Digitrax Mobile Decoders

Digitrax makes a large variety of DCC decoders for use in trains (mobile decoders). This page lists issues and capabilities of these decoders.

Additional Digitrax CVs

Digitrax’s current line of decoders contain what they call the FX3 set of functions. The name specifically refers to special-effects applied to the output lines (to provide things such as flashing ditch lights), but the name also designates the class of decoders supporting those features, whose model numbers end in 3, 4 or 5. The functions provided by these decoders are described in the Mobile Decoder Manual (PDF). If this link breaks, look in the Tech Support Depot section of Digitrax.Com for the Mobile Decoder Manual entry under Mobile Decoders. Digitrax has a table of their CVs as well, but it omits some of them (CV09 for example).

CV02, CV05, CV06: Basic Speed Table
Vstart & 28/128 Step Operation: Vstart (but not Vmid or Vmax) is also active in 28/128-step mode with Digitrax, and is used to set the minimum speed, as well as to set interpolated speeds up to the midpoint (step 15). In other words, for steps 2-14 the values are treated as divisions of the voltage range between the percentage represented in Vstart and that of step 15. (or at least, that’s how I read the documentation; need to check that and see how it works).

CV08: Reset
CV08 is a read-only CV containing the manufacturer ID, per the standard. Digitrax also uses it as a “reset to factory default” switch. If you write 8 into the decoder, it won’t actually change its value, but all programmable CVs will have their values reset to the original defaults.

CV09: Supersonic Operation (0 = maximum, which is the default; 255 = minimum, or off)
This is Digitrax’s name for a method to adjust the motor-control pulse widths so that the pulsed power fed to the DC motor does not cause it to hum at a frequency audible to humans (normal PWM pulses are at an audible frequency). This is on by default, and its use reduces the effective torque of the motor, which can cause problems with low-speed operation. Enabling Back-EMF is a way to counteract some of those problems. This appears to be implemented differently than the NMRA recommendation, see the CV09 discussion up above.

I’ve found that the default (CV9=0) produces the best result. While a model will run faster with higher values, models generally run too fast as it is. And in addition to running faster, they’ll run noticeably louder. However, you get more torque if you increase this, and for very long trains (or trains of older cars with more friction in their wheels) it may be worth the higher noise to get more pulling power.

CV15 - 16: Decoder Lock
If these are set non-zero, the decoder will be locked against resets and reading, and against some other decoder writes. If CV16 is set to the same value as CV15 the decoder should be unlocked. Also, at least some Digitrax decoders have a “disable” bit in CV54 that needs to be cleared before the decoder lock will work. See the feature description on the DCC Decoders page for more on this.

CV19: Advanced Consisting
Digitrax supports Advanced Consisting (CV19, 21-22).

CV33 - 47: Function Remapping
These allow the function outputs (wires) on the decoder to be controlled by other function buttons on the throttle. Each bit (power of 2 values: 1, 2, 4, 8, 16, 32, 64, 128) corresponds to one button, and each CV to one wire. Set all the bits (add the values) that you want to affect the output (e.g., if both Green and Violet wires should be controlled by F2, set CV36 (F2) to 12 (4+8). See the Digitrax manual noted above for a table showing which does what.

CV33, 34: Head/tail light reversal
Normally CV33=1 and CV34=2. To reverse the orientation of the lights, set CV33=2 and CV34=1. (this came from a tip on a Trainboard.Com thread). This fix is typically needed in conjunction with using CV29 to reverse the direction of travel if the motor was wired wrong.

CV49 - 52, 113 - 116: FX3 Functions
Each function output has a CV that can be set to enable a number of effects, none of which are really applicable to a Japanese prototype other than “Rule 17” lighting, which allows headlights to be dimmed when passing another train or in a station.

CV54: Switching Speed & Torque Compensation
In FX3 decoders (not the older FX ones), this CV controls two independent functions (the default is “switching speed” off, and “torque compensation” “on”). Switching Speed is used to alter the behavior of the throttle settings and to reduce momentum effects, to make switching cars around easier. Torque Compensation is an internal adjustment to compensate for some of the torque loss caused by Supersonic operation and is enabled by default.

Also hidden in CV54, at least on some decoders, is a “disable” bit that needs to be cleared to use the decoder lock capability. This probably exists because it’s really easy to lock a decoder and not know or forget how to unlock it, and they don’t want you doing it by accident. Bit 6 (value of 64) when set in CV54 disables the decoder lock feature.

Although enabled, it’s not clear if Torque Compensation does anything if BEMF is also enabled; in many decoders the choice is one or the other, since they’re different ways to achieve roughly the same end result: smoother running and more power when “supersonic” PWM is used.

CV55 - 57: Scalable Speed Stabilization (Back-EMF or BEMF).
These variables are used to adjust the behavior of Back-EMF, which may be needed when consisting locomotive as it helps the motor operate at a fixed speed for a given throttle setting regardless of slope or number of cars being pulled, however as noted above this can be problematic. Back-EMF is also important for low-speed operation. Back-EMF is not enabled by default(?) and needs to be “tuned” to the specific characteristics of the model it is controlling (see the BEMF page).

By default, BEMF is on for non-consisted operation, and off if Advanced Consisting (CV19) is used.

CV61: Transponding (and other features)
Transponding is Digitrax’s proprietary train identification feature. A decoder with this feature can be used in conjunction with a block occupancy detector that supports transponding to not only detect that a block is occupied, but to report to a central system (such as a computer) which train occupies the block. CV61 is also used to set a few other features.

CV66+: Speed Tables, Forward Trim (CV66) and Reverse Trim (CV95)
Digitrax has a 28-value speed table in CVs 67-94. This is enabled if CV29 is programmed to enable “advanced” (aka “28/128 speed step”) control (bit 1, value 2) and use of speed tables (bit 4, value 16). When the command station is capable of 128-step throttle commands, the decoder will interpolate the missing settings (e.g, CV67 is step 1, CV68 is step 4 or 5, and steps in between are estimated).

CVs 66 and 95 only apply if a speed table is configured in CVs 67-94 and enabled as above. The values here are used as multipliers on the values put in the speed table (e.g., if a locomotive can only go half as fast in reverse, this is a way to reflect that). A value of 80 (I think that’s meant to be 0x80 hex, so 128 decimal) means 100%. If the result of the multiplication exceeds 100% (values in the speed table are used to compute a percent of maximum throttle) then it’s clipped to 100%, since the decoder can’t output more than that.

The math, assuming I’ve read their knowledgebase article correctly, is M(Ti) = (CVn/255) x (CV66/128), meaning for the i’th throttle position (e.g., 1), the value in the n’th CV (e.g., CV67 for 0, CV68 for 1 in 28-step mode) gets divided by 255 (to compute percent of maximum throttle) then multiplied by the value in CV66 (for forward; CV95 for reverse) after dividing that number by 128. The result is the motor speed as a percent of maximum (or put another way, the motor voltage as a percent of the maximum track voltage after it gets through the decoder). As noted above, 128-step mode estimates the in-between speed steps and values above 100% mean 100%. Note: CVs 67-94 are limited to a range of 0x00-0xFE (0-254).

Other Notes

The FX and FX3 decoders implement speed tables for all modes of operation (except for the DN121/DZ121 decoders, which lack the processing required). Their “non-FX” decoders only support this feature in 14 and 28-step modes, not in 128-step.

I’ve seen several posts that suggest that on Digitrax CV02 (start voltage, part of the basic speed table CVs) is not enabled when speed tables are in use, and others that it is, but should be set to 0 to avoid problems. According to Digitrax (their “Decoders” manual, pg. 54), FX decoders (and presumably their FX3 also) enable CV02 even when a speed table is used (CV05 and CV06 are always ignored if the speed table is enabled) but it’s not clear how this differs from the use of the Kick Start CV. My assumption is that kick-start controls a momentary increase to start a stopped motor, but CV02 applies for as long as the throttle is in step 1; in short, it’s either additive to or a replacement for the first table entry, CV66. However, Digitrax rather oddly labels CV66 as the “Step 4 value” so I could be wrong about how the table is used (I suspect this is a typo).

I’ve also seen reports that Kick Start and the two Trim CVs are not implemented on all Digitrax decoders, but no detail on which lack them.