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FANUC Controller
Program/G-code · Intermediate · 30-60 minutes
Fanuc Alarm 087 - BUFFER OVERFLOW
Alarm 087 BUFFER OVERFLOW occurs when the CNC reader/punch interface receives more than 10 characters after issuing a read stop command, indicating the external device (tape reader, PC, DNC system) is not responding to XON/XOFF or DC codes properly. This is a hardware handshaking failure between the CNC and the external communication device on the reader/punch channel.
087
Fanuc Alarm 087 - BUFFER OVERFLOW
BUFFER OVERFLOW
Fanuc PS087: A buffer overflow occurred on the RS-232 serial communication interface. The receive buffer was filled faster than the control could process the data.
Root Cause Summary
Alarm 087 BUFFER OVERFLOW occurs when the CNC reader/punch interface receives more than 10 characters after issuing a read stop command, indicating the external device (tape reader, PC, DNC system) is not responding to XON/XOFF or DC codes properly. This is a hardware handshaking failure between the CNC and the external communication device on the reader/punch channel.
Safety
- Stop machining operations immediately - buffer overflow may cause program execution errors or unexpected machine behavior
- Ensure the machine is in a safe state before attempting any troubleshooting steps. Stop the program and power down the spindle.
- Be cautious when working with electrical connections. Power down the machine before disconnecting or reconnecting any cables.
- Refer to the CNC machine's safety manual for specific safety precautions.
- Incorrect parameter changes can lead to unexpected machine behavior. Back up parameters before making any changes.
- Be aware of moving parts when the machine is powered on.
Causes and Fixes
- 1
Navigate to SYSTEM > PARAM and verify Parameter 20 (input device selection) is set correctly: 0=tape reader, 1=RS232C reader/punch interface. Confirm Parameter 21 bit settings for stop code selection match the external device configuration.
- 2
Check Parameters 100-109 (RS232C channel 0 settings). Verify Param 102 bit 0 (DC code enable) is set to 1 to enable XON/XOFF handshaking. Confirm baud rate (Param 100), stop bits (Param 101 bit 0-1), and parity (Param 101 bit 2-4) match external device.
- 3
Test external device software settings. Ensure DNC software or communication program has XON/XOFF flow control enabled, not set to NONE or hardware-only handshaking. Send a small test file (under 80 characters) to isolate the issue.
- 4
If using RS232 cable, verify cable is properly wired for software handshaking with TX, RX, and GND connected. Check cable continuity and ensure no cross-wiring. Replace cable if suspect.
Call a technician if:
- ·I/O board hardware replacement needed
- ·Parameter corruption requiring system restore
- ·Multiple communication alarms occurring simultaneously
Prevention
- Regularly verify communication parameter settings match external devices
- Use quality shielded RS-232C cables
- Implement proper grounding between CNC and external equipment
- Keep DNC software updated with latest communication protocols
Common Mistakes
- Assuming all RS-232C devices use same handshaking method
- Ignoring baud rate timing requirements
- Using incorrect cable pinout for specific application
- Not verifying external device buffer capabilities
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