Re: Difficulting reading back CV's in N-scale locomotives

wirefordcc <wire4dcc_admin@...>


You asked some very good questions. I'm curious as to what Digitrax
told you. They should have answered your questions.

Your problem is reading back the CVs that you are programming in
your N-scale locomotives.

You were wondering if there was something that could be changed so
that a decoder could see a motor and read back the CV. Actually,
the process doesn't exactly work that way.

The decoder doesn't see the motor. When a decoder receives a
programming signal or a request to read a CV, ALL the decoder does
is briefly apply power to the motor to acknowledge that it has
received a request from the command station. The motor acts as a
load and draws current. That's all the decoder does. So there is
nothing to change in your decoder.

Your programming station, such as a Chief, sees the surge in current
drawn and reads that as an acknowledgement by the decoder. If the
Chief is programming, it displays 'Good.'

When trying to read back a CV, the Chief sequentally sends the
decoder every possible value for a given CV. When the Chief sends,
and the decoder receives, a value equal to that of a particular CV,
the decoder again briefly puts power to the motor to acknowledge
this particular value. In this way the command station figures out
what value is in the CV. Note that the decoder has no way to send
the value of the CV. The command station has to "guess" it.

It is not essential that an acknowledgement pulse be received by the
command station for a CV to be programmed. I frequently program
decoders before I put them in locomotives without a motor attached.
I get noPG indicating that the command station did not receive the
acknowledgment pulse. But the CV is programmed nonetheless.

So how come you can't program a long address? A long address is
programmed in two CV's - 17 & 18. The command station is probably
programming one of them and then looking for the acknowledgement
pulse. It never programs the second CV because the command station
didn't receive the acknowledgement. You can get around this by
programming CV 17 and 18 individually. You will need to figure out
what values need to go into them manually. With the original
Digitrax throttles, this is what you did, so they had how to figure
these values in their manuals. I don't know if they still do.
Don't forget, when programming a long address, you will also need to
program CV 29 for long addresses. The new Digitrax DT400, and I
believe DT300, throttles make programming CV 29 easy for you. Or
again, you can do them manually.

The above is how to program a CV without getting an
acknowledgement. So what can you do to get the acknowledgement?
Usually the motor will do the track. I admit I don't have
experience on N and Z scale locomotives. I suggest that you program
the decoder prior to placing it in the locomotive if you can. Use a
20 ohm, 2 watt resistor for the motor. Use the resistor only for
programming. If you try to "run" the decoder using the resistor as
a motor, you may smoke the resistor. (You don't actually need to
use a 2 W resistor for programming. The 2 W is only in case you
apply power and forget you have the resistor as a motor. It will
start to smoke, but you probably won't destroy it before you can
power down.)

I suppose it would be difficult to test some drop in decoders in N &
Z or connect a resistor to them temporarily.

The final choice is to change the sensitivity of the command station
to the acknowledgement pulse. I don't know exactly how to do this,
but I suspect it is just a resistor change. If you contact
Digitrax, they may be able to tell you how to do it or have you send
back your command station so that they may make the change for you.

I hope this gives you some insight into the reading CVs and
programming decoders

Join to automatically receive all group messages.