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Reference

Decoder plugs and sockets

Many ready-to-run locos have a socket so a decoder can plug straight in. This table shows the common plug and socket types, how many pins they have, where you’ll usually find them and what they can carry.

Current ratings are for each contact of the connector, not the decoder or the loco (NMRA S-9.1.1). “Suitable for” is as stated in the NMRA standard where it gives one.

Decoder plugs and sockets compared
InterfacePinsSuitable forFeaturesStandard
6-pin (NEM 651)Small interface, 6-pin direct 6 N or larger (non-sound decoders) No speaker pins NEM 651 · NMRA S-9.1.1.1
8-pin (NEM 652)Medium interface, 8-pin 8 HO or larger (non-sound decoders) No speaker pins NEM 652 · NMRA S-9.1.1.1
JST 9-pin9-pin JST 9 Non-sound decoders in most scales No speaker pins NMRA S-9.1.1.2
21MTC21-pin MTC 22 — Motor, baseline 8 function outputs (options up to 12), 2 input sensors, data bus (SUSI); decoder plugs straight onto the loco's board, no cable NEM 660 · NMRA S-9.1.1.3 · RCN-121
PluX22PluX 22 — Motor, up to 9 function outputs, 3 logic-level I/O, storage capacitor and speaker connections NEM 658 · NMRA S-9.1.1.4 · RCN-122
PluX16 16 — Subset of PluX22 NEM 658 · NMRA S-9.1.1.4 · RCN-122
Next18 / Next18-SNext18-S (sound) 18 N and TT, and smaller HO installations Contacts arranged symmetrically so a rotated fit should not damage anything; Next18-S size must provide speaker connection NEM 662 · NMRA S-9.1.1.5 · RCN-118
E24 28 N and TT, and smaller HO installations; also function-only decoders and SUSI modules Motor, directional lighting, baseline 12 function outputs; physically symmetrical but not keyed NMRA S-9.1.1.6 · RCD-124

6-pin (NEM 651) (Small interface, 6-pin direct)

Pins
6
Suitable for
N or larger (non-sound decoders)
Features
No speaker pins
Standard
NEM 651 · NMRA S-9.1.1.1
  • Layout: 1 x 6
  • Pin spacing: 0.050 in (1.27 mm)
  • Keyed: No
  • Current per pin: 0.5 A (0.75 A peak)
  • Status: Not recommended for new designs (NMRA); large installed base
6-pin (NEM 651) pin-out
PinSignalUsual wire colour
1Motor +orange
2Motor -grey
3Right railred
4Left railblack
5Front headlightwhite
6Rear headlightyellow

8-pin (NEM 652) (Medium interface, 8-pin)

Pins
8
Suitable for
HO or larger (non-sound decoders)
Features
No speaker pins
Standard
NEM 652 · NMRA S-9.1.1.1
  • Layout: 2 x 4
  • Pin spacing: 0.100 in (2.54 mm)
  • Keyed: No
  • Current per pin: 1.5 A (3.0 A peak)
  • Status: Not recommended for new designs (NMRA); very common in older OO/HO models
8-pin (NEM 652) pin-out
PinSignalUsual wire colour
1Motor +orange
2Rear headlightyellow
3Output 3 (AUX1), optionalgreen
4Left railblack
5Motor -grey
6Front headlightwhite
7Common (V+)blue
8Right railred

JST 9-pin (9-pin JST)

Pins
9
Suitable for
Non-sound decoders in most scales
Features
No speaker pins
Standard
NMRA S-9.1.1.2
  • Layout: 1 x 9
  • Keyed: Yes
  • Current per pin: 1.0 A, 50 V DC
  • Status: Current NMRA Standard (Nov 2020)
JST 9-pin pin-out
PinSignalUsual wire colour
1Output 3 (AUX1)green
2Right railred
3Motor +orange
4Common (+V)blue
5Front headlightwhite
6Rear headlightyellow
7Motor -grey
8Left railblack
9Output 4 (AUX2)violet or brown

21MTC (21-pin MTC)

Pins
22
Features
Motor, baseline 8 function outputs (options up to 12), 2 input sensors, data bus (SUSI); decoder plugs straight onto the loco's board, no cable
Standard
NEM 660 · NMRA S-9.1.1.3 · RCN-121
  • Layout: 2 x 11 (22 positions, pin 11 omitted as key; 21 used)
  • Pin spacing: 1.27 mm
  • Keyed: Yes
  • Current per pin: 1 A max per contact
  • Largest decoder (mm): 30 x 15.5 x 6.5
  • Status: Current
  • Speaker pins: No

PluX22 (PluX)

Pins
22
Features
Motor, up to 9 function outputs, 3 logic-level I/O, storage capacitor and speaker connections
Standard
NEM 658 · NMRA S-9.1.1.4 · RCN-122
  • Layout: 2 rows, pin 11 omitted on decoder as key
  • Pin spacing: 1.27 mm
  • Keyed: Yes
  • Current per pin: 1 A max per contact
  • Largest decoder (mm): 30 x 16 x 6 (NEM 658 allows up to 35 mm long)
  • Status: Current

PluX16

Pins
16
Features
Subset of PluX22
Standard
NEM 658 · NMRA S-9.1.1.4 · RCN-122
  • Layout: 2 rows (uses PluX pins 3-18)
  • Pin spacing: 1.27 mm
  • Keyed: Yes
  • Current per pin: 1 A max per contact
  • Largest decoder (mm): 20 x 11 x 4.2
  • Status: Current (PluX8/PluX12 deprecated by MOROP)
  • Speaker pins: Yes

Next18 / Next18-S (Next18-S (sound))

Pins
18
Suitable for
N and TT, and smaller HO installations
Features
Contacts arranged symmetrically so a rotated fit should not damage anything; Next18-S size must provide speaker connection
Standard
NEM 662 · NMRA S-9.1.1.5 · RCN-118
  • Layout: Encapsulated 18-pin plug/socket
  • Keyed: No
  • Current per pin: 0.5 A max per contact
  • Largest decoder (mm): Next18: 15 x 9.5 x 2.9; Next18-S: 25 x 9.5 x 4.1
  • Status: Current

E24

Pins
28
Suitable for
N and TT, and smaller HO installations; also function-only decoders and SUSI modules
Features
Motor, directional lighting, baseline 12 function outputs; physically symmetrical but not keyed
Standard
NMRA S-9.1.1.6 · RCD-124
  • Layout: Encapsulated 28-pin (24 contacts + 4 guide pins)
  • Keyed: No
  • Largest decoder (mm): 19.5 x 8.4 x 2.6 (height without socket/underside parts)
  • Status: New NMRA Standard (Feb 2026)

Decoder wire colours

Hard-wired decoders use a standard colour code for their wires:

Standard decoder wire colours
Wire colourConnects to
RedRight-hand rail pickup
BlackLeft-hand rail pickup
OrangeMotor +
GreyMotor -
WhiteOutput 1 front headlight
YellowOutput 2 rear headlight
GreenOutput 3 (AUX1)
Brown or violetOutput 4 (AUX2) or speaker
BlueCommon + for lights/outputs
Black/white stripeDecoder ground (common -)

Sources

Checked 18 October 2026. Written in our own words from:

  1. nmra.org (nmra.org)
  2. intermountain-railway.com (intermountain-railway.com)
  3. nmra.org (nmra.org)
  4. nmra.org (nmra.org)
  5. nmra.org (nmra.org)
  6. nmra.org (nmra.org)
  7. nmra.org (nmra.org)
  8. nmra.org (nmra.org)

Spotted something wrong or out of date? Tell us and we’ll check it against the sources.