DCC Theory: When does the base station stop saying "stop"?

[F2Andy]

Nothing here you need to know to use DCC, I am just curious about how the technology works.

When the user sets the speed of a loco, the base station sends a message across the layout, and the relevant loco picks up that message and responds. So far so good.

I recently learnt that the message is still being sent out some time later. A loco was set to full power while it was off the track, then put on the track, partly on a cloth, to clean the wheels (may not be best practice, but not relevant here), and even though some time had passed (perhaps 20 seconds, but that is a long time for electronics), the loco started.

This makes me suspect that the base station is continually sending these messages. If you have six locos running, then it will cycle though them, telling each one what it should be doing, even if no one changes the speed on any of them. The base station has to send out something all the time, so it might as well be doing this.

But what about the other locos, the ones that are not moving? Does it also send out messages to them all the time telling them they should be stationary? How does it decide which locos to send it too? Given an address range of up to around 10 000, it cannot being doing it for every possible ID, can it? So does it only do it for locos it has told to move this session? When does it stop sending "stop" messages to a specific loco?

[smokebox]

There is an "All Stop" message that can be sent out.  All decoders will (should) respond to it.  It is not directed at any particular address.  There is also a specific "Stop" message that is sent to a particular address.

Edited August 22, 2022 by smokebox
Typos

[TWG]

When a loco is running the base station is continually sending out messages, as you have deduced, to confirm to the loco what it is supposed to be doing.  When the loco is stationary it does not, as there is no point.

 

I do not know how other systems work (probably in a similar manner), but on my Uhlenbrock Intelliibox the refresh commands cease 2 minutes after the last change by the user or if the speed is set to 0.  The time can be set to anything between 1 and 240 minutes, but I do not suppose that many people find the need to change it.

[WIMorrison]

The constant chatter that is on the DCC Bus is why the practical limit for running trains is around 110-120 locos and not the 9999 addresses that can be used. You also need to remember that chatter that is caused by accessories which just adds to the volume of traffic on what is in reality a slow communication medium - but more than adequate for most layouts.

[F2Andy]

3 hours ago, TWG said:

When a loco is running the base station is continually sending out messages, as you have deduced, to confirm to the loco what it is supposed to be doing.  When the loco is stationary it does not, as there is no point.

...

The point would be in case the loco missed the message - just the same as for setting the speed in fact. I wonder if it uses the same two minutes you mention?

[RAF96]

DCC sends out several broadcasts, a global broadcast that every loco hears and obeys - emergy stop; a consist broadcast that only that consist address reacts to; and a nominated loco/accessory broadcast that only that address reacts to. These commands are streamed and repeated continuously as sent until a change of command is sent. Some controllers will prioritise the stream so that standing locos are virtually ignored (in electronic time), giving better reaction time to active locos. In addition some decoders have a time out setting whereby if they do not hear a command for them within set limits they revert - either go to DC running if allowed or come to a stop.

 

[Ron Ron Ron]

It's all techno babble to me, but reading through the various NMRA Standards, I see that NMRA Standard  S 9.2 says (in plain English)....

 

"Packets sent to Digital Decoders should be repeated as frequently as possible, as a packet may have been lost due to noise or poor electrical conductivity between wheels and rails."

 

I was under the impression it was up to the manufacturers to implement this, how they saw fit.

 

If I've got it right, the only command signal packets that are not repeated, are decoder functions and suchlike (e.g. triggering sounds, switching on an off (accessory) functions such as lights, uncoupling etc.

I've no idea about individual or global STOP commands.

 

Again, if I understand it correctly, there is digital signal transmission at all times (otherwise there'd be no traction power to call upon) with the spaces in-between command signal packets, being filled in with a steady stream of "ones".

 

I'm sure Mr C will have the answers?

 

 

.

[Crosland]

Loco commands have to be repeated in case they are missed due to dirty track, etc.

 

Speed zero (stop) is no different to any other speed and needs to be repeated some number of times to guarantee reception.

 

It's really down to the system designer's interpretation of the specs. A common scheme is to have a "stack" of currently active locos, even those that are stationary, that are refreshed.

[Crosland]

On 23/08/2022 at 10:22, RAF96 said:

 In addition some decoders have a time out setting whereby if they do not hear a command for them within set limits they revert - either go to DC running if allowed or come to a stop.

You are conflating two different mechanisms.

 

If no valid DCC signal is detected (whether addressed to the decoder or not) then the decoder may switch to an alternate power source, including DC.

 

If no command is explicitly addressed to a decoder after some timeout period it must stop. The timeout is mandatory, minimum of 20s, but may be disabled.