Monday, 8 March 2010

First Steps in DCC

I've finally taken the plunge and purchased an NCE Power Cab system (from Digitrains). This comes with everything you need to get started except a locomotive decoder.

As a first experiment I have converted a brand new Farish Class 24. This is an ideal choice as it is "DCC ready", meaning there is an internal circuit board with standard 6-pin decoder socket, and no need to modify the chassis. The socket comes fitted with a "blanking plug" for ordinary analogue operation.

I believe the idea is to buy a decoder with an integral 6-pin plug and simply plug it in. However, the decoder I bought at the same time as the system was a Digitrax DZ125 without a plug, just with 7 bare wires. These are colour-coded as follows, where the first column gives the numbers of the (labelled) sockets on the loco PCB, according to the NMRA standard:
1 (orange) motor right
2 (gray) motor left
3 (red) right rail
4 (black) left rail
5 (white) front light
6 (yellow) rear light
- (blue) light common
Note that there are 7 wires but only 6 sockets. The blue wire is not needed - the standard is apparently such that if it is not connected then the lights are supplied using the track as the common instead. As I had no plug I unsoldered the socket from the loco and soldered the individual wire ends instead (a bit fiddly and not what the maker intended!). The decoder sits happily where the blanking plug was initially located.

(Note: Sorry there are no photos as my camera is broken).

With some trepidation I assembled a circle of Setrack (the wheels have not been changed yet), put the loco on the rails, connected the Power Cab, and turned it on. It picked up the loco straightaway with the default factory address of "3," and I was able to drive it away with no further ado. One benefit of DCC showed immediately: I could turn the headlights on or off independently of whether the loco was moving (well, it impressed me anyway). When you change the direction on the controller, the lights change colour too.

I then followed the "Getting started" instructions in the Power Cab manual to easily program a long address for the Class 24. In the default state the operation was not smooth, increasing in 28 discrete steps from standstill to ridiculously fast. I expected it to be a lot of work to remedy this, but in fact it was simple. Following the instructions in the manual again (at least to begin with), I rapidly obtained fantastic running by programming the Class 24's decoder through the following procedure:

(1) Change the start voltage - configuration variable (CV) 2: I tried increasing this in single steps (these variables take values from 1 t0 255) until the loco just starts moving on speed step 1 (initially there was not enough voltage until speed step 2).
(2) Change the maximum voltage CV5: I lowered this to get a more reasonable top speed.
(3) Change the mid speed voltage CV6: I put this mid-way between my new values for CV2 and CV5 (not sure if it was necessary to set this independently).

With these settings the loco was able to run at a reasonable range of speeds, from an incredibly slow crawl to a reasonable pace. I then got adventurous and fiddled with CV3 (acceleration rate) and CV4 (deceleration). By default these are set to 0 so the loco moves instantly at the new speed, creating an undesirable jerky effect. By setting a non-zero acceleration I found that the loco could move very smoothly from one speed to the next, despite the fact that there are only 28 discrete speed steps on the controller. Nice. I opted for only a small deceleration rate so that the braking is also smooth, but is quick enough that I can control where the loco stops.

That concludes my DCC experiences so far. Unfortunately I only have one "chipped" loco as yet, so I can't experiment with controlling multiple locos at once. Although, on Inverneuk this will be something of a challenge. Time to order a decoder for the 08 I think...

1 comment:

Nigel Cliffe said...

Try swapping the controller handset to 128 step mode. It may work better for you. (No changes required in decoder chip). I think its one of the buttons on the bottom row of the NCE.

For the 08, try a CT Elektronik chip; will make the Digitrax one look very ordinary.

Medium term for setting up decoders, invest in the NCE box for computer interface (USB) and use JMRI/DecoderPro. Its a lot easier than messing with CV values on a handset.

The momentum button on the NCE works by changing CV3 and CV4 in the decoder. Its a simple automation of a number of button presses.