DCC the command station at work

[cleanerg6e]

Has anyone ever wondered what signals are sent by the command station along the bus wires, up the feeders along the track and eventually to the decoders in your locos?.

Well according to a Model Railroader book this is what happens every time you press a button on your throttle,

Each packet of data is a stream of binary digits(ones and zeros) transmitted at a rate of approximately 8,000 bits per second. Each packet of data consists of four elements.

1.The first element is called the preamble. It's like an introduction to the decoder, alerting each decoder by saying,"Hey look, I'm a packet of data from the command station and I might be addressed to you."

2.The second element of each packet of data is the actual address byte, telling each decoder whether or not this particular packet of data is addressed to it.

3.The third element of data is the instruction byte, telling the decoder what to do with it's loco. Turn on or off lights, sound horn, increase or decrease speed etc.

4.The fourth element of data is the error detect byte, and is used to detect errors in the data, if they should occur. Since the signals travel along the track in a very electrically "noisy" environment, the error detect byte is essential for making sure that the decoder understands the address and instructions from the command station.

The command station can transmit almost 8,000 packets of data per second along the rails of you layout. If there are five locos on you layout a single loco will receive approximately 40 packets of data per second and ten locos on you layout will receive 20 packets of data per second.

The length of time it takes for each packet of data to travel along the rails is what makes DCC so successful. If a packet of data has been contaminated by noise (signals interference) while traveling along the rails to a decoder equipped loco, the next packet of data is merely a millisecond behind.

I myself use NCE Procab 5amp wireless.

cleanerg6e.

[Dave]

DCC is something I'm hoping to expand into in the future, but as a software engineer who has done a good amount of electronics, I find the whole thing rather interesting. Your description prompted me to look up the DCC packet standards, which I found here: http://www.nmra.org/standards/DCC/standards_rps/S-92-2004-07.pdf

40 bits of data (well, 39, technically, I suppose) is quite a compact amount of data but gives more than enough range for the sorts of systems we are controlling. Makes me want to write my own software controller, although I've got more than enough to do already! Maybe I'll have a look at the open source DCC code if I get time before the weather improves

Very interesting, though - thanks for the prompt that made me look stuff up!

[cleanerg6e]

G'day Dave, yes it's really amazing how something so small as a command station can do so much. For outdoor railways I think that a wireless control is pretty well mandatory as with the unpredictable weather if you can have your command station indoors and there is a down pour of rain you only need to worry about your locos and not have to hurry to get your control equipment in first whilst your locos are getting saturated, especially sound diesels with speakers under radiator fan grilles on their roofs.

I know the Gauge Master does a wireless throttle and I'm surprised that NCE hasn't got a permit to sell it wireless products in the UK. At the moment as far as I know you can only by NCE wireless in the USA Canada and Australia. Even living in the UK you can't buy an NCE wireless throttle on the net from the USA to operate in the UK. The courts would have a field day with NCE for supplying a product with no license agreement with UK authorities on it's use in the UK.

I originally tested my wireless DCC system when I first bought it as they say that mobile electronic devices can interfere with the DCC signal. I got my brother to stand in the front of my house whilst I went right to the bottom of my back garden and operated the Horn/whistle function on the throttle. I had a Bachmann class 20 and he gave me a thumbs up signal from the back door indicating it worked. I also have a wireless modem and two mobile phones all of which were on and over my back fence is a double track electrified mainline railway. Some say that they've got over 150ft operating distance with NCE wireless.

Roy.

[Dave]

Sounds like far more than I'd need - 50ft at most is all I've got!

Still, baseboards before wireless control!