Resolving Alarm 085 / SR720 / SR730 / SR740 & Communication Errors
How to fix data format and transmission errors at the CNC interface.
Experiencing these errors during data transfer?
- Alarm 085 (COMMUNICATION ERROR): The control aborts because it cannot correctly decode the received data characters.
- Alarm SR720 / SR730 / SR740: These represent parity, framing, or overrun errors commonly found on modern Fanuc CNC controllers.
- Symptom: The error occurs either immediately when starting the transmission or happens intermittently mid-program.
Understanding the behavior (Troubleshooting key)
The timing of when the alarm occurs is the primary indicator of the underlying issue:
Scenario A: Error occurs at the start (Parameter Mismatch)
If the alarm triggers the moment data transfer begins, the communication settings between your PC and the CNC are mismatched.
- The Solution: Verify that the following settings match exactly between your DNC software (e.g., EasyDNC or RemoDNC) and your Fanuc parameters (Parameter 101-103 or I/O Channel settings):
- Baud Rate (e.g., 9600, 19200)
- Data Bits (7 or 8)
- Stop Bits (1 or 2)
- Parity (Even, Odd, or None)
Scenario B: Intermittent error during transmission (Interference or Cable Fault)
If transmission starts correctly but fails randomly during the process, it is typically a physical hardware issue.
- Electromagnetic Interference (EMI): Poorly shielded cables pick up electrical noise from shop floor motors, VFDs, or high-voltage power lines, causing bit-flips in the data stream.
- Loose connections or cold solder joints: A serial cable that flexes during machine axis movement can cause intermittent signal drops.
- The Solution: Inspect your RS232 cable shielding (ensure it is grounded at one end only!) and check all physical connectors and solder joints for integrity. Ensure flow control settings (XON/XOFF vs Hardware RTS/CTS) are correctly configured for your specific cable wiring.
Technical Insight: Parity and Framing
In serial communication, data bytes are transmitted as individual bits. A Framing Error occurs when the start and stop signal of a character arrives out of sync. A Parity Error indicates that the mathematical checksum of the received bits does not match the expected value, signaling that the data was corrupted during transit.
Special Case: Alarm 080 (TV Check / Block Parity)
Occasionally, the machine pauses not due to a communication fault, but because the TV Check (Tape Verify, Vertical Parity Check, or Block Parity) is active. The CNC expects a specific character count per line of code.
Since modern programs generated by CAD/CAM software often ignore these legacy paper-tape requirements, the TV Check frequently triggers false alarms. When active, this results in Alarm 080 (TV ERROR) or a corresponding SR700 code. Most DNC users resolve this by permanently disabling this check in the machine parameter settings.