Hal,
The original NMRA 'Standard' included 2 plug/socket configurations:
As is normal with high tech stuff, and the usual rush to miniature-ize, the 9 pin in-line JST connector started being used by manufacturers. This made it easy to get a thin connection between the decoder and the cable. Many decoders use this now and include a 8 pin NMRA connector on the other end - rather nice. Last summer I picked up some new very small Digitrax decoders with a 'mini' JST connector on the decoder end, and loose wires on the other end for 'hard wire' installations is small area.
Athearn has been releasing engines as of late with a NMRA 8 pin socket, and a 8 pin to JST cable. Very easy to just plug in a JST decoder and run!
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
The 8-pin and the NMRA connectors are the same, and are normally used between a decoder's wiring harness and the circuit or light board in the loco. They (usually) cover the track power, the motor, and the front and rear lights. All other functions have to be wired externally to that connector.
The 9-pin (or JST) connector usually goes between a decoder large enough to accommodate it (usually HO or larger) and the wiring harness. Some locos have 9-pin connectors in addition to, or instead of, the 8-pin connector on the circuit board. In that case you would unplug the wiring harness from the decoder and plug the decoder directly into the 9-pin connector on the loco.
There are other various connectors in use as well, such as the 6-pin one you mentioned, and I believe a 21-pin as well. As far as I know those aren't generally used in the US, but may be more common in products intended for the European market.
What are you connecting?
Each DCC system manufacturer has its own connections for the throttle bus, where you plug the hand-held into the controller, or into a jack that leads to the controller. The throttles themselves are not interchangeable between manufacturers, so that fact that the plugs and jacks aren't either is not important.
DCC decoders come in different flavors. All have 2 power wires from the rails, 2 motor wires to the engine, one wire to the front headlight, one to the back headlight, and a common to both headlights. That's 7. Other decoders feature additional lights, so that would add more wires.
I wire in most of my decoders anyway, so I'll leave the rest of the answers to those who know what they're talking about in that area.
It takes an iron man to play with a toy iron horse.
Being somewhat new the the DCC world , i am a bit cornfused about the connectors and how they interchange, There is 8 pin connector , 9 pin and NMRA connector. Saw a 6 pin connector. Are these all interchangable somehow??
Thanks Hal