# Inserting a Non-DCC block or section of track



## GrapevineFlyer (Sep 7, 2015)

If you have a DCC layout, and at some section you were to have a short block of track that was non-DCC (strictly DC current) would your DCC loco's performance be impacted when it reached this block? That is, would the loco stop or forget it's state as a result of driving over the non-DCC section of track?


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## Lemonhawk (Sep 24, 2013)

I think your courting disaster. If you need a Non-DCC section you want it totally isolated from any DCC tracks so there is no possibility of the wheels shorting between the DC and DCC tracks.


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## CTValleyRR (Jul 26, 2014)

Aside from not understanding why you would want to do this, you're risking the life of the decoder, the DCC power supply, and the DC power pack. Really just a high risk operation, no matter how you do it.

If you need to, the section of track must be totally isolated, and controlled by a selector switch, preferably one that is normally off and requires constant pressure to select either DC or DCC inputs, as required.


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## wvgca (Jan 21, 2013)

You could have a piece for DC, but when a metal loco wheel bridges the gap between DCC and DC sections with both powered up, it would not be good ....if the DC part was unpowered when that occurs, the DCC loco would just stop when it no longer has power to the wheels


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## DonR (Oct 18, 2012)

If you want to have the capability to run both DCC and
DC locomotives on the same layout the proper and
safe way to do it is using a double pole double throw
(DPDT) switch. One way the layout is DCC, the other
way it's DC and you avoid the damage to your gear.

It should be noted, DC locomotives should NEVER be
allowed to idle on a DCC powered track. You will burn
out the motor. You would want an isolated track with
an on/off switch where you can park DC locos when
running DCC power.

Don


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## gregc (Apr 25, 2015)

GrapevineFlyer said:


> would the loco stop or forget it's state as a result of driving over the non-DCC section of track?


assuming this is a hypothetical question and ignoring what happens when the wheel bridge the rail gaps ...

section-B of the S-9.2.4 NMRA standard describes switching modes says it may switch to DC mode in the absence of a DCC signal for 30 ms and that it should switch to DCC when it detects a DCC signal (several options).


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## Brakeman Jake (Mar 8, 2009)

You can't ignore the wheels bridging the gap,no way out of this one.If you use a DPDT switch as suggested,it becomes a "either A or B" situation where no two different systems get interconnected and thus get shorted.

With the wheels bridging the gap,you then have an "A and B" situation where two different systems get shorted together even for a very short sequence.Locomotives have generally have two four or six wheel trucks and,in theory,the short should last the time the loco takes to fully cruise over the gap.BUT!...in the real world,the loco will die instantly over the gap,keeping the short active until you react or either or both systems cook off.I can already smell the results.......

And many layouts,like mine,don't even have the DPDT option.I use DCC and my layout is separated in four sectors through a PM42 manager and I also have two loop polarity reversers....I don't think these devices would like DC anyway.


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## Lemonhawk (Sep 24, 2013)

And its not just the Loco wheels, its any car that has metal wheels crossing the cap between DC and DCC. I don't even like the DPDT solution as just forgetting to throw one switch would be a disaster. Keep them completely separate!


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## GrapevineFlyer (Sep 7, 2015)

Thanks everyone for the responses. Just so I learn as much as I can out of this thread, if a DCC locomotive in motion suddenly finds itself without a DCC signal, does the locomotive continue rolling or stop? If it continues rolling and the DCC signal is restored, what happens then? I'm trying to understand all these things at the theoretical level.


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## CTValleyRR (Jul 26, 2014)

GrapevineFlyer said:


> Thanks everyone for the responses. Just so I learn as much as I can out of this thread, if a DCC locomotive in motion suddenly finds itself without a DCC signal, does the locomotive continue rolling or stop? If it continues rolling and the DCC signal is restored, what happens then? I'm trying to understand all these things at the theoretical level.


A DCC decoder (the microchip that goes in the locomotive) doesn't have a continuous signal. DCC uses continuously energized track, powered from a command station. Each decoder has a unique address. When you send an instruction from your throttle, the command station sends the instruction out to the track with the unique address of the destination locomotive. The decoder picks it up and changes what the locomotive is doing, but only for the desired locomotive.

In the absence of new commands, the decoder just keeps doing what it was doing before. If power is interrupted, the locomotive stops, unless it has an additional power source (usually a capacitor). When power is restored, the decoder usually continues doing what it was doing before the power interruption.

So, if your loco encounters a stretch of unpowered track, it would just stop until you supplied DCC power. If it encountered a segment of track powered by DC, it would probably fry.


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## DonR (Oct 18, 2012)

GrapevineFlyer said:


> Thanks everyone for the responses. Just so I learn as much as I can out of this thread, if a DCC locomotive in motion suddenly finds itself without a DCC signal, does the locomotive continue rolling or stop? If it continues rolling and the DCC signal is restored, what happens then? I'm trying to understand all these things at the theoretical level.



CTValley has done a good job describing the way
locos respond to DCC signals.

I'm not understanding your concern about a DCC
loco running onto a power dead section of track. It
would stop instantly with no voltage getting to
the decoder.

The decoder takes the approximately 14 or 15 volts
of modified AC from the track, rectifies it and reacts
to digital commands from the DCC controller. If
told to go forward it would feed DC to the motor. If
told to back up if would reverse the DC polarity to
the motor. Speed is also set by the decoder responding
to throttle digital commands.
If the loco lost power then regained it, it would
continue doing what it's last command requested.

It is not unusual for a loco to temporarily lose power
going through a turnout, for example, or running over
a 'dirty' rail. You see the
headlight blink as the loco 'stumbles' then continues
on.

Don


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## wvgca (Jan 21, 2013)

CTValleyRR said:


> In the absence of new commands, the decoder just keeps doing what it was doing before. If power is interrupted, the locomotive stops, unless it has an additional power source (usually a capacitor). When power is restored, the decoder usually continues doing what it was doing before the power interruption.


The command set absence is easily simulated by simply 'unplugging' your handheld controller, everything will continue on, In my case I use a MRC and the handheld remembers speed / light / other functions .. until it's supplied power again and values can be changed ...
The locomotive doesn't need continuous DCC signals ... if it has a 'momentary' power interruption, [like bad track], it will continue on 'as before', unless the interruption is long enough that the decoder powers down completely, then it should stay stopped, the same as when you power up the system initially ..


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## Brakeman Jake (Mar 8, 2009)

Most newer generation decoders will run on DC (if CV29-bit 2 is enabled).If DC is disabled,it simply won't run,no damage done.

The discussion here is a completely different problem.The decoder will likely not survive carrying the load of the two power supplies (DC and DCC) being shorted together when the locomotive jumps across the gap.

Will the frying decoder act as a fuse and save the power supplies?May be...but I wouldn't try it.I don't think the DCC command station would enjoy a sudden surge of DC and vice-versa so either or both power supplies could be ruined in a split second.


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## ncrc5315 (Jan 25, 2014)

If someone has covered this already, I apoligize. I have not tried this, but it is my understanding, that a DCC equiped locomotive, can be set up, that when it comes in contact, with straight DC, will stop the locomotive, but, the lights will still lit. This would allow for a stop at a red signal, but the lights would stay on. Once the DCC signal is once again applied, the train takes off. I need to put in a removable bridge, and was thinking of doing just this, this way I would know that the train is waiting, and not just stuck.


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## Brakeman Jake (Mar 8, 2009)

You've been misinformed...don't try it.For a removeable bridge,having a switch that kills all power to a length of tracks both sides of the opening when the bridge is removed is a much better idea.


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## DonR (Oct 18, 2012)

ncrc5315 said:


> If someone has covered this already, I apoligize. I have not tried this, but it is my understanding, that a DCC equiped locomotive, can be set up, that when it comes in contact, with straight DC, will stop the locomotive, but, the lights will still lit. This would allow for a stop at a red signal, but the lights would stay on. Once the DCC signal is once again applied, the train takes off. I need to put in a removable bridge, and was thinking of doing just this, this way I would know that the train is waiting, and not just stuck.


As Jake has warned, don't even think about a junction between
DCC which is a modified AC and DC current on the same layout. 

A DCC locomotive may or may not be capable of running
on DC. I would not do it. Just as I would not run a DC
loco on a DCC track, though some of our members
do it regularly without harm.

There are several ways to depower a length of track
either side of a lift out or hinged 'bridge'. Some use
microswitches, some simply provide metal contacts
that make the connection when the bridge is in place.
And you can get more complicated by using a relay.

Don


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## ncrc5315 (Jan 25, 2014)

DonR said:


> As Jake has warned, don't even think about a junction between
> DCC which is a modified AC and DC current on the same layout.
> 
> A DCC locomotive may or may not be capable of running
> ...


Look at the Digitrax DH166P, decoder manual, CV29, page 37, brake generator. It describes doing just what I was talking about.


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## GrapevineFlyer (Sep 7, 2015)

This forum is great. So many informed people. Great learning environment.

Related question: If a DCC loco is traveling down the track and you flip polarity, will the loco notice or care?


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## gregc (Apr 25, 2015)

GrapevineFlyer said:


> If a DCC loco is traveling down the track and you flip polarity, will the loco notice or care?


by flip polarity, I assume you mean to quickly reverse the connections to the track. Bear in mind that DCC is constantly changing polarity of track voltage in order to send data. The time between the change in polarity represents whether the bit is a 1 or 0.

direction is determined by the last command. The decoder rectifies the power and applies the appropriate polarity to the motor to control the direction.


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## Brakeman Jake (Mar 8, 2009)

If you like complicated things,it can be done.You need a "DCC/DC breaker module" that works pretty much like a polarity reverser.It detects DCC and allows it until the loco has cleared the gap then instantly switches to DC,much like a DPDT switch.

What about multiple locos?Does the system flicker between each one?And lighted cars?

Anyway,what we've said remains true.The kill switch still is the cheapest,simplest and most reliable solution for a removeable bridge.


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## ncrc5315 (Jan 25, 2014)

Brakeman Jake said:


> Anyway,what we've said remains true.The kill switch still is the cheapest,simplest and most reliable solution for a removeable bridge.


 I agree, but what would be the fun in that.


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