Advance Consisting, Digitrax DN135 and Bachman HO GP9 (DCC) pairing.

[FONT=&quot]My issue concerns two identical Bachmann GP9s of the same series with Bachmann Factory Installed decoders replaced with Digitrax DN135 decoders. Both decoders are programed the same: Cv2 =2; CV3=1; CV4=1; CV5=154; CV6=128. One unit has reversed polarity CV29=35. Programing was done on the main, using MRC Prodigy Advanced system. (CV29 value per MRC PA²manual).[/FONT][FONT=&quot][/FONT]
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[FONT=&quot]Individually, these locomotives perform quite nicely at 128 speed step setting, crawling at SS#1 and gradually increasing speed at a rate and level comparable to other units. When controlled independently by two separate throttles, they maintain distance very well, and exhibit great speed matching, which varies slightly speed step to speed step – one will be slightly faster at a given speed step, but resynchronize at the next speed step. There is no issue here.[/FONT]
[FONT=&quot]However, when the two engines are consisted, using the MR C PA²[/FONT][FONT=&quot] [/FONT][FONT=&quot]Advanced Consisting – which is supported by Digitrax per the Digitrax Manual – they do not perform or sync well at all.[/FONT]
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[FONT=&quot]Both motors are energized at speed step 001, but will not move until throttle is advanced to a range of speed step 020 -027. Then one will move before the other, and at a higher rate of speed than the other. There is no apparent synchronization until the throttle is advanced to somewhere around speed step 040. Even at that point synchronization is erratic. One unit seems to be doing all the work, while the other is pushed or pulled around. [/FONT]

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[FONT=&quot]In other words, It appears that the two units consisted and controlled by one throttle, require a greater current draw than the two units operated separately on the same track, controlled by separate control cabs.[/FONT]
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[FONT=&quot]Using the 28 speed step mode, does not really improve the situation. The units will not move until around speed step 05, and will reach operating speed at about speed step 10, but with erratic synchronization.[/FONT]
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[FONT=&quot]I previously noticed this with a consist of a Bachmann RS3, and a Bachmann GP7. Again both with unmodified Bachman mechanism and use of DN135 decoders replacing Bachmann’s factory installed decoder. [/FONT]

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[FONT=&quot]I used the same consisting method, pairing Atlas with Atlas, P2K with P2K, Athearn Genesis with Athearn Genesis , – and Atlas + P2K with satisfactory results. These consists include locos using Digitrax, NCE, TCS, and Tsunami decoders, with both matched and brand mixed consists.[/FONT]
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[FONT=&quot]The only common point here is the Bachmann equipment / Digitrax decoder combination, but in all cases, the response to throttle settings with single not consisted engines is excellent. Performance only declines when the engines are placed in consist.

Any thoughts or solutions would be appreciated.
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The one thing that always killed me when I had speed matched something, was to have back EMF still on when they were consisted, some decoders disable this feature when units are consisted and some don't. Check your decoder manual and disable back EMF, see what that does for you.

 

Wojo,

Try programming CV57=102 for the two locomotives with Digitrax decoders while in an advanced consist.

From various sources, I have found that this is a um, feature, of Digitrax decoders. Here's how they describe it.

CV57 control the amount of intensity or speed loss as loadis increased, by limiting the amount of change or compensation that the decoderis allowed to implement. CV57 controls the intensity separately for bothregular addresses and consist addresses. The first hex digit controls theamount of intensity in effect when speed is controlled using the standarddecoder 2 or 4 digit address. The second hex digit of the CV value controls theintensity in effect when the decoder is part of an advanced consist for speed anddirection control. Values for each digit can range from 000/x00 to 015/x0F. Ifeither digit is 00, speed stabilization OFF. A value of 015/x0F is speedstabilization FULL ON. If the intensity CV value is too high, you may see locosjump from one speed to the next if they encounter an obstacle or problem withtrack work. If the value is too low, there will be very little speedstabilization effect at all. A higher digit makes the intensity or speedfall-off less. A typical value for most locomotives not part of an advancedconsist is CV57=005/x05, but the actual value that is best for a locomotive andtrain size must be determined by observation and experimentation. Note thatthis value of 005 for this example means that no speed compensation is usedwhen this decoder is in an Advanced Consist.

Somehow, all this translates to mean CV 57 needs to be 102 in each decoder in an advanced consist tomake them run as they should after speed matching, if CV57 individually was set at 5. The technical explanation as I understand it has to do with Digitrax using hex coding in their decoders andat one time their DCC systems. There are two positions in hex coding that are totaled for decimal coding, what we used in our MRC PA2s. From the Digitrax factory, CV 57 is set upin default for BEMF to be active in individual operations (why each locomotiveduring speed matching look great), but not for advanced consisting (which iswhy they won't run right, coupled together or disconnected, but consisted inyour DCC system).

Hope this helps.

 

Tried CV 57 = 102 and it worked well with both consists, the 2 GP9s and the RS3+GP7 consist. Thanks.

 

Thanks. I ended up using EMD E9's suggestion, which effectively disabled back EMF. Appreciate your help.

 

To be clear, if you set CV57 to 102 then you did not disable BEMF. The default setting has BEMF on when not in a consist and off when in one. By setting CV57 to 102, you are setting BEMF to ALWAYS be ON, both running alone and in a consist.

 

aha, poor choice of words. Did not mean Back EMF, was referring to the speed stabilizer E9 mentioned. Had the right idea, wrong words.

 

You had the right words, the speed stabilization that CV57 controls IS back EMF compensation.

 

CSX Robert is correct. Since the locos run well individually with BEMF on, CV57 = 102 adds the smooth running when consisted, and leaves them that way.

And sorry to all about the word spacing in my post! It looked ok when I inserted it, but reading it now, it's sort of jumbled.

 

Bottom line, is that the fix works.