Model Electronic Railway Group

Leading Model Railway Electronics since 1967


Copyright for items on this page is retained by the authors identified.

The use or misuse for the sale, design, supply, process, installation, delivery, test, repair, servicing, and alteration of the designs, instructions, computer code, photographs, circuits, and any other information that appear on this web site is entirely at the risk of the user or visitor to the web site. The information is made available in good faith without warranty of any kind strictly on the basis that no liability will attach to the Model Electronic Railway Group (MERG), its officers and members.


Revised 23 January 2011

The MERG DCC system has been largely developed by Mike Bolton with help from other members including Gordon Hopkins and John Eato. The system follows the NMRA standard and is designed to be easier to use than most commercial systems for those used to more conventional Model Railway controls. It is also reasonably easy to build, in fact we have successfully assembled decoders on our demonstration stand at exhibitions. MERG is registered with the NMRA as a manufacturer with the id #165.

The original DCC command station and handset are now rather old and a bit limited in relation to recent developments in commercial DCC such as decoders with sound etc. With the development of the MERG LCB, ie CBUS the opportunity has been taken to develop a new DCC command station and handset (CAB1) with up to date features whilst using the CBUS for communication between handset and command station simplifies wiring and allows for simple connection to a computer if desired. In fact the new command station has already been demonstrated driven by the computer using JMRI and by other software developed by members. Using JMRI also allows wireless operation using an IPOD touch or similar gadget as the handset. The new handset is shown on the right, more details of the CBUS system are on the LCB page and specific info for the new DCC items on the dedicated page.


Kits for the programmer, command station, handsets,accessory decoders and encoderare available to MERG members. Members should check the kit pages in the members area for availability of kits. For various reasons we cannot supply to non-members. However full details to allow construction from purchased components are available for downloading in the downloads area.

Programmer Programmer


The programmer is a stand alone device able to programme any NMRA compatible decoder and can be used in conjunction with the MERG system or with any commercial DCC system. The latest batch of programmer kits use the 16F628A chip and need theversion 2x of the software, downloadable below. The latest version, 2K has been revised to cope with the large current inrush of sound decoders and also can handle Digitrax and CT decoders in direct mode.(The original version 1x is still available for anyone using the obsolete 16F84).

Decoder Decoder Booster Unit Booster Unit

Locomotive Decoder

Mike advises that the new versions 10 and 11 replace all earlier versions. Version 10 has all components surface mount, eliminating the need to drill the board. It may be built single sided or double sided, ie long and thin or short and fat.

Version 11 is a similar but smaller version, it has all the same features but the current rating is smaller because of the small components used.

Version 12 now has both feedback and stealth features giving a silent drive suitable for can or coreless motors in a similar size package to version 11.

Version 13 has the feedback and stealth in a version 10 size.

The software for these has been upgraded to version Dec133 which combines the features of the original Dec131 and Dec132.(NB. Dec133 updated on 14/3/03 the original version works but the BEMF code was not modified as intended - the new version corresponds to the description).

Simple cutout module

Simple cutout module (BCO1)

This is a simple cutout without auto reverse and can be used to divide a layout into sections so that a short on one section does not shut down the whole layout.

Cut out and detector unit (BDC1)

This unit can be built as a cut out enabling a booster to supply a number of seperate power districts. Alternatively it can be built as a single block occupancy detector, or both at the same time.

Booster Unit

Booster (NB1A)

This unit provides a simple booster for use where a layout needs a bigger power supply than the system unit can provide. The rating of the unit can be 10 amps or more to suit whatever external Dc power supply you have. Files updated May 2010 to fix some typing errors.

Auto Reverse and Cutout module

Auto Reverse and Cutout module (ARC)

This module provides an auto-reverse feature for use on reverse loops etc. and can also be used as a high speed circuit breaker to allow several seperate sections to be fed from one booster.

Auxiliary lighting unit

FLASH is a small 'add - on' unit for use with DCC decoders to give realistic ditch lights, gyralights and dimming. This is achieved by high frequency pulse width modulation of the LED current and look-up tables for the varying brightness with time. This enables a much slower apparent increase and decrease in brightness or a dimming without flicker. Version 2 software now gives more realistic flash rates, timed from the real thing.

Alternative software for the locomotive decoder

Gil Fuchs provided the alternative available below as 'mbint105.asm', the CVs available are listed in 'INTDEC10version103.pdf' Click here for Gil's description of his software.

January 2004. Gil Fuchs has informed me that he has discovered a 'bug' in the code for mbint104 - the Decoder 10 /11 code. It only applied when selecting which functions operated when in 'consist' mode so most users will not have noticed it. This bug has been fixed in the mbint105 which also now has the newly allocated MERG manufacturers id included.

Karol Marcinczak has rewritten mbint104 to support the PIC16F628 (cheaper than the original 16F84) and to give slow down/speed up on direction change using the values in CV3 and CV4. Download from download area as 16f628.asm.

Accessory Decoder

Software for the accessory decoder

The accessory decoder code has been updated to version 4. This version is included in the kits.

A more detailed description of the software can be found on the supplementary page.

There is a new version of the code which allows a decoder to work in dual mode. This can give 8 separate outputs when used with a Lenz system (Acc5). The code is available for download below as well as a description (acc5txt.pdf). Mike Bolton will reprogram existing PICs if you send them with a SAE for return.

Stefano Curtarolo has provided an alternative, this is available in the download section. It supports the "Accessory decoder configuration variable access instruction". This instruction is similar to a OPS program mode for accessory decoders. NCE and SYS1 stations support this useful instruction, and they make life much easier. (dbacc5 is the version for PIC16F628). It should work with the PIC16F627 too. The F628 is cheaper, faster, and more powerful than the obsolete F84.

Accessory Decoder

Accessory Decoders for Tortoises etc.

This is a variant of the above decoder with an appropriate output for slow action motors, LEDs etc. It uses the same software. The schematic and board layout can be downloaded below. Note. this decoder needs a supplementary unit to give enough power for Fulgurex type motors. It can now be replaced by the Acc6 version which has a simpler design and does not need the supplementary unit. Details can be downloaded from the download page. (Note for members, Acc 6 is not available as a kit).

Update for the accessory encoder (ACE)

This unit has been fully revised and is now at version 2b. There is also an adapter board allowing the encoder to be operated from a PC in addition to or instead of the switch matrix. Note: The code on the website was incorrect and did not work. It has now been replaced. If you have downloaded dccace3.asm and/or dccace3.hexplease replace them with the current versions ace3a.asm and/or ace3a.hex. Sorry for the error. 18/1/2006.

The code and text file have been updated to correspond with those in the kits, 18/8/08.

Handset Handset Handset

Handsets (HS2)

The handset is now at Mark 2 This uses a single sided board and is much easier to build than the Mk1.

Handset Rack

Handset Rack

Bryan Knight offers this idea for Handset users.DCC Controller Rack.

When using the MERG DCC system, it would be useful to have somewhere to park any handsets not requiring immediate attention. There is a tendency for any not actually in a hand to tumble to the floor, assisted by the weight of the cable. A rack has been constructed which holds them conveniently. Basically the device is like a lectern or music stand in which the handsets can sit, the bottom surface having cut-outs to clear the cables. It is clamped to a convenient surface, such as a table top, using one or two woodworker’s clamps to hold sideways extensions.

The whole thing is made by gluing five pieces of 6mm plywood. The “reading” surface is 260mm long by 100mm high. The sill, the same length, is 26mm wide, with the four cable slots 11mm wide and 19mm deep, separated 68mm. A triangular piece at each end supports the “reading” surface so that it is leaning back by about 15 degrees. The fifth piece is the bottom of the structure and is 340mm long to provide room for the clamps.

The picture shows the front view, with the fixing extensions and the cable clearances. The surface against which the controllers rest is leaning back in this view.

8-Channel DCC Track Circuit/Block Occupancy Detector - ‘DTC’

8-Channel DCC Track Circuit/Block Occupancy Detector (DTC)

This design, now available as MERG kit No. 56, enables members to derive an indication of the location of trains on their layout. Crafted specifically for DCC systems, it employs a 'current sensor' front end to detect the flow of traction or decoder current in a block section. A train 'in section' induces a current sensor output giving a low output at both a bipolar ‘Output’ pin and a ‘Discharge’ open collector pin. When the train goes out of section, the output from the sensor disappears and provides a delayed release of the track circuit which is prototypical in operation. Its design enables the traction and train detection circuits to be kept electrically separate.

Capable of operation from a power source of 5 - 15v dc, it can drive a panel LED or small relay directly as well as CMOS logic. This unit has been designed by Gordon Hopkins as an integral part of his RPC system, hence on a complete standard module the eight bipolar outputs can be fed to a standard RPC shift register device to allow the module to be plugged directly into an RPC Stack if required. It can be mounted as supplied in a central location, or sliced up into four smaller sections each containing two detectors which can be installed adjacent to separate track feeds around the layout.

DCC Track Circuit/Block Occupancy Detector

Single channel DCC Track Circuit/Block Occupancy Detector (BOD1)

This design is a simple block occupancy detector for use on DCC systems only. It is based on the resonant transformer circuit by Wayne Roderick of Teton Short Line to whom full acknowledgement is given.