How to Choose an M12 Junction Box
| Decision |
Options |
How to choose |
| Number of ports | 4, 6, 8 | Count the sensors and actuators in that area of the machine and leave a spare port or two for future devices. |
| Pins per port | 4-pole, 5-pole | Match the pin count of the sensor cordsets that will plug into the box. |
| Signals per port | 1 or 2 | One signal per port for standard sensors. Two per port when a device has two outputs, such as a dual-channel sensor or a valve with two coils. |
| LED indicators | None, PNP, NPN | LEDs show power and the state of each signal at the box. Choose PNP or NPN to match the sensor outputs. |
| Home-run connection | Integral cable, M23, 1-1/8" minifast, terminal chamber | Integral cable for the fewest connections. A connector when the box needs to be replaced without pulling cable. |
| Home-run length | 5, 10, 25 m on WAGO integral-cable boxes | Measure the routed path from the box to the panel. For other lengths, choose a connector version and a separate home-run cordset. |
Check the current rating as well. Turck's 4-port eurofast boxes with a minifast home run, for example, are rated 2 A per signal and 7 A total.
Reading a Turck Junction Box Part Number
Turck multibox part numbers spell out the configuration. Using 4MB12Z-4P2-RSF120 as an example:
| Part |
Meaning |
| 4MB | Number of ports (4MB = 4 ports, 8MB = 8 ports) |
| 12Z | Port connector: 12 = eurofast M12, 12Z = eurofast low profile, M12 = miniature eurofast |
| 4 | Pins per port (4 or 5) |
| P | LED type: P = PNP LEDs, N = NPN LEDs, blank = no LEDs |
| 2 | LEDs: 1 = power LED only, 2 = one signal LED per port plus power, 3 = two signal LEDs per port plus power |
| RSF120 | Home run: RSF 120 / RSM 120 = 12-pin 1-1/8" minifast connector (mates with RKM 126 cordsets), CS12 / CS19 = M23 multifast connector, a number alone = integral cable length in meters |
M12 Junction Box FAQ
What is a passive M12 junction box?
A passive M12 junction box is a sealed housing with several M12 ports that wires each sensor or actuator signal back to a single home-run cable or connector. It has no network electronics, so the signals still land on I/O in the control panel. It replaces many individual cable runs with one, which cuts cable tray fill and wiring time.
What is the difference between a passive junction box and an I/O block?
A passive junction box only consolidates wiring; each signal still needs its own conductor and its own input or output point in the panel. An active I/O block has network electronics, so it sends all signals over one Ethernet or fieldbus connection. Passive boxes are simpler and cost less, while I/O blocks reduce panel I/O and support diagnostics.
Do I need PNP or NPN LEDs?
Match the LED type to the switching type of your sensors. PNP (sourcing) sensors need a box with PNP LEDs and NPN (sinking) sensors need NPN LEDs, or the signal LEDs will not light correctly. Boxes without LEDs work with either, but you lose the visual status at the machine.
Should I choose integral cable or a home-run connector?
Integral cable has the fewest connection points and is ready to wire into the panel, but the length is fixed and replacing the box means pulling the cable. A connector version lets you replace the box or the home-run cable separately, and choose any home-run length. Most maintenance teams prefer the connector version on machines that see frequent changeovers.
What should I do with unused ports?
Cover every unused port with the manufacturer's M12 sealing cap. An open port does not carry the protection rating of a mated connection, so dust and washdown can enter the box. Caps are listed in the M12 Adapters & Accessories category.
Planning a junction box layout?
Send us your sensor count, sensor types, and the distance to the panel, and the Automation Distribution team will help you size the box, home run, and cordsets. Contact Automation Distribution or call 1-888-600-3080.
