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What they up to now, the 4th rewrite of XpressNet protocols? Oh well I guess they'll get it right eventually. I think they should call it HalfNet, since it's missing half of it's functionality... still. Cut out the middle man and let the decoders comunicate directly with the throttles, ah LocoNet, peace of mind and future proof, and doesn't need constant upgrades and protocol rewrites like that XpressNet junk. Well that's just sunk Hornby's system into the bottom 10% hasn't it.
 

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XpressNet doesn't have the necessary functionality to process the data from other devices such as accessory decoders, block detectors and all those other ancillaries mentioned, this is why XpressNet systems need a second comunications BUS (the feedback BUS) running along side XpressNet. This second BUS is limited in it's functionality to that required for the accessory devices, however it is not as limited in it's network size so can therefore handle the many hundreds of devices required without the use of repeaters or any of that nonsense.
Incidently the feedback BUS XpressNet systems use is compatible with LocoNet.
So basicly XpressNet can't handle all the additional devices, but XpressNet systems do include a workaround for the problem.

Doug, ever seen an Ntrack convention? The Network size they run would be impossible with XpressNet.
 

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QUOTE (Makemineadouble @ 4 Jan 2006, 12:17)But while I'm talking: with Digitrax each addon to the cab bus be it: booster, reversing module, stationary decoder, PM42, BL16, Handheld & UP5, is connected to LocoNet or the cabbus system surely Lenz works the same way?.
Uh no.
The main difference is that with LocoNet all devices send data to all other devices, and all devices receive data from all other devices.
With XpressNet all devices send data only to the command station, and only the command station can send data to another device.
XpressNet uses a high data transfer rate because there is no constant updates like there is on LocoNet. Let me put it this way, let's say you have several throttles connected to the system, on one throttle you select loco#1234 and start running it, now here's the differance;
on a LocoNet system as soon as a throttle selects a loco it tells all other throttles that loco#1234 is now in use. So if you try to select loco#1234 on another it won't allow it because it knows that loco is in use already, and the throttle doesn't have to ask because it was told as soon as the other throttle selected that loco.
Put another way this what happens on a LocoNet system;
You select Loco#1234
Throttle tells the system that loco#1234 is in use
If you try to select loco#1234 on another throttle it doesn't have to ask the system because it already knows it's status.
Simple right?

on an XpressNet system it's a bit more complex, there could be up to 31 throttles connected (the command station is the 32nd device), each throttle has it's own ID# (1-31, with the command station being #00) so first the throttle tells the command station it's ID# then asks if it can select loco#1234, the command station then tells the throttle that it is now controlling loco#1234, but none of the other throttles know this because they haven't been told. So if you try to select loco#1234 on another throttle, first the throttle tells the command station it's ID# then asks if it can select loco#1234, the command station then tells the throttle that loco#1234 is currently in use.
Put another way this is what happens on an XpressNet system;
Lets assume you selected the loco on throttle#8, so here's what happens;
You select loco#1234
throttle#8 asks the command station if it can select loco#1234
the command station tells throttle#8 it is now controlling loco#1234
Then if you try to select loco#1234 on another throttle (lets say number 9);
throttle#9 asks the command station if it can select loco#1234
the command station tells throttle#9 that loco#1234 is in use

Now you should be able to see that for the same operation XpressNet is much busier than LocoNet, which is why XpressNet is limited to just 32 devices, anymore than that and the network gets too slow.

So what does this have to do with adding stationary decoders and other devices?

Well if XpressNet gets slow with alot of throttles connected how do you think it will handle having several (dozens? hundreds?) of other devices connected?

Put simply... it can't handle it... it would overload the system and slow it down too much.

So that's why XpressNet systems have a second (feedback) BUS for connecting stationary decoders, block detectors, and other devices to the command station. The XpressNet feedback BUS is purely one directional, the command station sends data to the device through the track, and the device sends data back to the command station through the feedback BUS.
Here's an example of what happens on the two systems when you change a point (we'll say point#01);
on LocoNet
You change point#01 to the left hand route on a throttle
throttle tells point#01 to change to the left hand route
point#01 tells the system it has changed to the left hand route
Now when you look at any throttle you know which direction the point is set

on XpressNet (let's say throttle#8 again)
You change point#01 to the left hand route on throttle#8
throttle#8 tells the command station via XpressNet to change point#01 to the left hand route
the command station tells point#01 via the track to change to the left hand route
point#01 tells the command station via the feed back BUS it has changed to the left hand route
the command station tells throttle#8 via XpressNet that point#01 has changed to the left hand route
the command station asks throttle#1 via XpressNet if it is connected
throttle#1 tells the command station via XpressNet it is connected
the command station tells throttle#1 via XpressNet that point#01 has changed to the left hand route
the command station asks throttle#2 via XpressNet if it is connected
throttle#2 tells the command station via XpressNet it is connected
the command station tells throttle#2 via XpressNet that point#01 has changed to the left hand route
and so on until all throttles have been told that point#01 has changed to the left route and then when you look at any throttle you know which direction the point is set

Hope that explains things.
 

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Ah good, I did write that in something approaching English then, trying to simplify the protocols and write the processes in plain English was begining to drive me mad!


I don't know if I'd call the feedback BUS a daisychain conection, mor like split at ech device, kinda like connect droppers to the track BUS, if you see what I mean?

Standard Cab BUS? LocoNet is the closest we've got, as the XpressNet feedback BUS actually uses part of the LocoNet protocol, so most systems use at least part of LocoNet. I don't know, it's nuts really, I've worked out how to completely eradicate the Cab BUS, but it would do funny things to Railcom equiped decoders, so I think I'll just go scream or something.
 

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My current Digitrax Command station is about 10 years old, I wouldn't have a clue how they update things because I've never needed an update, in fact I don't think there has ever been an update (other than when the NMRA changed the Function identity standards soon after the Chief was launched, and that didn't affect me). Actually I'm quite certain there has never been an update to LocoNet, hence the advantages of a future proof com-BUS, it's already compatible with all the bells and whistles that XpressNet users have been asking for for years, including those which no-one's thought of yet.

If you really want to know how Digitrax do upgrades, then an email to the tech department ([email protected]) should get you an answer in a couple of hours from those who build the systems.
 
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