How are decoder acknowledgements received by Decoder Pro?

[Dressendorfer]

I understand that there is a way for acknowledgements to be received from a decoder, so that a user who is using (for example) Decoder Pro can discover the value of CV settings. My guess is this could be conceptually accomplished by brute forcing through (for example) sending a speed command to all the possible addresses for a Decoder one by one till the motor runs, thereby "discovering" the decoder address. How do these acknowledgements make their way back to Decoder Pro? When on the programming track, is there a way the decoder sends messages back through the rail in the form of acknowledgements? 

[jrbernier]

  Feedback(reading) is done on the program track.  When Decoder Pro writes/reads a decoder CV value, you may see some loco 'stutter'...

Jim

[Stevert]

It's all done by timing and current pulses.

Basically, the programming device "asks" the decoder if the value in the CV being read is "1".  If there is no response (no increased current pulse) within the specified timeframe, the programming device assumes the answer is "No". 

The programming device then asks if the value is "2", and so forth until the decoder provides a "Yes" answer by increasing it's current draw (commonly done by pulsing the motor).

It's all (more or less) spelled out in the NMRA's standards.  Here's the description of the basic acknowledgement from S 9.2.3.D:

Basic Acknowledgment
Basic acknowledgment is defined by the Digital Decoder providing an increased load (positive-delta) on the programming track of at least 60 mA for 6 ms +/-1 ms. It is permissible to provide this increased load by applying power to the motor or other similar device controlled by the Digital Decoder.

[Dressendorfer]

Thanks Stevert. That sounds very clear. I'll read that part of the documentation.

[Anonymous]

Good answers all around. 

Dressendorfer, the best place for information on Decoder Pro and all JMRI systems is the JMRI Yahoo group. Join that and the very designers and programmers of the entire project are there to answer the most detailed inquiries.

[rrinker]

 How does DecoderPro get the acks? Via the command station. The program commands and ack pulses are generated and sensed by the command station. All JMRI does is act like a throttle - thus the tip always given to see if you can program a loco with the system's throttle, if not, there is zero chance of JMRI doing any better. If a throttle works, and your interface device is up and working properly, then JMRI should be able to program and run locos.

 JMRI literally does nothing except tell the command station to read a CV value. It 'pushes the buttons' automatically so you don;t have to use a throttle and press a specific key sequence to read values.

 The methods of programming and reading decoders are covered in the NMRA DCC standards and rp's, if you are looking for a technical explanation on how it works. JMRI does not do anything other than automate the keypresses you would have to do to perform the same functions with the throttle.

                           --Randy