Barry and Penarth Model Railway Club

DCC – the way forward?

– A discussion paper by Gwynne Chivers

Background

Most people are aware of the current rise in interest in DCC over the last couple of years or so. Systems have been produced independently in both Europe and the USA, so that currently there are essentially six systems readily available for purchase. By name Digitrax, Atlas and NCE are the main system manufacturers in the USA and Roco, Lenz and ZTC Controls (now defunct!!) are the European manufacturers. There is also some additional availability from some other sources.Bachmann’s recently introduced system is a basic set which is manufactured for them by Lenz, and the Model Electronic Railway Group (MERG) produce a full range of kits available for those who have the skill and equipment for surface mount soldering.

During 2005, the club embraced DCC, but has created a totally flexible system of operating its main layouts to allow members to operate any of their stock. There is a statement about DCC on a separate page.

Some basic information about DCC

DCC has three main components, and just to help there is sometimes different
terminology for each!

1 – the ‘throttle’ or ‘handheld’ controls loco speed, direction, functions and programming by identifying the loco to be controlled and generating the signals required for each operation. The number of available functions, and whether it is knob or button speed control (etc), are generally dictated by
this device.

2 – a ‘command station’ processes the signals from one or more throttles, and transmits them to the track via a ‘booster’. The throttles connect to the command station either directly via the socket on the unit or via a ‘network’ for more than one throttle.

3 – a ‘booster’ or ‘power station’ takes the a.c. supply from a separate transformer and regulates it to a fixed voltage (which may be different for different scales), and has built in short circuit and overload protection. A booster can be either combined with a command station in one unit or a separate unit.

There are two reasons for having separate boosters – one is if the total locomotive current draw exceeds the rating of a single booster, the other is to  create separate ‘power districts’ which have independent supply so that a fault on one doesn’t shut down the whole layout. Separate boosters do not process signals, but need to be linked to a ‘command station’ via 2 wires known as the ‘command bus’ in order to pass signals to the track in its power  district. Each booster is usually supplied form a separate transformer. One TF can supply more than one booster, but the maximum current is then  limited to the TF capacity.

(N.B. Some of the so-called starter systems can have several units combined – for example the Bachmann EZ DCC has everything in one box; the Digitrax
Zephyr’ has throttle and command station in one box; and the Lenz LV100 has the booster and command station in one box…)

Interchangeability

The essential feature of all of the DCC systems is that they all perform to within a set of standards for the control of locomotives. These standards are those set down by the NMRA as Standards and Recommended Practices (RP). These standards and RP’s define the way that a system sends information to the track and the format and content of the registers within each mobile decoder that holds the information about the locomotive and the way that it responds to the various commands.

By adhering to these standards any manufacturer’s DCC systems can control any decoder. Therefore one owner, or group of owners, is not restricted to purchasing all the locomotive decoders from the same manufacturer as the control system.

On the other hand there is an incompatibility between the US and European systems in the way that multiple hand throttles (hand controllers) can be used with a base station. The systems, known as LocoNet by Digitrax and XpressNet by Lenz,are networks that use different protocols and connections to
communicate, hence you can’t mix the two manufacturers. This means that a hand throttle from Digitrax will not be able to work with a Lenz system or vice-versa. However, Lenz, Roco and ZTC all use the same system and any of their hand throttles can be interchanged.

Hand Throttles and Functions

In general use any hand throttle can control up to a reasonable number of locomotives at any one time. There are various differences between the way that each locomotive is selected and some have rotary speed knobs whilst others use a push-button type speed selection.

Locomotive functions such as lights, smoke units, sound etc are selected from the hand throttles. Some of the smaller hand throttles will only support the earlier NMRA standard of 9 functions (f0 and f1-8) whilst the later units support the full latest version with f0 and f1-12 with the ability to select on-off toggle or momentary for all of these functions.

The number of functions in use depends completely on the decoder fitted to the locomotive. This will determine how many functions are available for use on that particular locomotive. In the majority of cases only 3 or 4 functions are available and used, sometimes only 1 or 2, but with the increasing number of specialised sound units becoming available all 13 functions are necessary to control all of the possibilities.

Programming a Locomotive

This refers to entering data into a locomotive to determine how it behaves when selected from the hand throttle. In general the locomotive will need an address entered, a loco number, and owners may typically want to set the amount of acceleration and braking delays. There are additionally many other items that may be set within the locomotive decoder. Each of the settings that can be programmed within a decoder is known as a Configuration Variable (CV).

There are two ways that programming can be carried out within DCC: Service Mode and Operations Mode.

Service mode programming is carried out on a dedicated programming track and has the ability to change any of the CVs for the decoder. In this method a separate connection is taken from the system control unit to the dedicated track. Only a locomotive on the programming track can be configured in this mode.

Operations mode programming is also known as ‘programming on the main’ in some documents. In this mode the locomotive to be programmed can be anywhere on the layout and must be addressed individually to ensure that any other locomotive on the layout is not affected. Some older decoders do not allow operations mode programming. Also some systems, and some decoders, will not allow locomotive addresses to be changed during operations mode programming.

Most systems restrict the amount of current draw available to the programming track. This is to prevent an accidental overloading or burn-out of a decoder whilst programming.

Power Districts and Boosters

Each system controller will allow only a maximum of 5 amps of current draw for the track. This limits the number of locomotives that can be used at any one time (including lights, smoke units and sound). If more current is needed on a layout then it must be split up into a number of power districts. Each power district must be fed from a power booster. There is no limit to the number of districts, and boosters, that can be used on a layout.

Each power booster is fed from its own separate mains transformer and has a set of control wires linking it to the main unit.

DCC and Non-DCC Locomotives

This is the area that interests a lot of members who have not yet ‘taken the plunge’. To put the issue into simple, straight-forward terms:

A loco fitted with a decoder will run on any DC layout provided the decoder is set to allow it.

A non-DCC loco can be run on a DCC system provided certain safeguards are taken:

  1. Both Lenz and Digitrax will run only one non-DCC equipped loco at any one time.
  2. A non-DCC loco will ‘sing’ when placed on the track but this is not damaging to the loco
  3. Both Lenz and Digitrax recommend that a non-DCC loco is not left on powered track for any length of time whilst not running.

Layout Wiring Considerations

As most people are aware the major difference in wiring between a DC and a DCC layout is that all tracks are fed together with DCC wiring. This means that all tracks are powered at all times. In conventional DC wiring sections are put in place to allow locos to be isolated or different controllers to run different sections. To allow operation on either DC or allow DCC to be plugged in then the traditional common return rail must be separated in a number of instances.It is not permissible to allow the connection, inadvertently or otherwise, of DCC output to the output of a DC controller. The DCC base unit would probably see the DC controller as a ‘short-circuit’ and shut down. However if it did not the introduction of 16v ac to the electronics in the DC controllers could damage them beyond repair.

Taking the suggestion of converting the goods loop on Holton to allow DCC operation this would mean the following:

  1. All common return wiring for the goods loop must be separate from the main lines.
  2. The double-slip on the shed entry road would have to be isolated from the DC circuit as would the loco shed entry from the fiddle yard.
  3. Switching arrangements would have to be inserted to allow for either DC or DCC operation at the controller entry and the connections to the other lines.

Recommendations

In concluding this paper it would not be correct to recommend the purchase of one system against another. This section is therefore intended to make a series of comments that the committee might consider when making their decision.

  1. The system must be capable of being moved between layouts easily. Ideally only two sets of wires for a single power district: one for the track current and one for the control cables.
  2. All hand throttles should be of the same type so that operators have no difficulty in moving between hand throttles on one layout at any time.
  3. Provision should be made for an amount of fixed wiring on each layout. This should include hand-throttle plug-in facilities at various points around the layout.
  4. All locomotives should be numbered in some form of standard manner to allow for ease of use and recognition by operators.
  5. A standard should be set by the club for speed data in locomotives. For example all locomotives should use the same number of speed steps and have common functions set from the same function keys where appropriate.

During 2005, the club embraced DCC, but has created a totally flexible system of operating its main layouts to allow members to operate any of their stock. There is a statement about DCC on a separate page.

Additional Web links

A useful external website is Allan Gartner’s ‘Wiring for DCC’ QSI manufacturers of sound systems for Broadway and others

South West Digital Ltd for friendly DCC help, advice and sales. They have a range of authentic sound chips for several British diesel classes.

Bromsgrove Models have a large range of DCC supplies from various manufacturers including
Digitrax, Lenz, Train Control Systems (TCS) and NCE both on mail order and
at exhibitions.