then try a similar value in CV4. Keep in mind that all these CVs will interact to a degree. For example, changing the speed CVs (CV2, CV5, CV6) will change the apparent acceleration, since the voltage applied at each of the steps has changed. Function outputs Photo 3: the Flying Scotsman carries the fleet number 4472. Using the last two digits (72) will typically be enough to uniquely identify a scale model of it on small layouts. The default function output mapping of our Decoder is typical. The F0 control has two aspects, one that is active when forward is selected, and one in reverse. By default, these are mapped to the white (CV33=1) and yellow (CV34=2) decoder wires, respectively, and would be used to drive something like a directional headlight. Our Base Station has controls for F1, F2 and F3, so CV33-CV37 are meaningful. Each of these CVs corresponds to a bit in the commands sent in the function packets. The values in the CVs dictate which outputs respond when the packet has a specific bit set. The behaviour is a logical ‘OR’, so that if any bit AND function combination gives a non-zero result, the corresponding output switches on. With four outputs, each CV has four bits that can be set, so the valid values for CV33-CV37 are 0-15. A value of 0 means that a command will have no effect, while a value of 15 means that the command will activate all outputs. A simple example that you might find useful can be applied if you find that the headlights are operating in reverse. Changing CV33 to 2 and CV34 to 1 will swap this, so that the yellow wire operates in forward and the white wire in reverse. One alternate configuration we have seen is to set CV33 to 5 and CV34 to 6, meaning that the F1 output (green decoder wire) is active any time that the F0 control is on, and it does not matter which direction the locomotive is operating. This could be used to control the interior lighting in a railcar at any time that the headlight control (F0 on the Base Australia's electronics magazine siliconchip.com.au Fig.4: the red line on this graph shows how the values of CV2, CV5 and CV6 can be used to change the speed mapping of a decoder. The blue line shows the mapping that occurs if CV6 is left at its default value of 0, while the green line shows the mapping if both CV5 and CV6 are left at 0. 54 Silicon Chip Station) is active. Table 3 shows this configuration. Table 3: function & output mapping CV33 F0F CV34 F0R CV35 F1 CV36 F2 CV37 F3 Notes 2: Yellow wire CV47-CV64 are set aside as CVs that manufacturers can use for custom purposes. CV49-CV52 are often used to control special effects on the function outputs, such as flashing. The Decoder project article describes how the values in these CVs work for our Decoder. We have used CV47 to control the voltage compensation feature, which is also explained in the Decoder article. Remember that fixed address CV programming (specifically CV1, CV17, CV18 and CV29 for our Decoder) can only occur on the programming track. This still gives the option of programming other CVs on the main track with operations mode, although without the luxury of read-back and verification. This means that the speed, acceleration and function mapping outputs can all be changed on the main track. This might also be handy for remapping the CV49-CV52 function effects and seeing the results immediately. CV19 (the consist address) is permitted to be set in operations mode. Operations mode programming packets are received without regard for the consist address. In other words, operations mode programming packets should be sent to the fixed short or long address for the Decoder. This means the consist address can be reset by writing ‘0’ to CV19 at the fixed address. This means that the locomotives in a consist can and must be configured separately. In a so-called triple-header, it would make sense to disable the headlight of the middle locomotive, which can be done by programming the function mapping or special effects CVs off for just that locomotive. If you find that the Decoder configuration has been corrupted, our Decoder has a factory reset option that sets all the CVs back to their original values. It simply requires programming CV8 with a value of 8. This is listed in the standards, so other manufacturers should offer this feature. Once you do get your Decoder configured to your liking, it is a good idea to check the CVs by reading them back and noting down the values. This can form the basis of programming other Decoders, or can be a reference if you need to perform a factory reset. Other manufacturers will offer other CVs with different features, but the siliconchip.com.au 1: White wire 4: Green wire 1 2 4 4 5 6 8: Purple wire Total CV value 8 0 8 0 This configuration has the white and yellow wires operating as their defaults, with the white wire driving a headlight in the forward direction and the yellow for reverse, as long as the Base Station’s F0 control is on. The bits for the green wire being set (using values of 4) mean that it is also active when the F0 control is on, regardless of the direction of travel. Photo 4: this layout design from Les Kerr (see page 85 of the February 2024 issue) has two loop tracks, plus some sidings, and would be perfect for having a handful of trains running at the same time using the DCC Base Station. majority listed here are standardised. Operations With all that out of the way, you are probably looking forward to operating trains! With our Base Station, the touchscreen only gives access to one set of controls at a time, but you can set one locomotive moving, then switch to a different control with L1-L5 buttons. The previously activated locomotive will continue operating as set. If you have a continuous loop, it’s easy to set one train running around that loop and switch over to a different locomotive and use it to shunt in the sidings. Even if only one train is moving at a time, DCC makes it much Australia's electronics magazine easier to switch between controlling different trains. Expansion Next month, we plan to present our DCC Remote Controller. This add-on connects to the Base Station and provides an extra set of independent locomotive controls. The Remote Controller has a daisy-chain feature, so multiple can be added. The protocol it uses is quite simple. Any device that can generate asynchronous serial data at 9600 baud and 3.3V can send data to the Base Station and command DCC packets to be sent to the track. We’ll explain this further SC in the project article. January 2026  55