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DCC BUS Terminators


Guest Moria

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Guest Moria

Whilst I don't think my BUS will exceed 30' (sems to be the critical point) for length, I was interested to read that BUS terminators can help protect Decoders etc in the case of a spike from a short. I think I will be about 20 - 25 feet per BUS line, (of which there will be three in a star pattern), so its getting close especially with twisting the BUS wires :)

 

I know there's many myths about DCC and wiring but I also believe there's some good practices that make sense for use even if they never seem to have much effect and this one may well be part of the latter. However, in reading about the terminators, I came across one sentence that I am not to sure about. I had assumed ( yeah assuming again) that a BUS terminator should just be applied to the end of the BUS furthest away from the booster, but in rereading some of the information, there was a suggestion that there should be a terminator at both ends of the BUS, ie at the Booster end and at the far end of the BUS. Both of the following could be read either way in my opinion, although the second one seems to imply both ends more directly.

 

This from one web site :-

 

Add a bus terminator/filter to each bus end point:This is a simple device that will act as a filter to improve the quality of the DCC waveform, act as a suppressor for voltage spikes that are generated by every single short circuit on the layout, and because of that, adding these low cost devices will extend decoder life and improve overall layout reliability.

 

This from another :-

 

One filter circuit is to be fitted at each end of the bus, so normally two or more of each component would be needed

 

Can anyone advise which of my assumed interpretations is correct please?

 

I will have to look for somewhere to order the components from here in Canada as I don't have a Maplin, and whilst I know that DCC Concepts do supply them, as usual, I have had no reply from them by email, and the time difference makes phoning them not realistic, and the only place I have seen them advertised is at Bromsgrove Models which doesn't ship to USA or Canada, so it's back to being self made :(

 

However, the components look pretty simple to try and source

 

The filter can be made from a 0.1µf ceramic capacitor and one 100 - 150 OHM 2 watt minimum wattage resistor

 

and will probably be a lot cheaper. Main question is one or two per BUS.

 

Thanks for any advice.

 

Regards

 

Graham

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I don't know about DCC bus termination, but I used to work with communications that required bus temination. Typically, a straight bus with the source tapped into the middle of it with a T connector would require termination at both ends of the bus, while a bus that originated from the source would only require termination at the far end - the source acted (or in some cases could be configured to act) as a terminator. I don't think there is any real disagreement between the two statements you quoted - one seems to asume a linear bus architecture with the controller tapped into the middle of the bus while the other seems to assume a radial bus architecture, with the controller as the endpoint of a number of linear busses.

 

Adrian

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I don't know about DCC bus termination, but I used to work with communications that required bus temination.

The components recommended for DCC are NOT terminators. A DCC bus does not have a controlled impedance and cannot be terminated in any meaningful way. Every single layout would need its own unique solution.

 

When a short occurs there is a very fast change in current flowing through the bus wires. This, combined with the inductance of the wiring can create voltage spikes. The capacitor and resistor form a simple suppressor circuit and no harm will come of fitting more than one set.

 

Andrew

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I've fitted the 'Ceramic Capacitor plus Resistor' filter at both ends of my wiring bus (which is a U shape) with the power being fed in the middle, ie at the bottom of the U. The 2 components are soldered together in series and then connected across the 2 bus wires.

 

In my case, I wasn't bothered with problems caused by 'shorts', but a loco fitted with a Hornby R8249 decoder. When it was running, some locos fitted with other makes of decoder would randomly stop and start, though allways at the same places. The solution was actually recommended by the nice Hornby man, who also pointed me in the direction of the Brian Lambert web site where it's covered in a bit more detail.

 

Oh, and my bus is nowhere near 30 foot in length either.

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Guest Moria

Many thanks all for the info.

 

Thanks for clarity Andrew, bit of a novice here with electronics, so was just going by the phrases used on the websites:)

 

I will just do the ends of the runs for now then.

 

Regards

 

Graham

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  • 2 weeks later...

HI

 

You may have problems with sections that use current detection if you use an RC snubber in those sections,otherwise no. If you have twisted the cable pairs of your bus at least 4 turns to the foot your are unlikely to need them at all though, even with a bus beyond 30ft.

However for a bus over 30 ft, voltage drop maybe an issue, dependent on starting voltage, number and type of joins and the guage of the wire used.

 

Regards

 

Kal

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I had 'terminators' for my DCC bus as like the OP I saw it as good practice but I had to remove them as they were giving me 'false positives' with my current-detecting block detectors. Having said that possibly if one uses the magnet based detectors instead of the diode based detectors as I did, this might not be the case.

 

Kenneth

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I had 'terminators' for my DCC bus as like the OP I saw it as good practice but I had to remove them as they were giving me 'false positives' with my current-detecting block detectors. Having said that possibly if one uses the magnet based detectors instead of the diode based detectors as I did, this might not be the case.

 

Kenneth

 

Hi

 

I am using diode based detectors with terminators without any issues. I assume you made the same mistake I did originally and fitted the terminators into one of the sections instead of the end of the bus wiring.

 

Cheers

 

Paul

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Excellent!!!! :laugh: :laugh: :laugh:

 

Serously though, as mentioned, it is good pratice to have Bus Terminators at the ends of the two bus wires, if you have a star shape with 3 sections, that would mean 3 Bus Terminators and so on, with your controller hooked up at the center. Yes, I can count, but I only have 4 toes to use, as in the avatar!!! :laugh: :laugh: :laugh: :laugh:

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  • 5 years later...

Looking for further advice please folks.

 

I purchased the capacitors and resistors from Maplins but before I wire them in, do they have to be at the very end of the bus wires or can they be fitted before the ends? (I have already have track connections to both ends of my existing bus wires).

 

Any help would be much appreciated,

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