DCC signal timing: Should the bus follow the track?

[RBTKraisee]

Should the main wiring bus need to follow the track?

 

For example, imagine a layout with a figure-of-eight configuration. Your main power bus could follow the track all the way around, or you could install a single pass across the long axis of the layout and your feeders then drop 'wherever' is closest.

 

In the latter scenario, don't you risk potential issues with DCC signals meeting at slightly different times, because the feeder drops could be out of order?

 

I'd imagine it wouldn't make much difference on a small layout, but on a larger one it might become more apparent. Do any experienced DCC hands here have any ideas of where potential "red-lines" are hiding?

 

I'm asking because for one of my layouts I've designed a triple-loop with a figure-of-eight on one of the loops, all fitting on an 2.4 x 4.8m (8 x 4 ft) board.   It means a single complete circuit is going to be around 10m (60 ft) in length, but I'm a little concerned that DCC signal timings could get a little 'interesting' if I connect different sections of the three loops, off a single main bus - or would it be worth considering three busses, one for each loop?

 

Any thoughts?

 

Ross.

[Dagworth]

Being as the signal travels at the speed of light 60ft is instantaneous, wire it however is convenient.

 

Andi

[Ron Ron Ron]

1 hour ago, RBTKraisee said:

Should the main wiring bus need to follow the track?  .......

Hi Ross.

If by “main wiring bus” you mean the DCC track bus, i.e. the power bus, then no it doesn’t need to follow the track or anything.

 

The track bus is simply a means of providing a robust and reliable, power and signal path to all parts of the layout.

You can configure it in any way you like, provided you take into account a number of practical aspects.

As Andi says, there won’t be any issues regarding the timing or “clashing” of DCC signals, if there are multiple electrical paths between the DCC track output and the onboard decoders.

Unless your layout is several kilometres in length, the signals will arrive almost instantaneously.

Something many people forget to take into account, especially when having misplaced concerns about multiple signal paths causing conflicts, is that the track itself is also part of the track bus.
Plenty of scope for multiple signal paths there!

Does it cause problems ?

No !

 

Cheers

Ron

 

 

.

[RBTKraisee]

Fair enough, good to know I'm not going to be screwing things up - at least not in that way

 

Thanks guys,

 

Ross.

[Crosland]

The only point to watch is the dropper length, if the bus is remote from the track. Generally you will use much thinner wire for the droppers. The farther they are from the bus, the thicker they should be.

[Parax]

300 meters of wire will give you a 1 Microsecond Delay.

a short DCC pulse is 58 Microseconds and a long pulse is 100 microseconds

So keep your Bus and Track lengths within 1km of each other and you'll be fine.

[jimwal]

A rail replacement bus should follow the nearest road . . . . .    

 

Hat, coat, . . . I'm gone!

[RBTKraisee]

6 hours ago, Parax said:

300 meters of wire will give you a 1 Microsecond Delay.

a short DCC pulse is 58 Microseconds and a long pulse is 100 microseconds

So keep your Bus and Track lengths within 1km of each other and you'll be fine.

 

That's really helpful to know.   Seeing the numbers definitely sets my mind at ease

 

Thanks Parax!

Edited April 21, 2020 by RBTKraisee

[RBTKraisee]

12 hours ago, Crosland said:

The only point to watch is the dropper length, if the bus is remote from the track. Generally you will use much thinner wire for the droppers. The farther they are from the bus, the thicker they should be.

 

Agreed.   I'm using 12 AWG for the bus and 18 AWG for the feeders, generally aiming to keep feeder length to 12-18 inches at most.   I'd be the first to agree that its overkill for this particular 8x4 ft board here, but I'm using this as a practice run for something a bit larger that I'm planning to follow up with!

 

I'm also planning to use a PSX-1 circuit breaker in addition to my Roco z21's own protection - again, overkill for this small layout, but gets me the practice I want. And the PSX can be used with my Arduino/DCC++/JMRI alternative controller, removing any worries about potentially smoking the motor shield board.

 

Ross.

[WIMorrison]

You should the PSX-1 to protect the track bus only and feed an accessory bus directly from the z21. you then feed all the accessories - points, signals, lighting, etc from the accessory bus and the trach from its own bus.

 

The advantages to this method are that when you get a track short e.g. overrun points, then you can still switch the point and after switching the PSX-1auto-resets and the train will start to run  without the hand-of-god

[RBTKraisee]

7 hours ago, WIMorrison said:

You should the PSX-1 to protect the track bus only and feed an accessory bus directly from the z21. you then feed all the accessories - points, signals, lighting, etc from the accessory bus and the trach from its own bus.

 

The advantages to this method are that when you get a track short e.g. overrun points, then you can still switch the point and after switching the PSX-1auto-resets and the train will start to run  without the hand-of-god

 

That's a good idea!

 

Thanks,

 

Ross.