Locomotive Interface/Next18

MOROP created the Next18 standard for extremely tight applications, such as TT and N scale locomotives. The NMRA has not embraced the Next18 interface, though some manufacturers have. In addition to specifying the electrical interface and connectors, the NEM 662 standard also mandates the maximum physical size of decoders. A fully compliant Next18 decoder is no larger than 15mm x 9.5mm - one would fit on a dime; a Next18S decoder is no larger than 25mm x 10.5mm. 
Connection	Length	Width 	Height
Next18 	15 mm 	9.5 mm 	2.9 mm
Next18-S 	25mm 	10.5 mm	4.1 mm
 There are two variants: Next18 and Next18–S. Next18 supports motor control, front and rear headlights and 6 auxiliary output functions; two of those function pins may be used to provide an on-board TrainBus serial interface. A Next18–S decoder sacrifices two auxiliary function outputs for the speaker connections. 
The interface specifies an extremely small 18-pin "encapsulated" connector. The entire connector is only 8mm long and 5mm wide. Because of their small size, connector pins are only rated for 500 milliamps, so some connections are allocated two pins. The result being that although there are 18 pins, some are redundant, and the 18 pins provide only 12 discrete connections. A feature of the interface is "smokeless symmetry". Pins are assigned in a somewhat symmetric fashion, such that if installed backward, no harm is done to either the locomotive or the decoder; it will not function properly, but nothing will be "smoked".
* This interface supports SUSI using pins 5, 6, 14 and 15. 
* A multifunction decoder can be used as a function only decoder, as the decoder has internal circuitry to allow this.
Doehler and Haass offers a range Next18 (and 21MTC) adaptors. See their website.
==== Pinouts of the Next18 and Next18-S ====
The pinouts are symmetrical, so the decoder can be plugged in either way. AUX 3 to 6 are logic level signals. If functions work incorrectly, the decoder is installed backwards.
Next18 / Next18–S Pin Assignments 	Next18 / Next18–S Pin Assignments 	Next18 / Next18–S Pin Assignments 	Next18 / Next18–S Pin Assignments 	Next18 / Next18–S Pin Assignments
Description 	Description 	Description 	Description 	Description
Name 	Next18 	Pin 	Next18–S 	Group
Track, Right 	Connection to right hand rail 	1 	Connection to right hand rail 	1
Motor + 	Motor connection 1, Positive/Forward 	2 	Motor connection 1, Positive/Forward 	2
AUX1 	Output 1 	3 	Output 1 	3
AUX3 	Output 2 	4 	Output 2 	4
GND 	Decoder Ground connection after rectification 	5 	Decoder Ground connection after rectification 	
V+ 	Decoder positive supply, after rectification 	6 	Decoder positive supply, after rectification 	
 	AUX 6 / Output 6 	7 	Speaker B 	4 or 5
F0f 	Headlight forward 	8 	Headlight forward 	3
Track Left 	Connection to left hand rail 	9 	Connection to left hand rail 	1
Track Left 	Connection to left hand rail 	10 	Connection to left hand rail 	1
Motor – 	Motor connection 2, Negative/Reverse 	11 	Motor connection 2, Negative/Reverse 	2
AUX2 	Output 2 	12 	Output 2 	3
AUX4 / TBDATA 	Output 4 	13 	Output 4 	4
GND 	Decoder Ground connection after rectification 	14 	Decoder Ground connection after rectification 	
V+ 	Decoder positive supply, after rectification 	15 	Decoder positive supply, after rectification 	
 	AUX 5 / Output 5 	16 	Speaker A 	4 or 5
F0r 	Headlight Reverse 	17 	Headlight Reverse 	3
Track, Left 	Connection to right hand rail 	18 	Connection to right hand rail 	1
S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface	S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface	S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface	S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface	S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface
Description of Signals, NEXT18 / NEXT18-S	Description of Signals, NEXT18 / NEXT18-S
Group 1 	Track, Right (pins 1 & 18) and Left (9 & 10) connect to vehicle wheels for its incoming power supply.
Group 2 	When a multifunction decoder is not present, Pin 2 (Motor +) normally connects to the right rail and Pin 11 (Motor –) to the left rail
Group 3 	Open Collector/Drain outputs which are switched to ground on the decoder's side. Voltage is supplied from pins 6 and 15 (V+) In situations where the taillights are not connected to the headlight circuit, taillights for the forward direction (Cab1) connect to AUX1, reverse direction (Cab2) connect to AUX2. Maximum current draw is 100mA per output.
Group 4 	Members of this group are TTL Logic Levels
Group 5 	Effective load presented by the speakers to the multifunction decoder must be 4 – 8Ω, and must be documented. Impedance of speakers provided with the decoder must be documented (NEXT18–S)
==== SUSI Interface ====
For SUSI, only four signals may be used
# GND
# V+
# Train Bus Clock
# Train Bus Data
Track connections are not to be used for this purpose. All other connections can provide functions to a SUSI module.
:See NMRA T1-9.2.3 for details.
==== Operations Without a Next18 Multifunction Decoder Installed ====
For minimal operations, a dummy plug can be used.
The plug must connect pins 1, 2 and 18 together, and the same for 9, 10 and 11.
Bridge Plugs: These can be constructed to power for head and taillights. 
See the associated NMRA Technical Notes and Standards for more information.
==== Usage as a Function Only Decoder ====
NEXT18 multifunction decoders may be used in an unpowered (unmotored) vehicle for the purposes of controlling functions; in this instance the decoder must have the necessary means of generating the required Service Mode Acknowledgement defined in S-9.2.3.
=== Videos ===
* Installing a Next18 Multifunction Decoder
==== Web Links ====
* S-9.1.1.5 Next18 Next18-s Decoder Interface.pdf
* TN-9.1.1.5 Next18 Next18-s Decoder Interface.pdf
* TI-9.1.1 DCC Interface Specifications
These links are in German:
* Normblatt RCN-118 des Verband der Hersteller Digitaler Modellbahnprodukte (VHDM)
* Normblatt 662 der MOROP
===== Next18 Adaptors =====
* Doehler & Haass Next18 Adaptors 
* ESU Adapter board Next18 for 6 amplified outputs. With soldering pads and wires