Observed FANUC 18i Communication State
The Star SR20R uses FANUC 18i channel 0 with 4800 baud, 2 stop bits, and ISO format. CNC-to-PC transfers through the selected I/O port produce a correct file in Cincom, but PC-to-CNC transfers leave LSK blinking without loading the program. Disconnecting the RS-232C cable while LSK is blinking clears that indication and produces B/PS 086.
| Transfer direction | Observed result | What it establishes |
|---|---|---|
| FANUC 18i to PC | File received correctly | The CNC transmit path and the PC receive path operate with the current connection. |
| PC to FANUC 18i | LSK continues blinking | The reverse direction remains unverified. |
| Cable disconnected during LSK | B/PS 086 appears | The control detects a communication fault when the connection is removed; the evidence does not define the code further. |
Why One-Way Transfer Does Not Prove the Cable
A successful CNC-to-PC transfer does not validate the conductors or connections used for PC-to-CNC data. The cable can therefore work in one direction while failing in the other. The available evidence does not include connector pin numbers, interface diagrams, or handshake settings, so do not infer a pinout or bridge terminals by guesswork.
Cincom also remains a possible variable, but the directional result makes cable wiring the first condition to isolate. A three-wire cable was identified as a practical correction for this communication problem; build or verify it from the actual connector-end definitions for the PC and machine interfaces.
Diagnostic and Correction Procedure
- Record the working setup: channel 0, 4800 baud, 2 stop bits, ISO format, selected I/O port, and the Cincom configuration.
- Check cable continuity end to end, treating the PC-to-control receive path separately from the already working control-to-PC path.
- Verify each connector end against the applicable interface definitions. The two ends may require different termination even though the cable works from the control to the PC.
- If the interfaces support it, test a correctly terminated three-wire cable. The evidence does not provide enough information to specify connector pin numbers.
- Retry the PC-to-control transfer without changing multiple variables at once. If it still fails, retain the verified cable and then examine the Cincom transmit configuration.
Verification Criteria
The correction is verified only when the FANUC 18i accepts a program sent from the PC while the same setup still transfers a correct file from the CNC to the PC. LSK must no longer remain indefinitely in the observed blinking state, and disconnecting the cable must not be part of normal operation.
FAQ
Why does FANUC 18i LSK keep blinking during an RS-232C receive?
In this case, LSK continued blinking because no usable PC-to-control transfer completed. Verify the receive-direction wiring first, then check that Cincom transmits with channel 0 configured for 4800 baud, 2 stop bits, and ISO format.
Can a FANUC RS-232 cable transmit in one direction but fail in the other?
Yes. A correct CNC-to-PC file proves only the control transmit and PC receive path; it does not prove the separate PC transmit and control receive path. Check both connector ends and every conductor used in the failing direction.
Should I use a three-wire cable for this FANUC 18i problem?
A three-wire cable is the evidence-supported cable correction to test. Confirm the actual connector-end definitions before wiring it because the available evidence supplies no pin numbers.