some dumb DCC questions

Posted By lownote on 01/03/2009 5:14 AM
DCC is very confusing. It's clear enough how it works in principle, but every manufacturer seems to give its products different fancy names, and the HO stuff isn't appropriate for large scale, and it's hard to know which is which. .





I'm currently using track power with remote control. I have some locos running Aristo's 75 mhz. train engineer, and some running QSI/airwire. I can see the advantages/appeal of DCC with what the QSI airwire system can do, and I've been thinking about going to full-on DCC.



So first, can I convert to DCC and continue to use the Airwire throttle? It wasn't cheap. Airwire makes, it seems, all the components needed for DCC. If I want to keep the Airwire throttle do I have to use airwire's command station/boosters etc.? Are they any good?



Second, the real advantage of converting to full DCC, for me, would be the ability to easily do reversing loops and switches. I've been thinking about making a wye or a loop for a while now, but want it to be automatic and to be honest I don't really entirely understand how to wire/control a reversing loop for automatic operation. We invite a lot of neighbor kids to ruin on our layout and I'd want something that was basically invisible. Controlling switches with a wireless throttle would be cool too.


Third, we have a lot of buildings and cars lit with leds via track power. Would going to DCC interfere with that? I don't think so but I want to make sure.

Thanks!



With any control system there are two parts, the signal and the power.

Airwire and Gwire sends the DCC signal over the air GWire can take the power from either the track or internal batteries while Airwire perfers batters. Track powered DCC sends the signal and power over the rails. There are also hybrid approaches that combine parts of each.

The AirWire throttle is actually a command station for sending the signal over the air. You can have it converted to work with several common track powered DCC systems. This conversion is a software replacement.

The reverse loop modules are easy to install, you isolate the loop at each end and connect the two track wires to one end of the reverse loop module and the other end of the module connects to the track.

Lights for buildings and cars may be a problem depending on your circuit but it is easily fixed. If your lights have a circuit to limit the voltage or current then you do nothing. If you are using raw track power then it may be that the lights work best at the common lower voltage used. DCC uses full track voltage all the time. If the lights are too bright simply place a diode on one lead and the voltage to the lights will be reduced by 1/2.

In selecting a DCC system select the one thaqt is the easiest for you to use.

Hope that helps

Stan Ames
http://www.tttrains.com/largescale/

Posted By Greg Elmassian on 01/04/2009 6:56 PM
I'll revise my last sentence, there may be some limitations or omissions.

Bill, have you found that all commands pass through, even speed up and down and momentum and consisting?

Regards, Greg


Greg

I have had a scope on the output of a Qwire receiver. The Gwire is simply the receiver chip and a very small booster (power station). The bit timing for the one bits is just a little off but other then that this is pure DCC. What DCC packets are supported is only dependent on the feature set supported by the Air Wire transmitter. For example the transmitter does not, at this time, support the functions from F12-F27 so the receiver will not transmit those functions. If some future command station upgrade is done the Gwire receiver would support this feature with no change.

Note these out of spec 1 bits may cause some problems with some decoders but most operate is this range.

A simple experiment for you would be to hook up the Gwire receiver directly to a normal DCC power station and operate your railroad through the Airwire command station.

As to passthrough for the AirWire receiver, the output must have a continuous packet stream so it would be much more difficult to try to filter out the packets then to simply pass them all through.

The only real idfference is that the booster is much larger on the AirWire receiver then the Gwire one. Thus I would expect all the packets to be available on this outpur as well.

Hope that helps

Stan
http://www.tttrains.com/largescale/

Posted By Greg Elmassian on 01/05/2009 12:34 PM
I was assuming the separate power input. The question will be the minimum DCC signal level on the TRKA and TRKB inputs. It's worth a try. I searched the Digitrax forum for an answer, but could not find one. Stan did run an ordinary Digitrax mobile decoder (right Stan?) from the Gwire, so I would assume the input design might be similar.

Regards, Greg




Greg

I think there may be some confusion here so let me try to explain it better. In classical track powered DCC the power is transmitted along with the signal. No signal no power. Thus the AirWire receiver must not only transmit the signal but must also be able to provide enough voltage for the attached sound decoder to sound good. It also has to ensure that the signal is sent continuously as it if stops sending the signal the power is removed as well. The reason this all works is because AirWire is indeed DCC. I fully expect you could hook most any decoder to an AirWire’s DCC output and all the functions including motor control would function properly. Note however that the power output in terms of amps is not sufficient to power Large Scale models which is why the restriction to sound or function decoders only.

The concept behind Hybrid DCC is that the signal and the power are no longer combined.

The Gwire receiver uses this hybrid assumption. It assumes the decoder has power from another source so it only is concerned with transmitting the signal. The DCC standards only describe the signal properties in the +- 4 volt range so any edge detection decoder should work just fine even though the +-5 volt Gwire signal is less than the minimum spec level of +-7 volts.

Sampling decoders should also work so long as the sample detection threshold is less than 5 volts.

As for bit timing there should be few if any problems.

“Digital Command Station components shall transmit "1" bits with the first and last parts each having a duration of between 55 and 61
microseconds. A Digital Decoder must accept bits whose first and last parts have a duration of between 52
and 64 microseconds, as a valid bit with the value of "1"”

My notes are old on this but I believe the GWire was a shad shy of 55us, less than the spec requires for command stations. Note it is also possible my notes are wrong on this and the signal is closer to 56us ( I did this several years back). Either way this should not be a problem because it is still well within the range decoders should accept.

I have indeed hooked up several decoders to a Gwire receiver and they work as one might expect since the signal is simply a very low power DCC signal.

There is no packet that cannot be transmitted, the only limitation being the feature set the AirWire transmitter has decided to support. Change the transmitter supported packet set and the dumb Gwire receiver will transmit those packets to its attached decoder.

Stan http://www.tttrains.com/largescale/