Summary: RailCom equipped decoders can transmit information back to the DCC System
RailCom® and RailComPlus® are registered trademarks of Lenz.
RailCom is a bi-directional data communications technology developed by Lenz, and found in NMRA Recommended Practices RP 9.3.1 and 9.3.2. RailCom can read data transmitted by a multi-function decoder. Examples include speed, motor load, contents of any CV, and its address. Lenz demonstrated the RailCom concept at a DCC Working Group meeting in the spring of 2000. It has been incorporated into an NMRA RP for Two-Way communications between the command station and multifunction decoders on the track. RailCom is completely backwards compatible with Digital Command Control.
RailCom Working Group
LENZ, KUHN, TAMS and ZIMO formed the RailCom Working Group to further develop "bidirectional communication" based on NMRA Draft RP's 9.3.1& 9.3.2. Work on the protocol began in 2006, with some decoders from 2005 already supporting RailCom.
Split with Lenz
In 2011 Zimo exited the agreement when Lenz terminated their licence. Zimo continues to make decoders which support RailCom only. In addition to Zimo, a number of DCC decoder manufacturers, as well as model railway manufacturers, declined to support RailComPlus in their manufactures.
Their stated reason for declining RailComPlus was related to the licensing and approval conditions demanded by Lenz and ESU, which these manufacturers found to be unacceptable, in that they prohibited the free exchange of ideas which would impede development and technical progress of digital control for model railways. There were opinions expressed that the inclusion of a patented and proprietary technology in the DCC Standard was a problem, one that could lead to the owners using their IP against other manufacturers.
Zimo introduced a technology, Zugnummernimpulse or ZACK (Train Number Impulse), supported by DCC in 1998, which is similar to RailCom.
RailCom and RailComPlus
Under Digital Command Control, multifunction decoders do not communicate back to the command station. The command station sends commands, and the multifunction decoder supposedly acts on it. Multifunction decoders supporting Railcom can communicate back to the command station its status.
- Ops Mode Programming is easier. Ops Mode or Program On Main is supported my many multifunction decoders and command stations. Although it is a blind programming method: a CV value changes, but the multifunction decoder cannot confirm it nor easily read existing CV values. Railcom can transmit CV values without a programming track, as well as provide confirmation of changes to a CV.
- Train number detection: With RailCom compatible detection modules the multifunction decoder can transmit their presence in a block along with the address, or direction.
- RailCom Cutout: A gap in the DCC transmission, created by a RailCom Cutout unit, of approximately 500µs, for the decoder to send RailCom Messages.
- RailCom Message: A message which a RailCom-capable decoder sends out in the RailCom-specified procedure. Consists of at least one, often several RailCom Datagrams i.e.: Four short (12 bit), or one short and one long (36 bit) datagram. Error detection 4:3 bits sent.
- RailCom Datagram: data word which consists of a RailCom Identifier (4 bits) and the data field of the datagram (8, 20, or 32 bits) There are three lengths for datagrams with a total of 12, 24, or 36 bits (Identifier and Data field together).
- RailCom Identifier: The first four bits of a datagram define its content. Example: Identifier 0011 introduces a short datagram (8 bit data field) for the speed report in km/h; Identifier 0110 indicates a datagram with a 16 bit data field for a position report (number of a track section, an electronic milepost, etc.); Identifier 1111 is at the beginning of a CV message, a response to a DCC CV readout command.
- RailCom Broadcast / RailCom Broadcast Datagram: The first datagram place (Channel, 12 bit) in every message is reserved for a Broadcast Datagram. The content of a broadcast datagram relates (in contrast to all other RailCom Datagrams) not to the address of the previous DCC package. In the case of a locomotive decoder, it is used to transmit the decoder address to Local RailCom detectors such as address display modules, each with an isolated track section monitor, and recognize the train currently located there with the help of the "Broadcast Datagram".
RailCom means, that within the framework of the NMRA DCC protocol, there is a flow of information not only towards the decoders, but also in the opposite direction. Not just transmitting commands, etc., to a Multifunction Decoder, but also messages such as receipts and status information from the decoders.
The basic functionality of RailCom is based on the fact that in the otherwise continuous DCC energy and data stream, short gaps (“cutouts”, max. 500µS) can be inserted into the DCC signal on the rails, providing decoders with the time and opportunity to send data bytes which can be evaluated by stationary detectors.
With the help of RailCom ...
- Any commands which are received can be acknowledged by the decoder, increasing the operational reliability and bandwidth of the DCC System, since commands that have already been acknowledged will not require repetition.
- Current data from decoders reported to the control center (the Global Detector) such as the real speed of the Train, engine load, routing and position codes, fuel supply, and current values of the CVs on request.
- Multifunction Decoder addresses can be recognized by local detectors connected to individual isolated track sections for train number recognition and address display.
The original intent of RailCom would enable available RailCom-enabled products to report the vehicle addresses on isolated track sections (which in the broadcast process reacts very quickly providing there is only one RailCom vehicle in the section), read out of the content of CV's on-the-main (PoM), and data from the decoder as a report to display speed, load, decoder temperature.
RailComPlus appeared in 2010. RailcomPlus is an enhancement of the original Lenz RailCom standard by ESU. Only multifunction decoders which are 100% compatible with the RailComPlus standard can carry the RailComPlus logo.
RailcomPlus is an improvement over Railcom. It allows a locomotive to be automatically recognized and registered by the command station, previously only possible with Märklin's MFX standard for their two rail trains.
The big improvement in RailComPlus was messaging speed. RailcomPlus transmits not only the locomotive's speed, but all the functions available at the same time. This makes the data read function much faster, in addition to new commands to read or write groups of CVs.
- A command station compatible with RailcomPlus displays the locomotive, its functions, and saves them for future use.
- If the DCC address is already in use by another multifunction decoder in your command station's database, the command station will automatically offer to change the new locomotive’s address to another address which is not in use.
To enable the bi-directional features of RailCom some additional components are required:
- A RailCom enabled decoder
- An external RailCom transmitter
- A detector or reader
- A cutout device that disconnects the DCC signal from the track. This cutout lasts for 488 microSeconds (0.488 milliseconds). This break is so brief it doesn't cause problems with multifunction decoders.
- During the "cutout" a +/− 30 mA signal consisting of two or four bytes is generated. A current of 10mA is seen as "zero", and a current of less than 6mA is a one.
- The detector has a very low input impedance (almost a short), any other impedances in the circuit will have little effect on resulting current signal.
Bi-directional communication is enabled in the multifunction decoder with CV29, bit 3. Some multifunction decoders use CV28 to determine the messages that are to be transmitted. The usual configuration of this CV is:
- CV28, Bit 0: Channel 1 used for address broadcast.
- CV28, Bit 1: Channel 2 used for data (CV values, speed, or other data)
- CV28, Bit 2: Channel 1 used for command acknowledge
The default for CV28 is 3 (Bits 0 and 1 are set to 1 or High)
What Can RailCom Do?
- Locomotive Identification. When a RailCom equipped multifunction decoder enters a section of track with a detector, its presence will be immediately noted. At the same time you'll know where your train is.
- Info Display: A number of items can be supplied by the multifunction decoder: Speed, fuel, water level, etc.
- Data read back: It is possible to display CV values during programming, and acknowledge commands
- Unknown multifunction decoder: If a new locomotive is introduced to the layout, you can find out its address, and even allow a throttle to acquire it automatically.
- RailComPlus adds the ability to read the multifunction decoder's function capabilities and automatically register the locomotive with the command station.
Immediately prior to the cutout occurring a RailCom compatible command station transmits an Idle Packet. This signals to a RailCom enabled multifunction decoder that the cutout will follow.
RailCom uses two channels or slots. Normally a RailCom enabled multifunction decoder will not transmit during the cutout period, unless it was addressed immediately prior to the cutout occurring. The two channels allow for a response from two multifunction decoders in the same power district. These transmissions occur at the same time. If the multifunction decoders have not been addressed, they won't respond.
By using the Address Broadcast feature, multifunction decoders will transmit their addresses, but only using slot one. If multiple multifunction decoders are active in slot one, the result is garbage. Any data on the second slot will be readable, being transmitted by a multifunction decoder which was addressed. Should an unknown locomotive enter a power district it can transmit its address and register its presence.
Railcom allows a device to interrogate the multifunction decoder and learn some details.
This can be used with block detection, where upon entering a block equipped with a detection system, the detector can report back which locomotive has entered the block.
Some command stations are equipped with Railcom, such as ESU's EcoS system, detect the presence of a Railcom equipped multifunction decoder, and imports a number of parameters from the multifunction decoder, such as address, sound file, function assignments and icons for the functions, without any operator intervention.
RailCom can be used for automation, allowing events to be triggered by a locomotive entering or exiting a zone with a detector. The detector can supply status information to an external computer running appropriate software, which can then trigger various events, such as headlight on/off and sound horn/whistle when entering or leaving a tunnel. Other possibilities include only having passenger trains stop at stations, or selected stations.
Another possibility is using the computer to calculate fuel/water usage, instructing a steam locomotive to halt and take on fuel and/or water. Diesels, which need less fueling can proceed past these facilities. Steam locomotives can also be instructed to stop at an appropriate fuelling location, for oil or coal. In addition to these functions, it could also be used to route a train onto the correct track for passenger or fuel operations. For example, southbound trains are routed to the south platform and northbound to the opposite platform.
This is all possible using the appropriate software and a computer. It will require writing routines or scripts to create and control these actions. The computer will create and transmit instructions to the command station to activate and control various functions in multifunction and accessory decoders as per the scripts.
What is RailComPlus?
Standalone Cutout Device for RailCom
The openDCC project supports Railcom via the Open DCC GBM command station. With PC model control software block occupancy and train identification are possible.
- DR5088RC Digidetect 16 Channel Feedback Module
- Sixteen RailCom detectors and a Global Detector
- LocoNet T
- USB 2.0
- Maximum addresses: 2048
- Powered from the track
ESU offers the 54680 RailCom transmitter. This transmitter, installed in a vehicle with a multifunction decoder which does not support RailCom, adds full RailCom compatibility. There is no need to upgrade the existing multifunction decoder. It is programmed to match the address of the vehicle and will respond to a RailCom receiver.
- These devices only work with specified ESU command stations and boosters.
The LRC100 is a small module which allows locomotives equipped with a multifunction decoder (without RailCom) to be retrofitted for RailCom.
Lenz offers the LRC120 address display.
The LRC120, which is a local detector, displays the address of a locomotive in a track section connected to the LRC120. This track section may be located at any point on your layout.
For the LRC120 to display the address, a RailCom capable booster must be used, such as Digital plus LZV100 or LV102, which currently have this feature. These units are equipped with the necessary cutout device.
When the LRC-120 is active the first digit of the display displays a - . When a locomotive equipped with a RailCom enters the section monitored by the LRC120, the address of the locomotive will be shown on the four-digit display.
If the locomotive is a member of a consist, the consist's address will be displayed. Thus, the display always shows the address which you can use to control the speed and direction of the locomotive from your throttle.
The LRC120 is also capable (in the future) of displaying CVs.
No products made by Märklin support RailCom. If you need feedback from there models use a RailCom sender unit or you have to use MFX.
Zimo multifunction decoders since September 2007 support CV readout, speed and load feedback, address broadcast.
The MX31ZL contains a global detector; for command stations MX1, MX1HS, MX1EC plug-in global detectors were planned for late 2007.
Accessory Decoders: RailCom will send feedback indicating the turnout position. The MX82W is a version of the MX82 with 4 outputs to turnouts or 8 signal lamps.
Most issues are related to the command station and the multifunction decoders in use.
Many, but not all, command stations generate a preamble with more than the minimum number of 12 bits. The extra preamble bits can be used for the RailCom cutout. Problems arise with multifunction decoders expecting a minimum of 12 or more preamble bits. Many multifunction decoders will function with only 10 preamble bits. Should the command station generate 16 preamble bits, 4 can be used for the RailCom cutout without impairing decoder operation.
When experiencing erratic operation by some multifunction decoders, verify RailCom is not active by checking the command station options. Turning RailCom off should eliminate the problem. If RailCom is desireable, any incompatible decoders will require replacement with a modern compatible version.
RailComPlus can be disabled, while leaving RailCom active. RailCom is enabled within CV29, RailComPlus is enabled using another CV. Check the documentation for the multifunction decoder for those CVs, as they can change.
Lenz DCC Issues
- Lenz DCC system software Version 3.6 has RailCom active by default.
If locomotives do not respond, verify the multifunction decoder installed. A number of non RailCom capable multifunction decoders offered do not handle RailCom signals properly and may not work with a Lenz DCC system. To solve this issue, RailCom must be disabled in the command station. See the manual for your system.
Lenz command stations and boosters accurately implement the NMRA DCC standards, often any problems encountered are related to multifunction decoders which do not correctly follow the DCC Standard.
TCS WowSound Multifunction Decoders
TCS WowSound multifunction decoders may not function correctly if RailCom is active. This is usually related to having a European command station, such as those from Lenz or ESU. Disabling the RailCom features should eliminate the issue.
Some TCS multifunction decoders do support RailCom. Firmware Version 89 and higher support RailCom and RailComPlus, provided they are purchased directly from TCS. Vendor stock may be a lower firmware version and results may vary. 
TCS has stopped supporting RailCom as of 2022, but will incorporate the technology in their upcoming WOWSound 5 multifunction decoders.
- Lenz website, in German. Unfortunately, Lenz no longer maintains a presence in North America.
- ESU RailComPlus FAQ
- locgeek.com RailCom-RailCom-Plus-What-are-These?
- RailCom Uses
- DCC4PC RailCom products
- RailCom page
- Some of the information presented was gleaned from newsletters published by Zimo.
JMRI also supports RailCom.
- See S9.2, Preamble: The preamble to a packet consists of a sequence of "1" bits. A digital decoder must not accept as a valid, any preamble that has less then 10 complete one bits, or require for proper reception of a packet with more than 12 complete one bits. A command station must send a minimum of 14 full preamble bits.