FANUC 16i/18i/21i hardware connection and troubleshooting

FANUC Series 16i/18i/21i MODEL B CNC Hardware Connection and On site Maintenance Guide

In the field of CNC machine tools, FANUC's Series 16i, 18i, 21i MODEL B (as well as enhanced models with the suffix "i") are still in service in a large number of devices due to their stable performance and powerful functionality. For on-site maintenance engineers, facing a FANUC CNC system that has been running for many years, every link from power supply connection, I/O wiring, servo and spindle connection, to emergency stop circuit design and battery replacement can become the source of faults. This article is based on the hardware connection manual of the CNC series, combined with on-site practical experience, to provide a complete technical guide from installation specifications to troubleshooting.

Installation and environment: Building a solid foundation for reliable operation

The FANUC CNC control unit is an open device and must be installed in a closed control cabinet with a protection level of not less than IP54. Strictly follow the following requirements during installation to avoid early failures caused by poor heat dissipation or interference.

1.1 Cabinet heat dissipation and spatial spacing

Control unit heat dissipation: The control unit is equipped with a built-in fan, allowing air to enter from the bottom and exit from the top. Therefore, a ventilation gap of at least 100mm must be reserved above and below the unit (this gap is still recommended for units with fans). It is strictly prohibited to install the control unit horizontally or upside down.

I/O module spacing: Connector Panel I/O modules also require a gap of 100mm above and below for wiring and ventilation. Devices with high heat generation, such as power supplies and transformers, should not be installed under the I/O module.

Cabinet surface: For small operator's panels, their heat dissipation capacity is approximately 8W/m ² ·℃ for coated metal surfaces and 3.7W/m ² ·℃ for plastic surfaces. If the total internal heat generation exceeds the cooling capacity of the cabinet, a heat exchanger or fan must be installed.

1.2 Anti noise measures: grouped wiring, shielding, and grounding

The signal lines of the CNC system are divided into three groups according to anti-interference requirements:

Group A: Main power line (AC 220V), secondary power line, servo motor power line, AC/DC relay/solenoid valve coil line. These cables must be separated from groups B and C by at least 100mm or use electromagnetic shielding.

Group B: DC 24V solenoid valve/relay wire, DI/DO wire from CNC to high-voltage cabinet. Try to stay away from Group A as much as possible and it is recommended to add shielding.

Group C: I/O Link cables, position/speed feedback cables, handwheel wires, RS-232C communication wires, battery wires, etc. between CNC and I/O units. Shielding treatment must be carried out.

Grounding points:

The protective grounding (PE) resistance should be ≤ 100 Ω (Class D grounding), and the cross-sectional area of the grounding wire should not be less than that of the power line.

The SG (signal ground) terminal of the CNC control unit must be connected to the grounding copper bar inside the cabinet with the shortest wire (recommended<30cm).

The shielded cables of servo amplifiers and spindle amplifiers must make extensive contact with the grounding plate through cable clamps, rather than being single ended grounded through thin wires. The grounding plate material should be nickel plated steel plate (thickness ≥ 2mm).

1.3 Noise Suppressor and Lightning Protection

For AC/DC relays and solenoid valves, CR spark suppressors (capacitors+resistors in series) must be installed at both ends of the coil. The resistance value is taken as the DC resistance of the coil, and the capacitance value is approximately I ²/10~I ²/20 μ F (I is the steady-state current). Varistor cannot be used alone as it cannot suppress voltage spikes.

To prevent lightning strikes, it is recommended to install surge absorbing components (such as Okaya's R · A · V-781B) between input power lines and between lines and ground. The total length of the connection between the absorbing component and the protected equipment during installation should be ≤ 2m.

Power System and Battery: Power on Timing and Data Preservation

2.1 Specification of 24V DC power supply

The CNC control unit, peripheral I/O modules, LCD units, etc. all require stable 24V DC input. The external power supply must meet the following requirements:

Output voltage:+24V ± 10% (including ripple and noise, 21.6V to 26.4V)

Instantaneous power outage holding time: ≥ 10ms (100% load drops to 0) or ≥ 20ms (50% load)

For machine tools with vertical axes, it is recommended to use a power supply with a longer holding time to reduce the distance of the vertical axis falling during power failure.

Not recommended power circuit:

The use of simple rectification filtering (only diode+capacitor) cannot maintain the output voltage during instantaneous power failure.

Connecting the CNC to a high-power load that frequently starts and stops (such as a large contactor) on the same 24V output line can cause the voltage to drop instantly below 21.6V.

2.2 Power on and power-off sequence

To ensure reliable system startup and avoid data errors, the recommended power on sequence is:

Machine tool main power supply (200VAC), separate detector (grating ruler) power supply

I/O Link slave station, separated detector interface unit, independent LCD unit (24VDC), servo amplifier control power supply (200VAC)

CNC control unit (24VDC)

When powered on, 1 and 2 should be completed within 500ms after 3 are powered on. The power-off sequence is the opposite: first cut off the 24V of the CNC, and then cut off other units. If the timing is violated, there may be residual alarm information in NC.

2.3 Battery Replacement: Memory Backup and Absolute Position Battery

FANUC CNC uses multiple batteries, and the most common problem on site is SRAM data loss or absolute position loss caused by low battery voltage.

(1) CNC memory backup battery (3VDC)

Model: Lithium battery A02B-0200-K102 (pre installed on CNC front panel)

Replacement timing: When the display screen flashes the "BAT" alarm, it should be replaced within 2-3 weeks (depending on the configuration). It is recommended to replace it proactively once a year.

Replacement steps (hot replacement):

Keep the CNC powered on for at least 30 seconds (charging the capacitor)

Turn off the CNC power supply

Unplug the old battery connector and remove the battery

Insert and connect a new battery within 30 minutes (for 16i/18i/21i series, memory data can be retained for 30 minutes after battery removal)

Power on again and clear the alarm

External battery box: Two D-type alkaline batteries (A02B-0236-C281) can be used instead of lithium batteries, and the CNC also needs to be powered on during replacement.

(2) Absolute pulse encoder battery (6VDC, used for separate detectors or motors with built-in absolute encoders)

Separate absolute encoder: using 4 D-type alkaline batteries (in series). When APC alarm 3n6~3n8 occurs, it should be replaced as soon as possible (within 1-2 weeks). When replacing, it is necessary to keep the CNC powered on, otherwise the absolute position will be lost.

The motor is equipped with an absolute encoder: the battery is installed on the servo amplifier. Please refer to the α i series maintenance manual.

I/O module and signal connection: from I/O Link to DI/DO

FANUC CNC connects distributed I/O modules (such as connection type I/O modules, operation panel I/O modules, etc.) through the I/O Link serial bus. Proper wiring and address allocation are key to avoiding on-site signal misoperation.

3.1 I/O Link Cable and Transmission Distance

Electrical signal cable: up to 10m in length (can be extended to 15m if all are in the same cabinet). Use recommended shielded twisted pair cables (such as A66L-00001-0284 # 10P).

Fiber optic cable: When longer distances or strong interference environments are required, use optical I/O Link adapters (A13B-0154-B001/B002). The total length can reach 200m (standard type) or 100m (high-speed type). Up to 16 high-speed adapters (or 5 standard adapters) can be cascaded.

3.2 DI/DO Module Wiring Example (Taking Connection Type I/O Module as an Example)

DI (Input): The module provides 24 input points/module (basic module+expansion module up to 96 points). The input voltage is 24VDC, the ON voltage is ≥ 14V, and the OFF voltage is ≤ 6.5V. The response time includes receiver delay (≤ 2ms)+I/O Link transmission time (≤ 2ms)+ladder scan period.

DO (Output): The maximum load current at each point is 200mA (continuous), and the saturation voltage drop is ≤ 1V. It is important to note that the total current of the DOCOM pin does not exceed 0.7A. The DO driver has overcurrent and overheat protection, and once triggered, the corresponding output byte will be forcibly turned off, and the corresponding alarm bit (such as Xm+15) in the DI address will be set to 1.

Important notice: For 24VDC input modules, DICOM generally recommends connecting to 0V (source input mode) to avoid mistaking the input as ON due to a short circuit to ground. The emergency stop signal must be assigned to a fixed address on the common terminal (such as Xm+1. x or Xm+2. x, etc.), and cannot be assigned to an address that allows the selection of the common ground (can be connected to+24V), otherwise there may be safety hazards.

3.3 Connection of manual pulse generator (handwheel)

The handwheel uses a 5V power supply, and due to voltage drop in the cable, the maximum length needs to be calculated. Formula: 0.2V ≤ 0.1A × R × 2L/m, where m is the number of 5V and 0V cores. When using the recommended cable A66L-00001-0286 (6 power cords), the maximum length of a single handwheel is approximately 76m (but FANUC limits it to ≤ 50m); About 38m with two handwheels.

Servo and spindle interface: Fiber optic serial bus (FSSB and serial spindle)

4.1 Servo Interface (FSSB)

The CNC and α i series servo amplifiers are connected through optical cables (COP10A interface). A single optical cable can control up to 8 axes (16i), 6 axes (18i), or 4 axes (21i). The total length of the optical cable is ≤ 100m, and the distance between units is ≤ 10m (using inner cables) or ≤ 100m (using reinforced outer cables).

High speed HRV function: When using high-speed HRV, the shafts on the same FSSB line must be used in pairs (1-2, 3-4, etc.), and amplifiers that do not support high-speed HRV cannot be mixed on the same line.

Separate detector interface unit: When connecting a grating ruler or external encoder, a separate detector interface unit (A02B-0236-C203/C204) is required, which is also connected via FSSB fiber optic cable and can serve as the last slave station.

4.2 Serial spindle connection

1-2 spindles: directly connected to the spindle amplifier through the CNC JA41 interface.

3-4 spindles (18i/16i): A serial spindle connection board (A13B-0180-B001) is required to expand one serial signal to four channels. When the cable length exceeds 20m or there is strong interference, a fiber optic I/O Link adapter must be used for photoelectric conversion.

Emergency Stop Circuit: Safety Standards and Redundancy Design

The Emergency Stop Signal (ESP) is the core of the safety circuit. FANUC CNC requires external design to meet standards such as EN60204-1 and EN954-1.

5.1 Signal Logic

*ESP is low level active (B contact). When the contact is closed (low level), the CNC releases the emergency stop and the servo and spindle can operate.

When ESP is disconnected (high level), CNC resets, servo motor stops through dynamic braking, spindle motor loses power but still rotates inertia.

5.2 Safety redundancy requirements

Main power contactor: A line contactor (MCC) should be installed on the input side of the servo amplifier power module (PSM), driven by the contactor control signal of PSM. But if the amplifier itself fails and the output relay cannot be disconnected, relying solely on internal shutdown of the amplifier is not safe enough. Therefore, it is necessary to design a redundant emergency stop path independent of the amplifier, such as directly cutting off the MCC coil through the emergency stop button.

Spindle stop delay: For the spindle, due to its high inertia, it is recommended to use an off delay timer in the redundant path to allow the spindle to slow down and stop before cutting off the power supply, avoiding free parking.

Limit switch: Although CNC has software limit switches, in order to prevent overtravel after servo feedback failure, a hard limit switch must be installed at the end of the stroke and connected to the ESP circuit.

5.3 Connection Example (α i Series)

One pair of normally closed contacts of the emergency stop button is connected to the * ESP input (fixed address such as X8.4) of the CNC, while the other pair of contacts controls the MCC coil of the PSM.

At the same time, connect the MCOFF signal of PSM in series to the MCC coil circuit to achieve dual turn off.

Common troubleshooting cases

Case 1: BAT alarm - CNC memory battery low

Phenomenon: When turned on, the display screen flashes "BAT", but the program is still normal.

Solution: Replace the lithium battery according to the previous hot replacement steps. If the battery is completely depleted (SRAM parity alarm 910 appears during startup), all memory needs to be cleared and parameters and programs need to be re entered.

Case 2: APC alarm 3n0- Absolute position loss

Phenomenon: The absolute encoder battery voltage is too low, causing the position to be lost after power failure. After powering on again, it prompts to return to the reference point.

Solution: Replace the battery (must be replaced when the CNC is powered on), and then perform manual or automatic return to the reference point operation. If the reference point position is unknown, the origin needs to be reset.

Case 3: I/O Link Communication Alarm

Phenomenon: The CNC displays an "I/O LINK" alarm, and all DO outputs are forcibly shut down.

Solution: Check if the I/O Link cable is loose or broken; Check if the power supply (24VDC) of the slave module is normal; Check if the last module has a terminal resistor installed (if using an electrical signal cable, the JD1A of the last module can be suspended, but pay attention to the adapter settings when using fiber optic).

Case 4: WDT alarm (watchdog timeout)

Phenomenon: RUN LED flashing, alarm code 22.

Processing: Read diagnostic parameter D9012 and check the scanning time. If the scanning time is too long, the program needs to be optimized (reducing loops, using CJ instructions to skip unnecessary segments). If it is caused by an instantaneous power outage, check the input power supply holding time.

Case 5: RS-232C communication failure

Phenomenon: Unable to transfer programs to PC.

Solution: Check if the cable connection follows standard wiring (SD → RD, RS → CS, etc.), and confirm that the baud rate, stop bit, and parity parameters are consistent. For the Remote Buffer interface, the handshake signal requirements for protocols A and B are different and need to be configured according to the device manual.

Summary and maintenance suggestions

FANUC Series 16i/18i/21i MODEL B is a mature and reliable CNC system, but the details of hardware connections often determine the long-term stable operation of the equipment. For maintenance engineers, it is recommended to include the following items in the regular inspection plan (every 6-12 months):

Environmental inspection: Control the temperature (0-55 ℃), humidity (10-75% without condensation), and presence of corrosive gases inside the cabinet.

Power check: Measure whether the 24V input voltage remains between 21.6-26.4V during load changes.

Battery status: Monitor the "BAT" alarm through the diagnostic screen and replace it in advance.

Grounding system: Check whether the PE and SG terminals are corroded, and whether the shielding clip is secure.

Fan and ventilation: Confirm that the control unit fan is operating normally and the ventilation holes are not blocked.

When a malfunction occurs, please follow the principle of "first check the light, then check the code, and finally measure the line". Use the diagnostic screen (DGN) of CNC to read error codes (such as D9008 displaying 22, 31, 32, etc.), and quickly locate specific modules or circuits based on relevant manuals. Through standardized installation and maintenance, this classic CNC system can still reliably serve modern production lines.