KB1031: Duplex Troubleshooting

This article was last updated on Jan. 23, 2016, 10:39 a.m. | Print Article | Leave Feedback


Digitrax Duplex Wireless Functional diagnosis and debug process:


Duplex Background:  The Digitrax Duplex radio system is a two-way RF radio link connecting wireless throttles (DT402D & UT4D) via a UR92 onto LocoNet.  It operates on one of 16 user-selected channels in the 2.4GHz Wi-Fi frequency band.  FCC rules requires that all WI FI devices including the RF24 Duplex transceivers (part of Digitrax Duplex throttles and the UR92) operate co-cooperatively in this 2.4GHz band and have to accept whatever wirelessenvironment is present.  This means that there is no guarantee that Digitrax Duplex will work in all situations or that it will work at all times.

The RF24 attempts to transmit data when there is no significant signal traffic on the channel selected.  Baby monitors, intercoms, cordless phones, Bluetooth devices etc. may operate in an almost continuous transmit mode, and may block a WI FI channel almost completely leading to poor or even non-existent Duplex capability.

Another possible issue is RF signal strength issue.  The RF24 typically has a power output that allows operation up to about 300'.  Any number of factors such as metal walls, HVAC ducting, reinforced concrete floors, local water table, human bodies, etc. can affect the performance of Duplex radio.

The UR92 is designed to operate as a single LocoNet Duplex access point on most small to medium layouts.  Reception issues can be resolved in many cases, as layouts get bigger with additional UR92's.


(1) Setup:   Locate the UR92 temporarily in a central part of the layout between waist to eye height.

a) Plug in the provided Digitrax PS14 DC power supply to the UR92.  Verify the UR92 GREEN "radio" LED is initially lit with no LocoNet connection, and occasionally shows a brief "heartbeat" blink off.  NOTE: UR92s produced after December 2015 will not operate without at least a 14volt DC power supply.

b) Connect a good tested LocoNet cable from the rear of the UR92 to an active LocoNet

c) Install a known good 9v into a DT402D throttle and plug the throttle into Loco Net.  Watch the display as it negotiates its connection.  If the DT402D correctly detects the UR92, and Duplex mode is available for use, it will join the Duplex network and flash up the; current Duplex Net Name in the center 8 characters of the LCD display, and the lower left side will show "ChXX", where XX is the Duplex channel number in use, in valid range from channel 11 to 26.

If you do not get the message, there is a problem with the LocoNet connection; UR92, PS14 power supply or Duplex Radio operation is turned OFF in the DT402D's Options.  Verify your connections and try again.

Plugging into LocoNet is the first logical step to join any Duplex LocoNet, and allows a DT402D or other Duplex throttle to "join" the Duplex network.  Once joined, a throttle will always remember these Duplex settings and tries to reconnect with these settings every time it is powered-up unconnected.

The next step is to perform a "RF site-survey" to get a sense of how well just this single UR92 and DT402D will operate in the current environment.

d) With no locomotives on the layout tracks, turn on track power and select address 0 on the DT402D throttle and then unplug the DT402D from the LocoNet.  If all is OK, you will see the same Net Name and "ChXX" display flash up as for step 1(c), and the Wireless active "lightning bolt" symbol will show in the upper left of the LCD indicating the DT402D has a good Duplex radio link. 

e) Set the speed of Analog Address 00 to 99%.  While observing the Command Station Track Status Indicator, press the direction change button (or double click the active throttle knob), you should see the color of the "track status" led changes.  This confirms that the Duplex Radio is functional.  If the white led in the DT402D blinks, this indicates that the throttle attempted to communicate with the UR92 and failed.  The throttle will try 32 times to send the message before failing and the white LED flashes to indicate the message failed.

f) Walk around the layout pressing the change direction button and verify that the color of the ‘track status’ led changes colors.  With a number of tests, you will begin to get a sense if there is generally usable reception and where there may be problem areas.  If reception is reliable then the UR92 location and operation is fine and you can operate with that configuration permanently.


For most layouts Duplex will now work fine, but in the case of issues, the following steps can help isolate the problems:

(2) Troubleshooting Duplex issues:  The following steps may be used to diagnose whether the general problem type is  (a) interference and/or (b) range/reception.

a) Change the Duplex channel number and see if the problem persists and is likely interference.  The procedure for Duplex channel change is in the DT402D and UR92 manuals. This procedure is easily preformed using the Digitrax DigiGroupSetup utility.

DigiGroupSetup is able to change the Net Name and channel numbers, and scan channels 11 to 26 to provide a peak signal graph for channel activity.  When channel scan is started instantaneous peak channel signal strength information is shown on the graph.  Channels with low or no activity are most likely to have lowest interference. Change the channel number to one of these channels.

 After changing the channel number, plug the DT402D in again to verify the throttle correctly re-joins the Duplex network and shows and uses the new channel number before trying attempting further tests.

Many interference issues are resolved after checking for the best channel coverage, and then using this best clear or low traffic channel to operate Duplex on.  In some cases a 2.4GHz device may need to be disconnected temporarily to see if it is the problem signal source, including Wi-Fi Router, Bluetooth devices such as a phone earpiece etc. Depending on how many other nearby 2.4GHz devices are operating at the same time, there is usually a free channel that will work, or other devices may be moved to other channels.

b) Changing range coverage is a possibility; if tests such as 2(a) show that there is no obvious interference issue.  Moving the first UR92 around the layout and testing, will often allow discovery of a best location that provides satisfactory coverage.

c) The majority of layouts following steps 1(a) to 2(b) will operate fine in Duplex mode. In some cases additional UR92's may be needed to provide desired coverage, particularly on large or complex layouts.

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