OMRON NX series system unit power configuration and troubleshooting

OMRON NX Series System Unit: Complete Guide to Power Architecture and Field Wiring

In industrial automation control systems, OMRON's NX series I/O platform is widely used in various scenarios such as NJ/NX series CPU racks, EtherCAT slave terminal blocks, and safety network controllers due to its modular design and flexible power architecture. Among them, System Units - including Additional NX Unit Power Unit (NX-PD1 □□□□□), Additional I/O Power Unit (NX-PF0 □□□□), I/O Power Connection Unit (NX-PC0 □□□□), and Shielding Connection Unit (NX-TBX01) - are the key to achieving stable distributed power management and reliable signal shielding. This article will provide a complete technical guide for on-site maintenance engineers based on the NX series system unit user manual, covering selection, installation, wiring, and troubleshooting.

Overview of NX Series System Units: Four Core Types and Applications

The NX series system unit is not an I/O module, but a dedicated unit used to build and expand CPU racks or slave terminal power systems. They do not process process data, only manage and distribute power, and provide auxiliary wiring. The four main types of uses are as follows:

Example of Unit Type and Model Main Functions

When the power supply of the NX unit embedded in the CPU or communication coupler is insufficient, an additional NX unit power supply is generated from the external 24VDC power supply to power the NX unit on the right side.

The additional I/O power supply units NX-PF0630 (4A)/NX-PF0730 (10A) provide I/O power supply from the NX bus to the I/O units; Can be used to expand the capacity of I/O power supply or separate I/O power supply segments of different voltage levels.

The I/O power connection units NX-PC0020 (16 × IOV)/NX-PC0010 (16 × IOG)/NX-PC0030 (8 × IOV+8 × IOG) provide additional I/O power terminals (IOV or IOG) for centralized power distribution to external sensors or loads, and do not form power isolation themselves.

The shielded connection unit NX-TBX01 provides 14 SHLD terminals, making it easy to connect multiple shielded wires to the same functional ground.

These system units all adopt a compact design with a width of 12mm, compliant with DIN rail installation, and use Screwless Clamping Terminal Blocks, which can significantly reduce wiring time.

Power architecture: The difference between unit power and I/O power

In the NX series system, there are two fundamentally different power sources:

2.1 Unit Power Supply

Function: To supply power to the internal logic circuits of CPU units, communication couplers, and all NX units.

Generation method: 24VDC is input through the CPU unit, communication coupler, or Unit power supply terminals (UV/UG) of the additional NX unit power supply unit (NX-PD1000). The internal power conversion circuit then generates the voltage required for NX unit operation and transmits it to the NX unit on the right side through the NX bus connector.

Key limitation: The entire CPU rack or slave terminal must use the same unit power supply. If different power sources are mixed, the potential difference may cause abnormal currents and damage the equipment.

2.2 I/O Power Supply

Function: Provide power for on-site signal circuits and external sensors/actuators of NX I/O units (digital input/output, analog input/output, etc.).

Two power supply methods:

Power supply through NX bus: I/O power is input from the I/O power terminal of the communication coupler or additional I/O power supply unit (NX-PF0630/0730), and then transmitted to the right I/O unit through the NX bus connector. This is the most common way.

Individually powered from the outside: Some I/O units have their own I/O power terminals, which can be directly powered by external 24V/5V without relying on the NX bus.

Important principle: The unit power supply and I/O power supply must use independent power supplies (cannot share a 24V power supply), otherwise noise may couple through the power line, causing I/O misoperation.

Additional I/O Power Unit: Solution for Insufficient Capacity

When the built-in I/O power capacity of the communication coupler or CPU unit (usually 4A or less) cannot meet the total current demand of subsequent I/O units, additional I/O power units need to be inserted into the rack. This unit has two main functions:

3.1 Expanding the Capacity of I/O Power Supply

NX-PF0630: Maximum 4A (suitable for most NX1P2 CPU racks, with an upper limit of 4A)

NX-PF0730: Maximum 10A (note: when connected to NX1P2 CPU, the actual maximum is still 4A; when connected to NX502, it can reach 10A)

The additional I/O power supply unit takes power from an external I/O power supply (5~24VDC, voltage needs to match the I/O unit used), and then delivers the I/O power supply to all NX units on its right side through the NX bus. Starting from this unit, the I/O power supply in the right paragraph is based on the input voltage of this unit, thus achieving power segmentation.

3.2 Separate I/O power supply segments (voltage isolation)

If different parts of a system require different I/O power supply voltages (such as 24V input units and 5V output units), or if it is necessary to electrically isolate the I/O power supplies in different areas to avoid interference, an additional I/O power supply unit can be inserted in the middle. The left paragraph uses one type of I/O power supply, while the right paragraph of the unit uses another type of I/O power supply (input through the unit's own I/O power terminal). This makes the power system very flexible.

3.3 Capacity Calculation Example

On site engineers need to calculate the total current consumption of the I/O power supply to ensure the selection of the correct additional I/O power supply unit. The typical formula is as follows:

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Total current consumption of I/O power supply=∑ (I/O power consumption of each unit)

Among them, the I/O power consumption of each unit includes:

Internal I/O power consumption of the unit (e.g. NX-PF0730 consumes 10mA)

Input current of I/O unit (per point x number of points simultaneously turned on)

Current of external sensors/loads

Calculation example: Configure an NX-PF0730 (10mA)+NX-ID3317 input unit (4-point ON, input current 6mA per point, external sensor 50mA per point)+NX-OD3121 output unit (4-point ON, load 125mA per point, output unit consumes 50mA per point). The total current is 10mA+(0+6 × 4+50 × 4) mA+(10+125 × 4+50 × 4) mA=10+224+710=944mA, which is much smaller than 10A capacity, but if using NX1P2 CPU, it cannot exceed 4A.

3.4 Precautions for Inrush Current

When connecting inductive or capacitive loads, the additional I/O power supply unit will experience higher surge currents. It must be ensured that the effective value (Irms) of the current does not exceed the rated value of the unit and related protective devices. Calculation formula:

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Irms = √( Ip² × D + Ia² × (1 - D) )

Where Ip is the peak impulse current, D is the duty cycle (pulse width/period), and Ia is the steady-state current. For NX-PF0730, the maximum allowable peak impulse current is 50A (pulse width of 1s), and when connected to NX1P2, it is 20A.

I/O power connection unit and shielding connection unit: wiring details

4.1 I/O Power Connection Unit (NX-PC00 □□)

This type of unit does not connect to the NX bus for power supply, nor does it generate power. It only provides additional terminals for distributing I/O power. There are three models in total:

NX-PC0020:16 IOV terminals (positive pole of I/O power supply)

NX-PC0010:16 IOG terminals (I/O power negative or COM terminal)

NX-PC0030: 8 IOV+8 IOG terminals

When providing 24V+and 24V - for a large number of three wire sensors or loads, using these units can avoid repeated wiring on each I/O unit. For example, connect the IOV terminal of NX-PC0020 to an external 24V power supply, and then extract multiple+24V from its terminals to multiple sensors, while using NX-PC0010 to provide a common 0V.

Wiring example: For NPN input units (such as NX-ID5342), it is necessary to connect the 0V line of the sensor to IOG and the+24V line to IOV. The use of I/O power connection units can simplify this centralized power distribution.

4.2 Shielded Connection Unit (NX-TBX01)

In industrial sites, shielded cables are usually used to suppress electromagnetic interference when connecting incremental encoders, analog sensors, or communication cables. NX-TBX01 provides 14 SHLD terminals and two functional grounding terminals. Just connect all shielded wires that need to be grounded to the SHLD terminal, and then connect the functional grounding terminal to the grounding copper bar of the control cabinet through a wire (grounding resistance ≤ 100 Ω). This ensures reliable grounding of the shielding layer and avoids the tedious work of making separate grounding clamps for each shielding wire.

Connection topology: encoder cable shielding layer → SHLD terminal → internal connectivity of the unit → functional grounding terminal → cabinet grounding.

Screw free clamping terminal wiring technique

The NX series system units all use Screwless Clamping Terminal Blocks, which do not require a screwdriver to tighten and can be simply inserted. But there are some key points in the wiring process, which often lead to on-site faults.

5.1 Applicable Wires and Terminals

Ferrule copper wire with sleeve: recommended for use. Insert the stripped multi strand wire into the sleeve, press it tightly with specialized crimping pliers, and then directly insert it into the terminal hole. No need to operate the release hole. Applicable sleeves include Phoenix Contact's AI series (0.34-2.5 mm ²) or Weidm ü ller's H series.

Multi strand bare wire: can be directly inserted, but only suitable for currents of 2A and below. If the current is greater than 2A, tin plated multi strand wire or conduit must be used.

Single stranded hard wire: It can also be directly inserted, but the grounding terminal (functional grounding) only allows the use of multiple stranded wires and cannot use single stranded hard wires.

5.2 Wire stripping length

The stripping length of all terminals (except for the grounding terminal) should be 8-10mm; the stripping length of the grounding terminal should be 9-10mm (because the grounding terminal hole is deeper). After stripping, the conductor part should not be scattered or bent.

5.3 Wiring operation steps (for multi stranded bare wires)

Insert a flat screwdriver (recommended Phoenix Contact SZF 0-0, 4X2.5) into the release hole on the side of the terminal at an angle of 10 °~15 ° until you feel the resistance of the spring.

Keep the screwdriver pressed in while fully inserting the prepared wire into the terminal hole.

Pull out the screwdriver, and the spring will automatically clamp the wire.

Gently pull the wire to confirm a secure connection.

Common error: Inserting the release hole vertically (90 °) will damage the terminal; Excessive force (over 30N) can cause the spring to deform; Rotating or twisting the screwdriver during insertion can also damage the terminal.

5.4 Secure Wires

When the wire diameter is thick or subjected to vibration, a cable tie (width ≤ 4mm, thickness ≤ 1.5mm) can be used to pass through the tie hole at the bottom of the terminal and tie the cable to the terminal seat to reduce the tension of the terminal on the wire.

Precautions for installation and disassembly

6.1 Installing NX Unit

The DIN rail must be installed first.

Two units are connected to each other through Unit Hookup Guides, and then pressed towards the DIN rail until the DIN Track Mounting Hook locks with a "click" sound.

It is strictly prohibited to install or dismantle units with electricity.

Do not touch the pins of the NX bus connector (located on the left side of the unit) to avoid contamination or electrostatic damage.

6.2 Disassembling NX Unit

Use a flat screwdriver to lift the DIN rail mounting hook upwards.

Then grab the protrusions for removing the unit on both sides with your hands and pull the unit forward. If there are other units on the right side that need to be disassembled, it is recommended to pull them out together with all the units on the right side, otherwise it is difficult to remove the middle unit separately.

6.3 Installation direction restrictions

When the system unit is connected to the CPU unit or communication control unit, it is only allowed to be installed vertically upright (with ventilation openings facing up/down).

When connected to the EtherCAT coupler as a slave terminal block, six directions (upright, flat, inverted, etc.) are allowed, but it should be noted that the output power of NX-PD1000 will decrease with increasing ambient temperature when not installed upright. For example, when installed upright at 45 ℃, it can still output 10W, but when installed lying flat at 45 ℃, it may only output about 6W.

Diagnosis and troubleshooting of indicator lights

7.1 TS indicator light (shared by all system units)

This indicator light (green/red) displays the status of the NX unit itself and the communication status with the higher-level unit (CPU or coupler).

TS LED status meaning countermeasures

Green is constantly on and running normally. I/O refresh is ready and working normally

Green slow flashing (2s interval) waiting for completion during initialization, restart, or project download, usually without intervention

If the hardware malfunction or watchdog error persists after re powering on the red light, replace the unit

Red flash (1-second interval) NX bus communication error check for loose connections between units and proper installation of units

The power supply of the unlit unit is not powered, or the unit is restarted/waiting for initialization to check the power wiring and 24VDC supply of the unit; If the power supply is normal and does not light up for a long time, the unit may be damaged

7.2 UNIT PWR indicator light (NX-PD1000 only)

Green constantly on: The unit power supply is normal.

Not lit: The unit power supply has not reached or the voltage exceeds the range of 20.4~28.8V. Check the external 24V power supply and wiring.

7.3 I/O PWR indicator light (NX-PF0630/0730 only)

Green constantly on: I/O power supply is normal.

Not lit: The I/O power supply has not reached or the voltage exceeds the range of 4.5~28.8V. Check the external I/O power supply and wiring.

7.4 Analysis of Typical Fault Cases

Case 1: All indicator lights on the NX unit on the right side of the CPU rack are not lit.

Reason: The unit power supply is not transmitted through the NX bus. It may be due to improper installation or damage of NX-PD1000, or insufficient unit power capacity of the CPU unit.

Check: Measure whether the UV/UG terminals of the CPU unit have 24V. If not, check the external power supply. If so, check if the UNIT PWR indicator light of NX-PD1000 is on. If it doesn't light up, check its UV/UG wiring. If it lights up but the right unit still has no power, it may be due to poor contact of the NX bus connector.

Case 2: The I/O PWR indicator light is on, but a certain analog input unit reading is abnormal.

Reason: The I/O power supply voltage exceeds the required range of the analog module (for example, a 24V sensor is connected to a 5V power supply segment).

Check: Measure the voltage between the IOV and IOG terminals of the module. If the voltage input from the additional I/O power supply unit does not match, it indicates that there may be an I/O power supply connection unit that has mistakenly connected to a different voltage.

Case 3: The TS indicator light flashes red and Sysmac Studio reports a "Non volatile Memory Hardware Error" (event code 00200000 hex).

Reason: Hardware failure of non-volatile memory in the unit.

Countermeasure: Try powering on or restarting the NX bus (via software reset) first. If the fault persists, the NX unit must be replaced. This fault cannot be fixed through the user program.

Case 4: The unit power supply is normal, but the right unit still cannot be recognized.

Reason: The NX bus connector was not fully engaged due to incomplete pushing along the hook rail during unit installation.

Inspection: Remove all units from the DIN rail and reinstall them in order, ensuring that each unit hears a "click" locking sound.

Maintenance and regular inspection suggestions

It is recommended to conduct the following checks at least once every 6 to 12 months (refer to the periodic checklist in the user manual):

Environmental inspection: The temperature inside the cabinet is 0-55 ℃, the humidity is 10-95%, and there is no condensation or corrosive gas.

Power quality: Use a multimeter to measure the unit power supply and I/O power supply, ensuring that the voltage is within the rated range (20.4~28.8V for 24V units, 5V power supply needs to be according to actual specifications).

Wiring inspection: Gently pull each wire to confirm that there is no looseness; Check if the shielding connection is secure.

Unit installation: Confirm that the DIN rail mounting hooks of all units are locked and the end covers are installed in place.

Cleaning: Wipe the surface of the unit with a dry soft cloth, and do not use diluents or benzene solvents. Do not touch the NX bus connector.