Summary: In order for decoders to know what to do, you have to program them. This applies to both multifunction and accessory decoders. Decoder programming is the action of entering suitable values into the configuration variables (CVs) in a DCC decoder to set their address, Speed Table, Functions and other parameters so they operate with the characteristics you choose. You should check your owner's manual for complete descriptions on the configurations available and which programming methods it is compatible with.
Movement & speed
|DCC Core components|
|Track work components|
- 1 Decoder Programming
- 1.1 Programming Modes
- 1.2 Service Mode Programming Methods
- 1.3 Operations Mode
- 2 Notes
- 3 See also
There are two primary modes of programming: Service Mode and Operations Mode.
- Service Mode Programming requires a programming track which is isolated electrically from the rest of the layout.
- Service Mode is also known as Broadcast Programming Mode
- Operations Mode Programming (OPS Mode) can occur anywhere on the layout since the programming instructions are directed to a specfic decoder's address. This method is sometimes referred to as Programming on the Main or POM.
As can be expected, all new vehicles2 equipped with multifunction decoders must be programmed using Service Mode on the programming track. They don't have an address yet (other than the default address of 031), which makes programming on the layout very dangerous. (See the section on Service Mode.)
Most DCC systems support all methods of programming, however some DCC decoders do not support all methods because of limited resources (amount of memory/features) available to the microcontroller.
- Some DCC systems may be limited by their software to CVs below 255. There may be additional steps required to program a CV greater than 255. Many multifunction decoders will not have this issue, but some of the more advanced multifunction decoders may.
Before programming, read the Address Range page for explanations about multifunction and accessory decoder address partitions.
The modes can be thought of in this manner:
- Operations Mode = Addressed
- Service Mode = Global
Operations Mode sends programming instructions to a specified address. Service Mode does not use an address, therefore its instructions are acted on globally by all decoders. Service mode can be used accidentally, which will program every decoder on the layout.
If the booster lacks a separate pair of connections for a programming track, both modes are present on the track. Boosters equipped with a programming track output should not transmit service mode packets to the track, this mode should activate the program track connections.
Operations Mode may be used while a vehicle is in motion, allowing for adjustments to motor and sound parameters. While you may change the Primary Address in this mode, altering the Extended Address is not possible.
Broadcast Mode is another term for Service Mode. Programming instructions are broadcast to all multifunction decoders on the track, and every decoder will accept the instructions. Broadcast Mode is dangerous as it will reprogram every multifunction decoder in every vehicle on the layout.
A programming track is either a dedicated length of track located off the layout, or a section of track on the layout which is electrically isolated from the rest of the layout. The booster's programming track outputs connect to it for Broadcast Mode programming. Only the vehicle on the programming track will be programmed.
Service Mode (Programming Track)
Service Mode Programming, also known as Broadcast Programming, occurs on a programming track that must be electrically isolated from your running track (see Programming Track for electrical isolation tips.)
The purpose of Service Mode is to allow Customization and testing of decoders.
Service Mode Programming Environment
Service Mode operations should be done on an isolated programming track.
- An isolated programming track is essential when using Service Mode programming, as the command station sends a broadcast message to all decoders on the track. If you did not use a programming track, you would essentially reprogram all your decoders, something you most likely do not want to do. By using a programming track, you isolate, or select, the decoder you want (re)program.
- External loads, such as constant lighting, smoke generators and other loads should be switched off. Decoders should shut down any loads connected to them when they enter service mode.
- Service mode operations should take place in a limited power environment. The NMRA specifies no more than 250mA for 100mS. The maximum current may be limited by the programmer using a resistor, but the manufacturer must state that it may not be 100% DCC compatible.
- Decoders equipped with capacitors, such as "keep-alive devices may pose problems. If possible they should be disconnected prior to programming operations.
- A programming track booster may be required for some sound decoders.
- A Program Track connection may or may not be part of the command station/booster. Boosters which don't have additional connections for the program track use Service Mode on the layout via the track power connection.
Entry and Exit from Service Mode
A decoder is required to enter the service mode when it receives a reset packet followed by a service mode packet. It cannot act on service mode packets when it is not in service mode.
The decoder will exit the service mode, but not enter operations mode if
- A non-service mode instruction packet is successfully received
- Twenty milliseconds have passed since the last valid reset or service mode instruction packet was received
After exiting service mode, the decoder will only re-enter operations mode when it receives a valid operations mode packet that is different from the previous service mode instruction packet. This is done to prevent execution of service mode packets as operations mode packets.
- Service Mode instruction packets have a short address in the range of decimal 112 to 127.
There is an acknowledgement (ACK) mechanism specified. This allows the decoder to act on instructions regardless of the ability of the programmer to detect the ACK signal.
There are two mechanisms defined:
This is defined by the decoder providing an increased load on the programming track, of at least 60mA for 6mS. One method is to apply power to the motor, or a similar load connected to the decoder. Many decoders apply a short burst of power to the motor, causing the locomotive to lurch forward.
The decoder should not send an ACK pulse until all the CV bits have been written into non-volatile memory.
The programmer should scan for the pulse after the end bit of the second service mode instruction packet, or as many packets as needed to send the instructions, as well as the specified decoder recovery time. The programmer must continue to send packets to the programming track (which turns supplies power to the decoder) until the end of the decoder recovery time.
Service Mode Programming Methods
There are three methods of service mode programming:
- Physical Register Mode
- This mode is the most commonly used programming mode and is the preferred method.
Paged Mode introduced the term "CV" (Configuration Variable).
Paged Mode simply queries the decoder with "Does CVxx have a value of Y?" It continues to increment the value of Y until the decoder acknowledges the value matches the contents of the specified CV. There are only two states: NACK (Not Acknowledge), where the command station times out waiting for a response, or ACK (acknowledge), where the decoder responds. This method can take a long time to read the CV settings of a decoder, as it must step through up to 256 (0,1, 2, 3, 4..... 255) possible combinations.
- Paged Mode can be problematic with some DCC Systems. Direct Mode is a better choice, especially with modern decoders.
- Direct Mode is another mode allowed by the NMRA DCC Recommended Practices.
- Direct Mode is becoming more popular.
Direct Mode is similar to Paged mode, except when querying a decoder it asks "Does Bit 'X' of CVx = 1?" It is faster, as it can read an eight bit CV with 8 queries (1, 2, 4, 8, 16, 32, 64,128) instead of a possible 255 reads required by Paged mode. It can then calculate from those responses what the content of CVx is. In the worst case the command station will have to wait for 8 timeouts to occur, or 9 to determine no reply.
- Direct Mode is the preferred method with ESU decoders, and is much faster.
Physical Register Mode
Physical Register Mode is an older, obsolete method of programming that is not used in current production decoders. Some decoders still support this mode for legacy reasons.
- This mode allows you to program a decoder no matter what state it is in. You do not even need to know the address (hence the danger of programming all your decoders at once). It will also work if the decoder's memory has become corrupted.
The only CVs which can be accessed and changed via Register Mode are:
- CV1 Short address
- CV2 Start Voltage
- CV3 Acceleration Rate
- CV4 Deceleration Rate
- CV29 Configuration Register
- CVs 7 (Version) and 8 (Manufacturer ID can be read in this mode.
Service Mode also includes a CV Readback capability. The decoder can be read and its settings determined. Not all command stations support readback.
Service Mode Instruction Packets
- Bits are numbered right to left, Bit 0 is the rightmost bit and the Least Significant Bit.
- Long Preamble: The packet will have a preamble of at least 20 bits, to allow the decoder time to process the instructions.
- Power On Cycle: After applying power to the track, the programmer will send at least 20 valid packets before service mode operations commence. This allows the decoder to power up and stabilize.During this time a current draw greater than 250mA lasting more than 100mS after initialization of packet transmissions is considered a fault condition. A decoder should not draw more than 100mA of current, except when processing an acknowledgement.
Operations Mode Programming occurs anywhere on the layout. With Operations Mode programming, the programmer directs programming commands only to the decoder address you selected. Some command stations may allow programming while the locomotive is in operation.
Operations Mode can also be referred to as:
- Program on Main (POM)
- Ops Mode
This mode of programming is useful if you cannot program the locomotive's decoder on a programming track. Or adjusting CVs while the locomotive is in motion, such as tweaking sound operation or speed matching.
You cannot change the decoder address in this mode. (See Below).
For Ops Mode programming to work, the decoder you are using must be capable of this feature.
This programming mode is based on the Direct Mode to provide quick programming capability.
Many DCC systems will perform a blind write to the decoder in this mode. The system will respond indicating the write was successful, even if it failed.
Changing an Address in Operations Mode
Changing an address using Operations Mode is fraught with problems.
If the current active address is an Extended (two byte) Address, any attempt to change the address will fail.
The reason is that an Extended Address is stored across two CVs (CV17 and CV18) and that specific address is used to direct the Operations Mode commands to the correct locomotive. When the first CV (CV17) is altered, the extended address is now a new, unexpected result (until CV18 is changed), but the original extended address is still being transmitted to change the value in CV18, thus the change instruction fails.
1: While the default address for a multifunction decoder is specified by the NMRA Standard as a primary address equalling 3 (three), this rule does not apply to a vehicle with a multifunction decoder installed at the factory. This exception allows the manufacturer to set the default address to any value they wish.
2: A vehicle can be a locomotive or other form of rolling stock, such as a caboose or a sound equipped freight car.
This is a device designed for NCE's Power Cab. It has one input from the Power Cab and two outputs, one to the track, the other to a programming track. When the Power Cab is switched to programming mode, the service mode packets received by the Auto-SW cause it to change the output from the track to a programming track. When exiting programming mode it reverts to the default state of power to track.
This device can be used with other manufacturers boosters which lack a programming track output.