JMRI Hardware Guide: Connecting to a Digitrax PR3

The Digitrax PR3 can be used as either a standalone decoder programmer much like the PR2, or as an interface between your computer and a LocoNet®, much like an MS100. It can only do one or the other of these at a given time. You select the mode of operation in the JMRI preferences during setup.

When the PR3 is acting as a programmer, it can be used to set CVs in all decoders 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 PR3 will work with many common decoders)

When acting as an interface, the PR3 allows JMRI full control of the LocoNet and the devices attached to it. That includes programming decoders on a programming track attached to a LocoNet command station, or on the main track. The PR3 can also serve as the "LocoNet terminator", which allows it to drive a LocoNet without an attached command station, e.g. if you want to have a LocoNet dedicated to signaling.

For more information on the PR3 itself, please see the Digitrax PR3 page and the PR3 page in the Digitrax Knowledge Base.

Drivers

The PR3 needs to have a driver installed when used with Microsoft Windows. See the Digitrax PR3 page for more information. 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 PR3 as a LocoNet Interface

PR3 Connection to Digitrax Command Station

To use the PR3 as an interface, connect a LocoNet cable to the LocoNet connector on the PR3. (Although not required, it's also OK to connect a power supply to the PR3). Finally, connect a USB cable between the PR3 and your computer.

Recommended setup

To set up a JMRI application to use the PR3 as an interface:

Start the program.
Open the Preferences window from the Edit menu and configure the connection by:
- Select LocoNet PR3
- Select the serial port that's connected to the PR3
- The speed will be fixed at 57,600 baud; don't try to change this in your computer's settings
- The "Flow Control" option defaults to "Use Hardware Flow Control", which uses the status leads in the serial connection to determine when the PR3 is ready to send and receive. You should try that first, as it is the most reliable mode. Only if you can't connect to your LocoNet with that setting, and only after checking that you're using the right port, should you select "None" for this preference and try again.
- In the "Command Station" option, select the type of command station you've got on your LocoNet, e.g. DCS100 (Chief), DCS050 (Zephyr), etc. The "Stand-alone LocoNet" option should be selected if you don't have a command station connected.
Click Save at the bottom of the preferences window, quit the program and restart.

Using the PR3 Programming Track

To use the PR3 programming track, connect the PR3 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 PR3 and your computer.

The PR3 does not necessarily provide reliable readback of CVs for some decoders, including many sound decoders, when the power supply is a 12VDC or 14VDC power supply. Some users repot that using a 18 VDC regulated power supply enables them to reliably read sound decoder CVs via the PR3 programming track.

Recommended setup

To set up DecoderPro to use the PR3:

Start DecoderPro.
Open the Preferences window from the Edit menu and configure the connection by:
- Select LocoNet PR3
- Select the serial port that's connected to the PR3
- The speed will be fixed at 57,600 baud; don't try to change this in your computer's settings
- The "Flow Control" option defaults to "Use Hardware Flow Control", which uses the status leads in the serial connection to determine when the PR3 is ready to send and receive. You should try that first, as it is the most reliable mode. Only if you can't program decoders using that choice, and only after checking that you're using the right port, should you select "None" for this preference and try again.
- In the "Command Station" option, select "PR3 Programmer".
Click Save at the bottom of the preferences window, quit the program and restart.

Usage suggestions

You can use a JMRI throttle to control the locomotive connected to the PR3 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 PR3 can only provide very limited current, so Digitrax has built a "neutral mode" into their sound decoders. When JMRI and the PR3 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 PR3 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 PR3 with Non-Digitrax Decoders

To implement that "neutral mode" described above, the PR3 automatically writes CV128 to 0 or 1. This happens when a JMRI throttle is used to run the locomotive on the PR3 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 PR3 with a decoder that uses CV128 for something, you should either not use a JMRI throttle to run the locomotive on the PR3 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.