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DCC BEGINNERS: BASIC INFO

18K views 177 replies 28 participants last post by  MichaelE 
#1 ·
I wanted to start this thread due to some mis information and confusing theories being posted lately, and in response to others' suggestion to do so. Simple, helpful comments welcome.

I do not have any Model Railroad DCC experience, which is why I feel I can comment without partiality and with a naïve viewpoint. Based SOLELY on information learned on this forum, here is my take.

All model trains require control of some sort, unless static models. Power pack or transformer control has been around for decades, whether AC or DC. The object being to control the speed and direction of the model train. The subject at hand is currently available DCC control- DIGITAL COMMAND CONTROL.
The BASIC idea is that the train (locomotive) direction and speed control is accomplished by powering the motor through a circuit board (decoder) which responds to command signals generated from a Command station.
For the sake of discussion, the subject example will use a dc motor. The following list is deliberately basic.
ITEM #1. The supplied power to the rails, thence to the Decoder, is AC voltage. The power supplied to the locomotive motor is DC, Rectified through circuitry in the Decoder circuit board.
ITEM #2. The AC supply voltage remains constant to provide power to the decoder, the Digital CONTROL SIGNAL is "carried" by the AC signal to affect various DC outputs from the Decoder.
ITEM #3. Due to the fact that Digital control signals are used to command the decoder functions, excellent wheel to rail contact is necessary. The AC power supply to the decoder board is equally important.
ITEM #4. While the object is to control speed and direction of the locomotive motor and many different systems exist, quality of course will vary. Decoder quality and performance will differ as well. Things like motor hum, programming difficulties, erratic performance and sporadic activation do occur.
ITEM #5. A benefit of the digital control is the ability to add "functions", such as sound and lighting effects.
ITEM #6. Programming. Decoders must react to specific commands for the aforementioned speed and direction, but also the sound and lighting effects. Additionally, multiple locomotives and therefore multiple decoders,must be able to respond to specific individual commands. Each decoder must therefore be programmed to respond to its specific desired control via the address assigned to it from the command station.
I may be way off on some of these points, but look forward to commentary in any case. :)
 
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#2 ·
Hi,

You have the essentials correct.

One item that you might find interesting is the a digital control system like this does not need to use AC as the power on the rails.

A system could have been devised that worked fine with just DC power on the rails. Of course you still need to impose the command information on the DC power but that is not difficult.

One advantage of the DCC approach is that the average DC voltage on the rails is zero - so when you put a DC loco on the rails it just sits there and hums/buzzes rather then runs away down the track at full speed - which is what would happen in a DC power based design.

Also the DCC approach allows varying the waveform so that the average DC voltage is other then zero allowing control of a single DC loco. I have heard that this feature doesn't work well with all DC locos. I don't know if all command stations implement it and I don't know if it is used very much.

Frederick
 
#4 ·
Is the "general" industry standard AC power with DC signal? Makers like Digitrax or MRC for example.
Is the control signal typically square wave "bits"? I'm thinking the decoder must receive it's signaling in the form of data, rather than FM. Hence "D" CC.
 
#5 ·
the signal can be roughly described as alternating square wave [ideally] , with changes in the duration of each portion of the full square being the data ..if you increase the length of the top, or 'positive' portion relative to the lower, or 'negative' unchanged portion, you basically get a DC offset, which can drive a non decoder equipped DC loco ...
all of your post looks good ... i'm glad to see the lack of the term 'fact' ... :)
 
#7 ·
My knowledge of this material is testimony to the helpfulness of members here, so thanks to all of you.
So fcwilt, you and W are saying the same thing; the frequency of the control waveform influences the decoder functions. Noting that it is of constant amplitude.
It seems inevitable that there is a likelihood of motor heating. I used to have trouble with transistor power packs overheating my old 3 pole Motors at lower voltages. Looks like a somewhat similar problem( potentially) here.
 
#14 ·
Thanks Mark for the links. I looked at the wiki one and was afraid to go to the NMRA one.
Don't be afraid to go to the NMRA spec. The spec is broken up into multiple parts, so the above link is just to the packet definition pages of which there are just a couple. Skimming through the document you'll see there are provisions for error detection and that commands are sent multiple times. So the designers took into account dirty rails and other real life problems.

Mark
 
#13 ·
Thanks, Mopac. I wanted to clarify that point based on some earlier misinformation from other threads.

Okay, so if locomotives are run in multiple, are they assigned the same address?
I can see where mismatched power would need individual control, but what about similar units?
 
#16 ·
Multiple locos are programmed into a "consist".
The consist has an address. So all locos in consist
receive the same command. Pretty neat. When removing
one or more of the multiple locos they must be removed
from the consist by programming and given their
unique address back.
 
#18 ·
Just in the interest of clarity when removing a loco from an consist setup using CV19 you only need to set CV19 back to zero and the loco will then respond to its normal address, either the short address in CV1 or the long address in CV17/CV18, depending on the setting of the long address bit in CV29.

These values are not changed when setting/clearing the consist address in CV19 so there is no actual need to set them again.

Frederick
 
#22 ·
My Bachmann EZ DCC controller emergency STOP button cuts
the power to the tracks. Since it is NMRA compliant,
I assume that all (or most) DCC controller makes
would do the same.

This is a much needed thread and the information
has been very correctly presented.

I would add, however, that most of the various
DCC makes are much the same, some are
easier to use, some are limited in the number
of locos they can have in their database and
some, like my Bachmann, cannot change
the individual CVs that the decoder uses
to control the motor, lights and sound.
However, in general, you can run any make DCC loco
with any make DCC controller.

Don
 
#27 ·
ITEM #3. Due to the fact that Digital control signals are used to command the decoder functions, excellent wheel to rail contact is necessary. The AC power supply to the decoder board is equally important.
momentum allows a locomotive to continue to run despite temporary loss of power due to dirty track. (obviously keep alive circuits allow longer losses in power).

capacitance in the decoder circuit allows it to maintain operation during temporary loss of power, as well as continue to perform the last commanded operation

dirty track will corrupt transmission of commands, but they are sent periodically, so many can be lost. Error detection bits sent in the command allow corrupted commands to be ignored.
 
#29 ·
The "items" as listed are based on my impressions formed by information I've read in several different threads here, so I'm glad that these things are being clarified.
My impression is(was) that the command signals were not dependant on a return signal or feedback. I'm still not sure about this in light of the previously mentioned error codes. Are error codes read by the command station via feedback?

Also, I was aware of sound equipped DC locomotives, but thought perhaps DCC control with sound was different.
Now I see that output from the decoder would be for direction and speed , plus lighting. Sound isn't activated with control bits, It is merely incorporated into the decoder board. Or at least can be.
 
#30 ·
My impression is(was) that the command signals were not dependant on a return signal or feedback. I'm still not sure about this in light of the previously mentioned error codes. Are error codes read by the command station via feedback?
there is no standard mechanism for a decoder to transmit information back to the command station in normal operation. (S9.3.2 is proposed, by I'm not familiar with it).

The NCE PowerCab is able to read back CV values from a single decoder/locomotive on a programming track because the decoder momentarily applies power to the motor, momentarily increasing the current, which the PowerCab can measure. The decoder transmits CV values by modulating the current.

The error detection bits I mentioned earlier are transmitted in the command. These are the last group of data in the command along with preamble, address and instruction. These bits are used to verify that the DCC command is not corrupted. The command is ignored if it is corrupted. The command station is unaware that a command is corrupted. The command station will repeat the command periodically.
 
#31 ·
Perfect, thank you. That confirms my initial thinking that the transmitted commands were sent regardless of whether or not a decoder received it.

Regarding rail/wheel contact, the impression I've had was that DCC was far more sensitive to dirty track than DC. While track certainly needs to be clean, the presence of redundant control signals would lead me to believe that it's not as critical as I thought.
In other words DCC will work in the real, imperfect world.
 
#40 ·
Next question: compatibility.
One would assume that most brands of DCC equipment that is NMRA compliant would be compatible. So a certain brand of command station should communicate reliably with other brands of decoders, and perform reasonably well.
Also, is there evidence of performance problems associated with a lower priced decoder that you wouldn't experience with higher priced units? Comparing cost to quality.
The problems of excessive hum or buzz and sporadic performance are things I've read and heard about. Maybe these problems were more evident in older equipment?
The focus of these comparisons should naturally be on currently available components.
 
#41 ·
Lot of truth in that statement. For example, I purchased a number of Bachmann engines that came with a (non sound) decoder. They ran "ok", but not all that great. I ripped out the decoders and installed a comparable TCS decoder and the difference was night and day. They were now actually running as well as my Atlas engines !

The same rule applies as always .... you get what you pay for ....

Mark.
 
#42 ·
I seldom use the Emergency Stop button on my
Bachmann EZ DCC controller or hand held. But
what I've discovered is that the STOP button
on the main controller shuts off the power to
the tracks, but the STOP button on the hand
held stops only the loco it is set for. The
others continue running.

Is it possible that the NCE Main controller STOP
kills the power and the hand held stops only
the loco it is set for?

Don
 
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