Technical Corner

DIGITRAX THROTTLES ALWAYS LOAD RAILSYNC

Digitrax throttles always draw current from RailSync, regardless of whether they have batteries or not and regardless of whether the batteries are new or run down, unless an adequate auxiliary power source is provided for the throttle panels used around the layout. The RailSync outputs of the command station are limited in the amount of current that they can supply.

If a sufficient number of throttles are used without batteries, or with run down batteries, the RailSync signal lines will be loaded down to the point of causing the voltage regulator in one or more of the throttles to fall out of regulation. If this happens the throttles may logically ‘lose their minds’ and kill LocoNet by jabbering uncontrollably. This is the "Unstable LocoNet" that is sometimes mentioned on Internet chat lists.

RailSync is the command station’s logic level DCC output signal. RailSync is a one way, output only signal from the command station to boosters and some Digitrax layout accessory devices. RailSync is quite literally the command station signal that the booster(s) “BOOST” to NMRA DCC compatible voltage and current level suitable for powering mobile decoders on the rails as well as stationary decoders.

It should be noted that I have no “in” with DIGITRAX or any of their employees or suppliers. I know only what can be found on the Internet or arrived at following careful observation and deductive reasoning. The material here is my opinion, my work, and nothing more.

To begin to understand RailSync and how it is used by throttles connected to LocoNet, refer to the following diagrams. Note carefully that the circuit identifiers for diodes, resistors, and any other components are my own, and do not in any way relate to what may or may not exist in any Digitrax product documentation. The wire colors, pin numbers, and signal names in the LocoNet interface agree with Digitrax documentation.

The diagram on the left represents a throttle without a battery. The throttle may be any throttle, whether made for use with a battery or not. The six wires in the Throttle’s LocoNet cable logically become just three at the throttle electronics. Pins 2 & 5 are both common, and pins 3 & 4 are tied together as the bi-directional LocoNet signal. One or the other of the differential RailSync signals at pins 1 & 6 is always positive, and these digital signals are rectified by diodes D1 and D2 to become the DC voltage for the throttle. The throttle will draw current continuously, first from one and then the other of the differential RailSync signals.

The throttle electronics includes a voltage regulator to regulate the bulk rectified RailSync to the (5 or perhaps less) stable DC voltage required by the throttle’s micro-processor. The voltage regulator requires some head room to assure correct operation. The minimum acceptable voltage at the regulator input is about 7.5 to 8.0 volts. The forward voltage drop of either D1 or D2 must be added to that resulting in a minimum acceptable voltage at either pin 1 or 6 of about 8 to 8.5 volts. Note that this is the minimum acceptable voltage; a robustly wired LocoNet installation should deliver 12-15 volts DC to the throttles through an auxiliary power source.

The diagram on the right represents a throttle with a battery. As in the throttle without a battery, the six wires in the Throttle’s LocoNet cable logically become just three at the throttle electronics. The 9 volt battery, with an added diode D3, connects from common to the throttle voltage line. The throttle will draw current from one or the other of the RailSync signals only when the RailSync voltage is greater than the battery voltage.

These two diagrams and descriptions reveal the use of the differential RailSync signals to power the throttles that are part of LocoNet. The following diagram will delve into the command station’s RailSync signal source and how the same is passed along to the throttles.

The Command Station is represented on the left and a throttle on the right. For this discussion, the only thing of interest is the RailSync source. A proper electronics diagram symbol might be some clever thing such as shown at the top of the command station diagram. For sake of better understanding, a double pole, double throw switch or relay is shown. The switch is shown in the logical ‘off’ or digital ‘zero’ condition. RailSync “-“ on pin 1 is connected to +12 Volts and RailSync “+” is connected to 0 Volts. When the switch is moved to the logical ‘on’ or digital ‘one’ condition RailSync “-“ on pin 1 will be connected to 0 Volts and RailSync “+” will be connected to +12 Volts. The voltage levels of 0 & +12 volts are simply for discussion, and in fact vary greatly depending on the number of throttles, boosters, and other LocoNet devices loading RailSync.

Normal operation of the DCC system will see the RailSync signal reversing 7-8 thousand or more times per second. The alternating digital RailSync outputs are rectified and provide throttle voltage as previously described. Turning track power off puts RailSync in the logical ‘off’ or digital ‘zero’ condition. This condition puts +12 Volts on pin 1 and 0 Volts on pin 6. Booster circuits use a ‘missing pulse detector’ to interpret the absence of the RailSync differential data stream signal as a command to turn off the booster’s output.

Note that the RailSync outputs are passed through resistors R1 and R2, the value of which are not defined. These series resistors are what limit the number of throttles that can be connected without loading to the point of “Unstable LocoNet.” These current limiting resistors also protect the command station against wiring problems that might short circuit the RailSync outputs.

Most layouts require the use of multiple throttle operating locations and this requirement is satisfied by UP5 throttle panels, and previously with the long discontinued but functionally similar UP3.

This diagram shows a UP5 connected to a command station. The UP5 has diodes D2 and D4 that take the RailSync “+” & “-“ signals on the rear RJ12 connectors, and rectify them to raw DC that is connected to pin 1 of the two front panel connectors and the single side connector. As previously described, a throttle will get its voltage from whatever source is the more positive; the battery if there is one or the voltage at pin 1 or 6 of the RJ12 connectors on the UP5. Pin 1 of the RJ12 will always be raw rectified DC from “+” & “-“ RailSync and pin 6 will be only the RailSync “+” signal.

There are other diodes on the UP5 and other sources of voltage as well. The following diagram of a UP5 with a throttle connected provides added description of the operation of a UP5.

The above diagram reveals the UP3 or UP5 diode arrangement that determine which source will actually provide power for the throttles that are plugged in. The panels have five diodes, with all their cathodes connected together, creating a local throttle power bus. The local throttle power bus connects, through resistor R2, to pin 1 of the front panel connectors and also the side connector. The anodes of diodes D2 & D4 connect to pins 1 & 6 of the rear panel LocoNet connectors. The anodes of diodes D1 & D5 connect to the screw terminals on the rear of the UP and may be connected to nearby track power. The anode of diode D3 connects, through resistor R1, to the Auxiliary Power input for the UP and to the + COM (+ COMMON) point on the UP5.

Throttles plugged into the UP5 will draw their operating voltage from the most positive of the voltages connected to these five diodes if/when one of the sources goes somewhat more positive than the throttle’s battery voltage. If the throttle does not have a battery, then it will draw operating current continuously from pin 1. At any instant, a throttle plugged into the front of a UP5 will draw power from the more positive of any one of SIX voltage sources, SEVEN if the throttle has a battery.

If a 14 Volt DC auxiliary supply is used to power the throttles used with UP5 panels, and if the power supply output is more positive than the most positive level of the RailSync lines, then loading RailSync will never be a concern.

If the rear terminals of the UP5 panel are connected to DCC track power then the throttles plugged into that panel will not load RailSync when the track power is on. Connecting the UP5 to track power gives indication of track power present through the LED on the front panel, and that indication is a nice feature. If track power is lost and there is no other auxiliary power source for the UP5 panel the throttles plugged into that (those) panels connected to the track will load RailSync and could result in “Unstable LocoNet” operation following a derailment.

Perhaps the worst possible combination of tethered throttle operation is to connect the UP5 panels to track power, and never use batteries, and not provide any other auxiliary power. If the throttles overload RailSync when track power is off and the throttles do not have power, then the throttle cannot be used to turn track power on. Relying on track power to provide throttle power is also problematic if stationary decoders are used to control switch machines. If an operator runs a switch aligned incorrectly the track power will be shut off and that removes power from the throttle and the stationary decoder making it impossible to align the switch correctly.

Digitrax instructions describe ‘daisy chaining’ the auxiliary power from one Universal Panel to another and another up to 10 UP5 panels via a wire soldered to the ‘+ common’ terminal on the printed circuit board. This single wire carries the power to the several UP5 Panels that may be daisy chained, and the return is through the LocoNet cables back to the UP5 where the auxiliary power supply is connected.

A different, and admittedly more expensive way, is to wire a two wire DC throttle power bus from UP5 panel to UP5 panel. The connection from this DC throttle power bus to each Universal Panel is via a short power cable that plugs into the coaxial power connector on the panel. This method eliminates soldering a wire to the tiny hole in the printed circuit board. The following diagram illustrates this method of providing throttle power.

I installed fixed DC power supplies for the UP3 (now discontinued) panels on the Denver HO Model Railroad Club layout in early 1999 and the club has never experienced unstable LocoNet. The fixed DC power supplies are left on day and night and function as throttle battery savers when the layout power is turned off. None of the power supplies for the command station, four boosters, and four PM-42 power managers are left on when the layout is unattended. Since the throttles have power present before the DCC system is turned on the system comes up clean every time.

The Digitrax documentation states that the voltage shown when plugging in a DT-400 (or similar) throttle may be as low as 9 volts. Throttle voltage that low is moving toward a voltage at which the attached throttles may become inoperative or unstable. I convinced that it is better to provide a regulated 14 volt DC throttle power bus for throttles and avoid loading the RailSync lines.

This paper has addressed the six wire Digitrax LocoNet only as implemented with UP5 throttle panels. The command station’s RailSync output, combined with the UP5 and its three ways of delivering throttle voltage results in an unknown number of wiring configuration combinations quite beyond anticipation by this writer. Suffice to say that the objective of this paper has been to provide some insight into the workings of the hardware when implemented as noted.

FOLLOW ALL LOCAL ELECTRICAL AND BUILDING CODES.

PURCHASE GOOD QUALITY POWER SUPPLIES WHICH CARRY THE APPROVAL STAMP OF NATIONALLY RECOGNIZED TESTING AGENCIES SUCH AS U/L, CSA, VDE AND OTHERS AS APPROPRIATE AND RECOGNIZED BY THE CONTROLLING BODY AT THE PLACE THE POWER SUPPLIES ARE TO BE INSTALLED AND/OR USED.

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.