DCC cab interchangability between mfg's

[fwright]

Short answer:  As long as all the DC blocks that share a common rail/common return are in the same DCC power district, there are no problems.  You should insulate both rails and feed both rails separately in each power district - power district defined as having its own booster. 

Typically, small layouts tend to be all one power district.  There is no need for more than one booster until total DCC draw approaches 5 amps, especially if the layout was previously wired for DC block control.  The existing block wiring, even though internal to one power district, enables you to switch off individual blocks for trouble-shooting in DCC (a good thing).

Explanation:  Common rail/common return wiring works in all circumstances until a metal wheel bridges the gap or insulated rail joiner with different throttles or boosters supplying either side of the gap.  When that happens, the two throttles or boosters are directly electrically linked together - one side through the common rail/common return and the other through the metal wheel bridging the gap.

With both rails gapped, both gaps must be spanned simultaneously, and different power supplies must be powering opposite sides of the gaps, for the power supplies to be electrically tied together.  This is why it is sometimes recommended to slightly stagger the gaps - it lessens the chances of both gaps being spanned simultaneously.  A 1/4" stagger is enough to be successful.

Note that if the same DC throttle/DCC booster is powering both sides of the gap(s), there is no problem.

In DC, when the 2 throttles are tied together by gap spanning, the 2 transformers provide sufficient isolation from the house common source that the voltages are free to float to whatever is appropriate.  The locomotive motor may see a big increase in voltage, a decrease in voltage, reversed polarity, or a combination.  But the float stops the 2 DC throttles from damaging each other, and the motor isn't likely to be damaged, either.  Obviously, you don't try to have a locomotive being powered by 2 throttles at the same time, but it does happen.

If you don't have separate transformers for each DC throttle, the isolation and float is not present.  If set to opposite polarities, the 2 throttles short circuit each other out, and can cause damage.  If on the same polarity, a fault current flows between the 2 throttles proportional to the voltage difference between the outputs.  This fault current has no load other than the resistance of your wiring and wheel spanning the gap (hopefully all very low), and so can be damagingly high. 

In the case of 2 DCC boosters powering adjacent blocks, spanning the gap(s) again causes the booster outputs to be directly tied to each other electrically.  Since the boosters do not produce straight AC or DC, and have self-protecting shut down circuits, describing exactly what will happen is difficult.  If the signals are in phase and neither booster shuts down, the power voltage on the track could double, blowing your decoder.  One or both boosters could shut down in self-protection (most likely outcome).  If there is a phase difference, the decoder could become confused as to what commands are being sent to it.

Summary:  Since common rail requires the spanning of only one gap, the interconnection of the 2 throttles or booster will happen every time a metal wheel crosses the boundary between the 2 devices.  Hence, common rail is not recommended for DCC with more than one booster or in DC where the throttles do not have independent transformers.  With both rails gapped, both rail gaps have to be spanned simultaneously for the interconnection to occur.  Slightly staggering the gaps further increases the odds of a wheel set not spanning both rail gaps at the same time.

hope this helps and makes sense

Fred W

[DeadheadGreg]

fred, can you easily convert a layout wired with Common Rail to DCC?  Or must you make it straight 2-rail wiring?

[betamax]

Are you old enough to remember the compatibility issues of early pc's? Today you can use software for one pc on virtually any other pc but it wasn't always that way. You could read a Tandy system disk on an IBM pc but you couldn't boot the IBM with it. The system software wasn't compatible. The same went for the hardware. A disk drive from a Tandy often wouldn't work in anything else. The same is true for todays DCC systems. Every system has it's makers proprietary hardware and software to drive said hardware including any and all components to fit that system. The only thing that's the same is the information it has to send out to the decoders.

Tandys were just plain weird. Only because they wanted to lock you into a specific vendor. They even sold hard drives labeled SCSI that were not, because they would not work on a SCSI controller, unless it was also a Tandy labeled product.

But the old days of minicomputers and the like were the same. The next model would have different connectors for the peripherals, so you couldn't recycle the ones you already had. Plus it kept out the third party vendors for a year or two.

Since the DCC standard doesn't dictate every little detail, manufacturers are free to do as they wish, as long as it will control a DCC decoder properly. Many standards are very explicit about certain things, but after that, you can do what you want.

[fwright]

ok, here's some thinking outside the box -

I have a medium sized layout, 12x18 modular that the 2 grandsons each want their own cab- what harm would come if I got a couple of Bachmann DCC controllers and wired both of them to a single buss?   

...and one ring to rule them all....

Most of the following information is contained in the November 2008 Model Railroader (a plug for our sponsors).  The primary components of a DCC system are:

• throttle or cab which provides provides a means to control a particular locomotive.  Inputs are your desires - faster, slower, reverse, which loco, etc.  Your inputs are translated into computer commands for the command station to execute.  Depending on system limitations, there can be more than one throttle.  Usually additional throttles are linked to the command station through a throttle or cab buss. 
• booster - takes the digital output of the command station and adds the power component.  There can be multiple boosters running separate power districts.  A train can cross from one power district to another because all have the same digital command data.
• command station - a computer which translates the throttle commands into digital commands to the decoders.  There can be only one command station for all the locomotives on a given section of track, and normally there should be only one command station for an entire layout.  Mixing of multiple command station signals will cause chaos for the decoders, and a chance of smoking some electronic components.  Hence, the phrase, one command station to rule them all.

Many lower cost systems combine the 3 components in a single package (like Bachmann).  Digitrax (as one example) allows you to turn off the command station portion of their systems; the system then becomes another booster.  If you cannot turn off one of the Bachmann command stations, then you can't use the 2nd unit as a second throttle or cab (or booster).

Some possibilities, assuming you cannot shut off the command station of one of the controllers:

Have each controller power a totally isolated and separate loop of track.  Trains cannot cross over the boundary between the loops and DCC controllers.  This is feasible with totally separate loops - you are operating as 2 separate layouts.  I could implement this on my HOn3 and HO layout so long as I had no dual gauge track.  Somehow, I don't think that's what you wanted.

Implement DC block control using the DCC systems as cabs.  Common rail wiring cannot be used.  A major drawback is that any operator error, such as driving a train into a track section being controlled by the other controller, can have rather dire consequences.  Highly not recommended. 

The correct solution is to use one Bachmann controller and add on a second throttle - I am assuming Bachmann makes add-on throttles for their system.  If Bachmann doesn't make an add-on throttle, you will have to invest in a different system with an add-on throttle.  MRC Prodigy Express would be the next least expensive system to achieve this goal.

my thoughts, your choices

Fred W

[ds137]

ok, here's some thinking outside the box -

I have a medium sized layout, 12x18 modular that the 2 grandsons each want their own cab- what harm would come if I got a couple of Bachmann DCC controllers and wired both of them to a single buss?   

[jeffrey-wimberly]

Are you old enough to remember the compatibility issues of early pc's? Today you can use software for one pc on virtually any other pc but it wasn't always that way. You could read a Tandy system disk on an IBM pc but you couldn't boot the IBM with it. The system software wasn't compatible. The same went for the hardware. A disk drive from a Tandy often wouldn't work in anything else. The same is true for todays DCC systems. Every system has it's makers proprietary hardware and software to drive said hardware including any and all components to fit that system. The only thing that's the same is the information it has to send out to the decoders.

[Vail and Southwestern RR]

The price has a lot more to do with development cost, and a tiny market, than it does with actual hardware cost.

The way the "lines" were drawn makes perfect sense to me, what is one the track is standardized, so anyone's decoder will work with anyone's system.  I think that even if an effort had been made to standardize at a different level, it would not have worked, there are too many opportunities to be clever!  Of course, even with the standardization, there is still MTH doing their own thing, but I just don't see how that works in the long run.

[Paul3]

ds137,
You shouldn't be surprised at the amount of reponses, really.  When one advocates changing the status quo, one shouldn't be surprised if others feel differently.  If anything, I'm surprised it hasn't garnered more replies.

In your IBM vs. Apple comparison, I would liken the DCC decoder to a simple .txt or .jpg file.  It's something that can be used by all systems.  But by bringing DCC throttles into the mix, now you're talking about substantial hardware.  Is it reasonable to expect to be able to install an IBM-type vid or audio card into a Mac?  Or vice versa?  Same goes for DCC throttles.  They may produce the same results, but go about it in very different and non-compatible ways.

In a VHS/Beta comparison, the TV is the DCC decoder...something that can be used by any system of video playback.  But the player is the brain while the tape is the throttle.  Both do the same job (record and play video & audio), but do so in very different ways.  Was it reasonable to expect someone to have made a tape that could be played on either VHS or Beta?

BTW, "totally propriatory" designs hasn't damaged Digitrax and their propriatory Loconet.  Of course, their license is minimal and designed to get others involved, but only in the correct way (there are more 3rd party Loconet users than there are for Lenz or NCE).

FYI, with Digitrax, one can actually use a universal TV remote as an IR throttle.  With Lenz, you can use a cordless phone as a throttle.  Neither, however, is universally popular.

Paul A. Cutler III
************
Weather Or No Go New Haven
************

[johncolley]

The NMRA focused on the decoder capability conformance to the standards and left the throttle end designs optional to the manufacturers, as long as the code packets were conforming. So we end up with some using 4 pin RJ11, one using 6 pin RJ12, some using 5 pin DIN connectors, and who knows what else? Now make a universal handheld remote for that? Doubtful there is enough market. jc5729 John Colley, Port Townsend, WA

[CSX Robert]

...The CFO at my house cringes when the 5 year old Grandson grabs the $140 Power pro to "play trains" but wouldn't worry about a $40 cab (as much)...

While it is still more than $40, a NCE Cab-04P at $72 is a lot cheaper than a Pro Cab and might not worry the CFO as much.

[ds137]

Wow, Guys, thanks for the input!

I got more responses than I thought I would get, but in essence what I am hearing is Demand is driving force behind the cost, rather than the actual hardware costs.  As to the compatability issue, the closest analogy I can draw is IMB PC vs. Mac- regardless of the totally incompatable operating systems and hardware, they can still share files at their lowest common denominator, the internet or specific text/graphic conversion programs.  Or closer still the age old VHS/Betamax battle.  Both products produced an RF signal that a standard (US) television could understand and decode.  (Please, let's not get into discussions of which technology is superior to the other - we all know that the dollar made the decision as to which technology thrived.  Totally propriatory designs seem to die out in today's marketplace)  My statement about cost of a cab in hardware was just that, for hardware only.  I fully realize there are more costs in delivering a finished product to market, (if it wasn't for one of those costs - transportation, I would be totally out of a job!) but some fixed costs for tooling and design would be lowered to the point of affordability when spread out over a larger number of units sold.  How many more modellers would buy a second cab at a $40-50 price point as opposed to the $85-140 cost that seems to be all that is available today?   The CFO at my house cringes when the 5 year old Grandson grabs the $140 Power pro to "play trains" but wouldn't worry about a $40 cab (as much)  Thanks for the link, Mr. Beasley!  That somewhat supports my position about the "Halmark" sound chips - I have not yet looked into their product, and I can only guess that while it may not be as slick and sophisticated at the $39.95 model I referred to, it just may be the thing I was looking for.  I realize that I may be comparing Apples to Oranges, but then again in some cases, fruit is fruit.     

[Stevert]

Oh, and there's no way a cab has just $20 worth of parts in it.  When the throttle costs $100-$225, the costs are a lot more than $20.  Especially when you realize that any company that could put out a $20 throttle would probably dominate the market.

Paul A. Cutler III

************
Weather Or No Go New Haven
************

  Paul is absolutely correct.  The enclosure for the Digitrax DT400, for example, is a variation of the Serpac M-8.  Without any of the drilling, facemask, etc. they go for over $6 each in lots of 50 at Mouser.  Larger lots would likely garner further discounts, but we're still probably looking at $4 or $5 each for blank cases alone.

  Then you have the encoders, two of 'em for that DT400, at about $2.00 each in lots of 100.

  So now we're up to $8.00 or $9.00 for a blank case and two encoders, and we still haven't even started to talk about the display, the micro-processor, the circuit board, or any of the other dozens of parts it'll take to complete that DT400.

  Don't forget the IP (Intellectual Property), either.  It took time and effort to design, test, and debug both the hardware and the firmware, and I don't think it's out of line to include the cost of that time in the cost basis of the product.

   Now that you have a finished, tested throttle, you have to put it into production which means you have an investment in reflow soldering machines, labor, packaging, manuals (writing and printing), advertising, and so forth, not to mention a building to house all that, utilities, insurance, etc. etc. etc.

  The bottom line is that Paul is right on target with his statement that "there's no way a cab has just $20 worth of parts in it" (emphasis mine).  It takes a whole lot more than that to put a viable product on the market...

 Steve

[betamax]

Oh, and there's no way a cab has just $20 worth of parts in it.  When the throttle costs $100-$225, the costs are a lot more than $20.  Especially when you realize that any company that could put out a $20 throttle would probably dominate the market.

For a throttle that retails for $100, the bill of materials will probably cost out at about $10. All you get is a small pile of parts, which are completely useless as-is.

The chances of a finished unit selling for $20 are pretty remote. As to building something that can interface any throttle to any system, the final cost of such a device would make it completely unnecessary, as it would be cheaper just to buy the proper throttle in the first place.

[MisterBeasley]

Well, for a cheap "moo," try these guys:

http://www.bakatronics.com/shop/item.aspx?itemid=361

$6.99 for this kit, and lots of other sounds.  Roosters, dogs, even a dinosaur for those Jurassic Era modellers.

[Paul3]

The NMRA made the decision a long time ago that DCC would be compatible at the railhead, and not at the throttle.  This allowed more competition into the market, and allowed for more rapid development of DCC.  I think it was a wise decision.

Currently, the only two companies that are cross compatible at the throttle level are CVP and Lenz.  NCE and, IIRC, MRC uses the same type of technology (RS-232 standard technology) for throttles, but these are not compatible with Lenz.  All of these systems are "master-slave" or polled bus systems, where you have a central "brain" that is the master, and that it asks each device plugged into it what it's doing, and the device then tells it (even if its doing nothing).  When new devices come out, the brain software must be updated if these haven't been forseen.

Digitrax, OTOH, uses Loconet, which is sort of like LAN.  It's a peer-to-peer network where the devices communicate with each other, and only talk when they have something to say.  The Loconet is sort of like a highway where the brain doesn't really care what the info is, just as long as it fits on the highway.  There have been no updates for the Digitrax Chief system's brains for 10 years or so because it really doesn't need any, while other manufacturers are constantly upgrading theirs because they have to.

Oh, and there's no way a cab has just $20 worth of parts in it.  When the throttle costs $100-$225, the costs are a lot more than $20.  Especially when you realize that any company that could put out a $20 throttle would probably dominate the market.

For Hallmark cards, how many of these are made in a year vs. the number of model railroaders who would buy stationary sound effects?  Costs are also based on units made, not just the cost of the materials.

Paul A. Cutler III
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Weather Or No Go New Haven
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[jrbernier]

Earl,

  A good question.  The original Lenz/MNRA 'standard' only defined what the information 'packets' should look like.  Each manufacturer could build any 'system' or 'decoder' as long as it would read and/or send packets in the same format.  This allowed each manufacturer to develop 'features' that would set them apart.

  In the case of the 'cab bus', there is lots of different types of throttles, and few are compatible with one another.  The NCE/Ramtraxx/Wangrow throttles were developed by NCE.  The Digitrax throttles use their own 'Loconet' throttle bus system(and I think there are OEM campatible throttles in Europe).  The Lenz system can use Easy DCC throttles, and their wireless network.  I beleive the Atlas system is a Lenz 'clone'.  The MRC series appear to be very propriatory and do not work with any others.  Would it have been a good idea to set a 'standard' for the cab bus/throttles?  Maybe in 20/20 hindsight,but I will take what we have right now, compared to the cab control limitations of 2 previous layouts!

  It would seem that there is an an opening for someone to market a device that can plug into any command station throttle bus - but I have not seen or heard of it.  Your analogy  of a $40 sound unit and a $4 Hallmark card does not hold water.  DCC is a very small market compared to the card business.  And $20 of 'parts' in a throttle is about right for a $100 throttle(there has to be mark up for the manufacturer/distributor/dealer to make a profit).

Jim

[ds137]

Not being and electrical or computer engineer, I don't understand why DCC throttles or cabs could not be used on competeing systems.  I mean, they are all required to "talk" to and be compatable with any decoder on the market, so why don't they talk to other control systems interchangably, ie PC based computers and all the interchangeable parts that fit most any other brand of pc computer.  If the output signal to the decoder has to be the same to make the decoder respond, how is it that cabs can't talk to each other?   It can't be something as simple as pinout configuration or someone would have already brought it up and and how to get around that.  One would think that someone could come up with an aftermarket cab that works with any of the most popular DCC control systems for a reasonable price.  Looks like to me that there isn't $20 worth of parts involved in a cab unit, so why don't we have a "plug and play" cab out there?  Same tupe of thing applies to sound effects (cows mooing in barn, street noises, etc that are selling for $39.95 when Halmark can put it in a $3.98 card?

Enquiring minds want to know.

Earl