Hardware Support: Connecting to a Digitrax PR4


The Digitrax PR4 can be used as a standalone decoder programmer with its own dedicated programming track (much like the Digitrax PR2 and PR3), or as an interface between your computer and a LocoNet®, much like an MS100 and PR3. It can only do one or the other of these at a given time. You select the PR4 operating mode in the JMRI preferences when setting up s "connection" in the JMRI "preferences".

PR4 as a stand-alone decoder programmer

When the PR4 is configured to act as a programmer, it is connected to a dedicated programming track via the screw terminals on the PR4. When JMRI is properly configured, JMRI can use the PR4 programming track to set CVs in any decoder that DecoderPro supports.

It can also be used to download sound information into Digitrax sound decoders and, within its limited current capability, run one locomotive to test it. When used with Digitrax SFX decoders, "neutral mode" allows you to vary the speed without the motor moving the decoder, so you can hear how the locomotive sound reacts to varying speeds.

Note that Digitrax only guarantees programming for Digitrax SFX decoders with properly installed loads, etc, but our testing has found that the PR4 will work with many common decoders.

PR4 as a LocoNet Interface

When acting as an interface, the PR4 allows JMRI to interact with devices connected to LocoNet. This includes programming decoders on a programming track attached to a LocoNet command station. Where the LocoNet command station supports "programming on the main", JMRI is able to make use of that feature.

The PR4 can also be used to connect JMRI to a "standalone LocoNet", which may be useful when LocoNet is used for some aspects of layout control while some other control system is use for other aspects. An example may be track occupancy detection and signaling using Digitrax BDL168 and SE8C boards while locomotive control is provided by a non-LocoNet DCC system. More information on this advanced topic is available on the "Standalone LocoNet" page.

For more information on the PR4 itself, please see the Digitrax PR4 page and the PR4 Support page.


The PR4 needs to have a driver installed when used with any Microsoft Windows Operating System. See the PR4 support articles at the web links for the PR4. Make sure to follow the instructions exactly, particularly about the order of installing the drivers and plugging in the device.

You don't need to install a separate driver when using current versions of Mac OS X or Linux.

Using the PR4 as a LocoNet Interface

PR4 Connection to Digitrax Command Station

To use the PR4 as an interface, connect a LocoNet cable to the LocoNet connector on the PR4. (Although not required, it's also OK to connect a power supply to the PR4). Finally, connect a USB cable between the PR4 and your computer.

Recommended setup as LocoNet interface

To set up a JMRI application to use the PR4 as an interface to a LocoNet which includes a LocoNet command station:

Using the PR4 Programming Track

PR4 Connection to Programming Track

To use the PR4 programming track, connect the PR4 to its power supply and to a dedicated piece of track which you'll use as the programming track. Finally, connect a USB cable between the PR4 and your computer.

Recommended setup as dedicated programming track

To set up a JMRI application to use the PR4 as a dedicated decoder programming track:

Usage suggestions

You can use a JMRI throttle to control the locomotive connected to the PR4 Programmer. Use the JMRI Power Control or the "Power" menu item on the Throttle's menu bar to turn on track power, and then you can operate the locomotive normally.

The PR4 can only provide very limited current, so Digitrax has built a "neutral mode" into their sound decoders. When JMRI and the PR4 is running a Digitrax sound decoder, the motor is disconnected to reduce the current load. You can still ramp the speed up and down with the throttle and listen to how the sound responds, but the motor won't move the locomotive.

Other decoders, including decoders from other manufacturers, may draw enough motor current to trip the PR4 internal circuit breakers. This will cause JMRI to mark track power as "off". Just wait a couple seconds and turn the track power back on to recover.

Using the PR4 programming track to program Non-Digitrax Decoders

To implement that "neutral mode" described above, the PR4 automatically writes CV128 to 0 or 1. This happens when a JMRI throttle is used to run the locomotive on the PR4 programming track, and there's nothing JMRI can do to prevent it. Unfortunately, this can interfere with the CV programming on some non-Digitrax decoders. For example, it will set the "Master Volume" CV128 on some SoundTraxx decoders to a very low volume.

If you are using the PR4 with a decoder that uses CV128 for something, you should either not use a JMRI throttle to run the locomotive on the PR4 programming track, or remember to rewrite CV128 with the right value just before removing the locomotive from the track.

LocoNet® is a registered trademark of Digitrax, Inc.