Optionally we can fit an expansion connector to the NCC series controllers. This connector is mainly used for 'double heading' on model railways - two locos are connected, one at each end of a train, and driven as one. In this application, the two controllers need to react as one from a single set of controls.
The expansion connector can be retro-fitted. This page describes the retro-fit to earlier controllers, in case you prefer to do it yourself.
The information here applies only to controllers of the NCC series with issue numbers earlier that 18. Issue 18 controllers have the expansion connector modified slightly. A separate page describes retro-fitting to issue 20 or later NCC series controllers.
The diagram below shows part of the circuit diagram of the NCC, with the expansion connector. For comparison, the later circuit is also included. Notice the change in function of pins 2, 3 and 4.
The table shows the pin functions of the Pro-120, with the corresponding pin numbers on the NCC and VTX controllers.
| Pro pin | Pin, early NCC | VTX/NCC pin | Function | Comments |
| 1 | 1 | 1 | Ignition or 9v | Low=On, high = off |
| 2 | +9v | . | ||
| 3 | 2 | Modulator timing | do not use | |
| 4 | 3 | 4 | Modulator timing | do not use |
| 5 | 2 | Speed O | 0 - 3v = 0 - full speed | |
| 6 | 4 | 3 | Speed I | . |
| 7 | 5 | 5 | Direction | High (9v)=forward, low (0v) = reverse |
| 8 | Current limit | . | ||
| 9 | 6 | 6 | 0v | . |
Generally connections should be pin for pin between the two controllers. Pin 1 to pin 1, 2 to 2 etc. All very simple, except for pin 4, the demand speed line. The reason for this is that there is a 10K resistor in circuit from the ramp circuit to the connector. This is the 10K marked with a hash '#' sign. It is present only so that the half speed reverse signal has an impedance to 'bite' on. If it is left in circuit, then the two controllers will load each other when coupled, so the gain will need altering when they are coupled. It is therefore best to link this out of circuit on both controllers. This will of course stop the half speed reverse function working - but this is not normally desirable on locos!
So: Link out the 10K resistor, on both controllers. It is arrowed on the drawing, below. 10K: colour code brown-black-orange.
Slightly above the arrowed 10K is shown a wire link. This must be fitted to enable the pin 3 connection. Also, just above the expansion connector are shown two red pads: these are for through-board links to be fitted and must be made when fitting the connector. Use a piece of component lead or copper wire and solder it to pads both sides of the board, on both links.
Once done, when the expansion lead is fitted, the front end of either controller will work both. This applies to ignition, reversing and speed pot, so you only require one set of controls which may be plugged into front or rear loco as required.
Other pages with relevance to this subject