Technical Corner

BREAKOUT BOXES FOR DCC THROTTLE/CAB INTERFACES

It should be noted that I have no “in” with either DIGITRAX or NCE or any of their employees or suppliers.  I know only what can be found on the Internet or divined from simple observation of the various products.  The material here is my opinion, and nothing more.

Any DCC user may have a problem with DCC cab wiring.  The problem may be trivial or terrible, but when strange symptoms present themselves, the wiring and connector panels must be dealt with.  DIGITRAX™ and NCE™ DCC systems typically use flat satin wire, similar to telephone wire. The common connector is the 6P6C modular type.  6P6C is a simple way to say 6 position, 6 contact in reference to the connector.  

6P6C CABLES

This type cable and 6P6C connectors are seen in the above photo.  One thing to note is that the wiring of the cables is 'straight through', or 'pin-to-pin'.  ‘Straight through’ or ‘Pin-to-pin’ means that pin 1 of one connector connects to pin 1 of the other, and so forth through pin 6.  The industry standard places the white wire on pin 1.  The short cables made for this photo show how the locking tabs on the connectors are on opposite sides of the cable.  This is necessary to have 'pin-to-pin' wiring.  

NCE UTP and DIGITRAX UP-3

The cables connect to throttle panels which are installed in appropriate locations about the layout.  DIGITRAX systems may include infrared (IR) or radio panels as well as LOCONET repeaters or other devices installed in the cab wiring.   NCE systems may have radio receivers or radio repeaters installed in the cab bus wiring.  This article addresses only the DIGITRAX UP3/5 and NCE UTP throttle panels because I don’t know what is inside the other stuff.  

Getting access to the individual wires in the wiring and connectors is the problem.  The connector contacts are quite small and there is no practical way to connect a test instrument directly to them.  There are makeshift means involving various connector arrangements and dangling wires, but perhaps the best idea is a ‘break out box’ of some sort.  A breakout box provides a stable means of connecting measuring instruments to the various wires in the cable.

BREAKOUT BOX WITH NCE PIN ASSIGNMENT LABEL

The above photograph shows a breakout box assembly.  The breakout box has provision for holding an interface pin assignment label that is unique to the interface being evaluated. A breakout box is simple to build: a board, a 6P6C modular connector box, a EURO style terminal block, and a label to identify the connector pins and signal names for the interface in question.  

RJ RECEPTACLE & PIN NUMBERING 

This close up image of a 6P6C receptacle shows the correct position of the wire contacts.  The pins are numbered from right to left and the locking tab goes up. DIGITRAX panels have the receptacles oriented in this way, while NCE suggests installing their panels so that that the locking tab is on the bottom and the contact wires positioned so that dirt & gunk cannot accumulate on the contact wires.

DRAWING OF BASEBOARD

This drawing photo shows the foundation of the breakout box, a simple board measuring 3-1/8 by 6-1/4 inches.  The overall dimensions are not critical; the board could be larger if desired.  Right click on this image, select ‘save as picture’ and save it as a JPG file.  Print the file to the correct scale and tape the print to a board to mark out the location of the mounting holes for the various components.  I used 8-32 flat head screws and actually threaded the wood block to hold the screws that secure the interface pin assignment labels.  Countersink the holes for the four screws, on the backside of the board.   The nuts and washers used to hold the laminated labels can be simple hex nuts or fancy brass knurled nuts as in the photo.  Number 2 round head wood screws 5/8 inch long are just right for the Euro style terminal block, and note that they are on 8 mm centers – not 5/16th inch!  Number 4 or 6 sheet metal screws ½ inch long will hold the modular connector.  I cut small notches in the Euro style block and used 1 inch # 18 brass escutcheon pins to provide something to grab with test clips. Bend the pins ¼ inch from the end and fasten them securely in the terminal block.

BREAK OUT BOX WIRING

Wire from the modular connector to the Euro style terminal block, being sure to connect the wires in the correct sequence.  The wires in the surface mount connector are color coded, and the labels are color coded as well.  The surface mount jacks are available at Radio Shack, though I purchased mine from JAMECO Electronics (stock number 71984).  I used AWG 24 wire that matched the assigned color for the different positions but matching the RJ color code is certainly not essential.  Velleman has an assortment of wire called “Mounting Wire Kit”, with all ten of the common colors (stock number K/MOWM). Velleman makes a lot of neat stuff, and their neat stuff is available from many brick & mortar dealers as well as on line.  I purchased my Velleman wire kit from at J.B. Saunders in Boulder, CO.  

NCE LABEL

Only one label is required for the NCE system.  The NCE Universal Throttle Panel (UTP) front panel connectors carry exactly the same signals as are on the cables that are plugged into the rear of the panels.

The interface pin assignment labels are laminated and trimmed to size, with holes punched for holding them to the board.  Right click on the desired label image, select ‘save as picture’ and save it as a JPG file.  Print the file to the correct scale and trim to size.  A single letter size lamination pouch will hold a three label set for only about a $2.00 charge.  I use the laminating machine at the FED EX store, and trimming machine as well.  A simple ¼ inch hole punch lined up on the ‘+’ marks on the artwork will fix the laminated label to fit on the screws.  

DIGITRAX UP SCHEMATICS

NCE UTP SCHEMATIC

NCE INTERFACE TEST

Let us start with the NCE system because only one pin configuration is involved and only one interface pin assignment label.  

It should be understood that the measurements are subject to some variation because of component tolerances.  Allowing perhaps +/- 10 to 20% tolerance from these measurements might be appropriate. 

NCE system test conditions:

NCE System: Power Pro 5 ampere system.

A proper power supply attached and the system turned on.

Layout and Program track outputs are disconnected; no load on either one.

A single UTP is connected to the command station.

There is no external power connected to the UTP.

A ProCab is connected to one of the UTP cab bus receptacles.

A single ProCab is necessary to provide a driver and a receiver circuit and ‘complete’ the RS-485 interface.

The Breakout Box is connected to the second UTP cab bus receptacle.

Measurements made with a high impedance digital meter.

All measurements made in reference to pin 2 (COMMON)

Pin 1: SPARE

Pin 2: COMMON

Pin 3: +0.98 volts DC

Pin 4: +4.09 volts DC

Pin 5: +11.86 volts DC

Pin 6: SPARE

My opinion is that only modern switching mode 12 Volt DC regulated power supplies should be used to power the cabs, unregulated wall adapters are sometimes of low quality and may contribute to irregular operation.

The following information was gleaned from NCE documents: 

Wall adapters should be placed about every 40 feet (of wire) or so.  There are no hard rules, closer if several cabs may be used within that distance or farther apart if fewer cabs are used.

Any power source between 10 to 16 Volts DC is OK.  Voltages below 10V may cause problems as addition cabs are hot plugged in the cab bus.  Absolute minimum cab DC voltage is 7.5 volts.

Current requirements for NCE cab bus devices.

Each master cab will draw 125 mA (0.125A) with the display backlight on. Each intermediate cab draws 40mA (0.04A)

Radio base station RB02 draws 60mA (0.06A).

Repeater RPT1 draws 35mA (.035A).

The command station can supply only 0.5A of cab bus power.  Overloading the command station cab power output can cause the DCC system to crash or become unstable.  Overloading the cab power output of the command station will not cause permanent damage to the command station.  Budget cab bus power supply need and locations accordingly.

DIGITRAX INTERFACE TESTS

The DIGITRAX interface tests are more complicated because the command station connectors on the back side of the UP/UR panels and the connectors on the front of the same panels do not carry identical signals or voltages values on pin 1.  

Pin 1 on the front connectors of a UP3 or UP5 (and some other DIGITRAX devices as well) is DC power for the throttles and is the full wave rectified output of the RAILSYNC voltages on pins 1 & 6 of the rear panel LocoNet connectors.

Two things about the DIGITRAX LocoNet must be understood.

1. Although pins 3 and 4 are given names that are different, they are in fact connected together internal to the DIGITRAX throttles as well as in some LocoNet products.

2. The voltage at pins 3 and 4 changed effective with the implementation of the throttles that replaced the UT1/2 and DT100.  The later throttles clamp the voltage on pins 3 & 4 to approximately 10 volts DC. 

It should be understood that the measurements are subject to some variation because of component tolerances.  Allowing perhaps +/- 10 to 20% tolerance from these measurements might be appropriate. 

LOCONET INTERFACE TESTS

WITH TRACK POWER ‘ON’

DIGITRAX LocoNet interface test conditions:

DIGITRAX System: DB150 command station.

A proper power supply attached and the system turned on.

Track power is ON.

Layout track outputs are disconnected, no load.

MODE switch set to RUN.

SCALE switch set to HO.

Address 0 disabled or set to zero speed.

A UT1 throttle is connected to one of the DB150 LocoNet receptacles.

A throttle is necessary to provide a driver and a receiver circuit and ‘complete’ the LocoNet interface on connector pins 3 & 4.

The Breakout Box is connected to the second DB150 LocoNet receptacle.

Measurements made with a high impedance digital meter.

All measurements made in reference to pin 2 or 5 (COMMON).

Pin 1: 6.16 Volts DC (RAILSYNC -)

Pin 2: COMMON

Pin 3: 11.77 volts DC (LocoNet -)

Pin 4: 11.77 volts DC (LocoNet +)

Pin 5: COMMON

Pin 6: 6.16 volts DC (RAILSYNC +)

WITH TRACK POWER ‘OFF’

Test conditions same as above but with track power turned off.

All measurements made in reference to pin 2 or 5 (COMMON).

Pin 1: 11.36 volts DC (RAILSYNC -)

Pin 2: COMMON

Pin 3: 11.77 volts DC (LocoNet -)

Pin 4: 11.77 volts DC (LocoNet +)

Pin 5: COMMON

Pin 6: 0.78 volts DC (RAILSYNC +)

LocoNet “TERMINATION” TEST

The LocoNet ‘Personal edition’ specification states that “This is using the "standard" Long Distance LocoNet ® termination of a 15 milliamp positive current source on pins 3 and 4 of the connectors.”  The specification does not provide a tolerance on the current of the current source.  

To test the LocoNet termination, set up the system as for the track power on test above.  Connect a milliammeter with the positive terminal on pin 3 or 4 and the negative terminal on pin 2 or 5.  

The termination current in the DB150 used for these tests was 12.6 milliamperes.

LOCONET UP/UR TESTS

WITH TRACK POWER ‘ON’

DIGITRAX UP/UR interface test conditions:

DIGITRAX System: DB150 command station.

A proper power supply attached and the system turned on.

Track power is ON.

Layout track outputs are disconnected, no load.

MODE switch set to RUN.

SCALE switch set to HO.

Address 0 disabled or set to zero speed.

A UP-3 Universal Throttle panel is connected to one of the DB150 LocoNet receptacles.

A UT1 throttle is connected to one of the UP-3 throttle receptacles.

A throttle is necessary to provide a driver and a receiver circuit and ‘complete’ the 

LocoNet interface on connector pins 3 & 4.

The Breakout Box is connected to the second UP-3 throttle receptacle.

Measurements made with a high impedance digital meter.

All measurements made in reference to pin 2 or 5 (COMMON).

Pin 1: 10.85 Volts DC (+ DC VOLTS)

Pin 2: COMMON

Pin 3: 11.77 volts DC (LocoNet -)

Pin 4: 11.77 volts DC (LocoNet +)

Pin 5: COMMON

Pin 6: 6.16 volts DC (RAILSYNC +)

WITH TRACK POWER ‘OFF’

Test conditions same as above but with track power turned off.

All measurements made in reference to pin 2 or 5 (COMMON).

Pin 1: 10.69 volts DC (+ DC VOLTS)

Pin 2: COMMON

Pin 3: 11.77 volts DC (LocoNet -)

Pin 4: 11.77 volts DC (LocoNet +)

Pin 5: COMMON

Pin 6: 0.78 volts DC (RAILSYNC +)

My opinion is that only modern switching mode 12 Volt DC regulated power supplies should be used to power the UP/UR panels, unregulated wall adapters are sometimes of low quality and may contribute to irregular operation.

There is a great problem with writing something like this.  There is a limited amount that can be said about something that is working correctly, but a near infinite amount of blather that might be written about how stuff can fail to work.  I would like to write about how to use symptoms to find problems, but that might go on for a long time, and I would doubtless leave some mistake unfound or some fault condition undefined.  Better to just provide this tool, this “Breakout Box” and hope that it will help someone who is having difficulties with their cab bus.

Rex G. Beistle

Submitted for publication by the Rocky Mountain Region of the National Model Railroad Association.

This original work is donated to the NMRA without expecting compensation of any sort.

If this is published by any other NMRA body, just give me credit.