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

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):