A Review of the Digitrax PR1 Decoder Programmer Interface
by Larry Maier

Digitrax Product Page

(Not compatible with Windows 2000, ME, or NT.)

One of the benefits of DCC is the ability to control several different functions on the locomotive and to customize each of these functions by engine number.  The functionality of today's DCC decoders turns out to be a double-edged sword, however.  As the Genie in Walt Disney's Aladdin said "Phenomenal cosmic powers!  Itty bitty living space"  The awesome powers of DCC must be programmed one CV at a time.  For those unfamiliar with DCC terminology, CV's are Configuration Variables.  They are numbers stored in various memory locations in the decoder that tell the decoder how to operate.  Some of the manufacturers are better at CV programming than others, but in all cases it involves stepping through each CV or CV menu and entering the appropriate data.  If you mess up one CV or one bit of data, you can render your engine immobile, or at least assign it an address whose value is a complete mystery to you.  Even if you are reasonably successful at programming, there is always the uncertainty of whether you really loaded the speed curve you wanted or the proper ditch light effect.

Digitrax has addressed this problem with their PR1 Decoder Programmer Interface for IBM Compatible Computers.  This product allows you to program the decoder in many locomotives using an IBM compatible PC and a programming track.  The significant benefit is that you can do the programming using a Windows Graphical User Interface (GUI).  You can look at a single window, and see the values that you have assigned for each function.  For the more complex functions, you are given a pull down menu from which to select. If you have a mystery engine, you can read back the data and see exactly where things are different than what you intended.

The PR1 is a DB25 female connector that plugs onto the serial port of your computer.  On the backside is an RJ12 female connector that mates with the supplied CCA 2 cable.  The cable terminates on the end opposite the connector with four wires: Red, Black, Blue, and White.  The Blue and White wires are connected to an isolated programming track, while the Red and Black wires are connected to DC power.  There is software included with the PR1 that supplies the basic GUI programming.  This software is easily loaded onto your computer.  There is a DOS version included for those of you that have salvaged an older PC to use with your layout.  The instruction book included with the PR1 is clear and easy to follow.  It also contains a good reference section on decoder functions, CV assignment, etc.

You can power the PR1 from either a dedicated DC supply of 15 volts, or power it from two nine volt batteries.  Although it appears that the unit will operate on 12-20 volts DC, buried down in the instructions in the online help is a statement that the read back function may not be reliable outside the voltage range of 14-16 volts DC.  I found this statement to be true.  Two fresh 9-volt alkaline batteries provide about 18.5 volts right off the shelf.  When I first fired things up, I got what looked like random numbers from my locomotive with known CV values.  After playing around for a little bit, things started to work correctly, and I could read and write reliably.  After taking a break for lunch, during which time I had disconnected the batteries, things were unstable for a while.  I checked the battery voltage, and it seems that the batteries need to be pulled down to less than 17.2 volts before the read back function starts to work correctly.  When you disconnect them for a while, they will spring back to a voltage above this value (close to 18.0 volts).  In fairness, the manual does recommend that you start with USED 9 volt batteries.  In this way, they assure a voltage in the range where read back works correctly. For the home layout where you are doing the occasional engine, the 9 volt batteries work just fine.  Turn things on and let them sit for a couple of minutes.  If you are going to use the PR1 with a large layout where a lot of programming is done, then a simple DC power supply is probably in order.  Make sure it is well filtered, since the PR1 wants less than 0.1 volts of noise, and the output voltage should probably be set to 15.0 volts to optimize the read back accuracy.  If you need information on how to build this type of supply, contact me through Tony's for further information.  One final word of caution: if you have a previously programmed/operated locomotive, make sure that all functions are turned OFF before connecting to the PR1.  Current flow from lights, etc. interferes with the read back communication between the decoder and the PR1 interface.

I started with two snap-on 9-volt battery terminals from Radio Shack.  I wired them in series by connecting the Red lead of one terminal to the Black lead of the other terminal.  I insulated this joint with 1/16" heat shrink tubing.  The remaining Red wire from one terminal connected to the Red wire of the CCA 2 cable (the cable supplied with the PR1).  The remaining Black wire from the second terminal connected to the Black wire of the CCA 2 and was insulated with 1/16" heat shrink tubing.  I extended the Blue and White wires using a Blue/White pair that I twisted together in the chuck of my power drill.  The Blue wire of the twisted pair connects to the Blue wire if the CCA 2 cable, and the White twisted wire connects to the White wire of the CCA 2 cable.  The polarity of the Blue/White pair does not make any difference.  Since I already had a 12" programming track with coaxial power jacks, I terminated the Blue/White twisted pair in the mating coaxial power plug.  I used Radio Shack part number 274-1569A, but there are several different varieties that will work fine.  When all the wiring was finished, I used a tie wrap (Digi-Key RP107BL-ND) to tie the batteries directly to the cable.  I plugged the twisted pair into the programming track, the PR1 onto my computer's serial port, and the CCA 2 cable into the PR1 (Note: disconnecting the CCA 2 from the PR1 will disconnect the batteries with no current draw).  My finished installation is shown in the picture.  I had previously loaded the software onto my machine, so I hit the icon for the PR1 system and opened the PR1 window.

When you first start the PR1, it looks for the serial port and it looks for a locomotive on the track.  If either is missing, an appropriate icon with a red line through it appears on the screen. If it doesn't find your serial port, you can access a menu for serial port setup.  Make sure you know if you connected to COM1 or COM2 and set the speed to 19200 if your machine is capable of this speed (most recent machines are).  When the port is correctly set, the red line through the connection icon will disappear.  If you have a locomotive on the track (start with one you know is OK), the red line through the locomotive will disappear.  At this point, you need to install a programmer.  This is the specific interface window for the unit you are programming.  As expected, programmers are available for all Digitrax mobile and stationary decoders.  There are also programmers for Lentz, Soundtraxx, Zimo, NCE Corporation, Hag, and EasyDCC.  There is a generic programmer for registers, and if all else fails, a CV programmer that allows access to CV's one at a time.  It is recommended that you go to the web site www.digitoys-systems.com to download the latest available programmer interfaces.  This site will keep the programmer interfaces current as decoder features evolve.

Most of the features are self-explanatory and require you to enter a number (such as the address of the unit you are programming). If you want to read the decoder data, hit the SEARCH button.  To program the decoder, hit the SEND button.  To modify a decoder, SEARCH the unit, modify the items you want, then hit SEND.  Good enough, but the best feature is the user speed table.  This table defines a custom relationship between the throttle setting and the engine speed.  Previously, this required you to enter a series of numbers that could appear meaningless to human beings.  In the PR1 interface, the speed curve is graphed showing each speed step and the associated percent of full speed.  As if that it not spectacular enough, if you want to change it, you grab the curve with your mouse and move it as you desire!  This feature alone makes the acquisition of the PR1 worthwhile!  Once you have the curve you want, you hit SEND and your unit is programmed.  The interface is also smart enough to not let you see the custom speed curve unless you have checked the box that causes the custom curve to be enabled in the decoder.  If you check 14 speed steps, the graph shows 14 steps, if you check 28/128, the graph shows 28 steps.  I have included a screen shot from the NCE SW9/1200 programmer to give a feel for the graphic nature of the programming interface.

Once you have developed a program sequence that works well for a particular engine, you can store the data.  Later, if you get a second engine, or if you accidentally reprogram your unit, simply setup the programming track, recall the programming file, and hit SEND.  The original unit is duplicated or repaired!

One of the main complaints against many DCC systems is the rain dance that you need to go through to program the DCC decoders.  With the PR1, decoder programming is quick, easy, and exact.  The software and software updates are free.  The PR1 just needs a computer and a programming track.  It makes no difference whether or not your main layout system uses Digitrax or one of their competitors.  If you are serious about operating your layout with DCC, the Digitrax PR1 is well worth the investment.

Addendum : In order to solve the readback data reliability problem, I revised the power source for the PR1. I purchased a pair of battery holders: an 8 AA cell and a 2 AA cell . I used Radio Shack part numbers 270-407A and 270-382A. These have snap on terminals identical to a 9 volt battery. After filling the holders with fresh AA bateries, I snapped them on my PR1 setup in place of the 9 volt batteries. The voltage was 15.8 volts, right in the middle of the recommended range. When I used the PR1 with this set up as a power souce, all of the readbacks appeared to be reliable and correct. I would recommend this approach in place of the twin 9 volt batteries called out in the PR1 instructions.

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