Setting CV 5 and 6 values

[Brew Man]

Good morning folks

I recently bought a pre-owned Bachmann Pannier Tank, lovely loco in great condition. After DC testing and fitting a decoder I turned it loose. It ran really well - except - if I turned up the throttle past a certain point it ran uncontrollably at full speed in either direction, requiring the the track power to be removed to get it to stop. After some head scratching I had a look at the values of CVs 5 and 6. They were each set to zero which I believe is the default. I changed these to 255 and 128 respectively and it has cured the problem. I'll probably fine tune them later. Not sure why the problem arose as I have not experienced it with any other decoder and I have a few of the same make in other locos. Has anyone else?

[96RAF]

You didn't say which decoder make and model BM.

[Brew Man]

Ah, yes, it's an 8 pin LaisDCC. As I said, I've got a few of these in other locos and none have displayed this behaviour. I even swapped it with another LDCC 8 pin direct.

I know you don't think much of LDCC, Rob but I've never had any issues with them before now. Not that I would say this is a particularly bad issue as it was resolved relatively easily.

[96RAF]

I seem to recall way back a post about a decoder having an apparent default of zero for these CVs. The normal default is logically 255/128. Regardless of that you have sussed out the problem.

[Will Hay]

Crikey.

I have a number of LaisDCC decoders and have never thought to check CV5 or 6.

I guess I would have had a problem by now, if they warranted an edit?

[Fishmanoz]

Struggling to put some logic to this??? By rights if CV5 = Vmax = 0, the loco shouldn’t run at all?

And do I have it correct CV6 = Vmid. If so with it 0, then loco shouldn’t run until throttle is over 64 speed steps?

But BM, you seem to be describing DC Runaway and I suspect you’ll find it starting as soon as your throttle exceeds 64.

As Rob says though, problem solved with correct defaults, so I’ll stop my verbal head-scratching too.

[Jim-R855]

Struggling to put some logic to this??? By rights if CV5 = Vmax = 0, the loco shouldn’t run at all?

And do I have it correct CV6 = Vmid. If so with it 0, then loco shouldn’t run until throttle is over 128 speed steps?

I have seen in a decoder manual (ESU or Zimo probably) where some CVs can be set to 0 where 0 isn't a practical useful setting, but instead tells the decoder to revert to the factory default for that CV. The decoder would read back 0 if it was last written to as 0, even though internally it knows to interpret 0 as 80 or 255 or something.

Perhaps in this case the decoder had some random values in those CVs from the factory (instead of proper factory defaults) but was reading as 0 because they had never actually been written to with another value.

[96RAF]

NMRA spec explains it.

[Brew Man]

The NMRA table I have seen does not specify a default value for CVs 5 and 6. I'd previously mis-read the second column, (Required) value in the table of 'O' to mean default, whereas it signifies, 'Optional'. Also, for those two CVs if the values are set to 0 or 1 the functions are not used. They are also ignored if the speed table in CV 29 is enabled, (it isn't in my case). It would still be nice to know why I was getting runaway with this particular loco and not others, but I won't agonise too much over it.

EDIT: Rob posted as I was typing.

[Fishmanoz]

Note I’ve edited my post for 2 instances where I wrote 128 speed steps and should have written 64, the mid value.