4151

Check CNC Communication Settings

Navigate to SETTING > I/O > RS-232C and verify baud rate, data bits (typically 7 or 8), parity (even/odd/none), and stop bits match your DNC software exactly.

Most common settings are 9600 baud, 8 data bits, no parity, 1 stop bit

Verify Handshake Protocol

Check the handshake setting in RS-232C menu - ensure it matches your DNC software (XON/XOFF, RTS/CTS, or none). Mismatch here is a primary cause of 4151.

XON/XOFF is most reliable for long program transfers

Test Communication

Use a simple text editor or terminal program to send a small test program first. If successful, the issue is likely buffer-related rather than basic communication parameters.

Inspect Physical Connections

Check RS-232 cable connections at both CNC and computer ends. Ensure connectors are fully seated and locking screws are tight.

Power down both devices before disconnecting cables

Test Cable Continuity

Use multimeter to verify continuity on pins 2 (RXD), 3 (TXD), and 5 (GND) at minimum. For hardware handshaking, also check pins 4 (RTS) and 5 (CTS).

Swap with known good cable if available - cable issues are more common than CNC hardware problems

Verify Cable Type

Confirm you're using correct cable type (null modem vs straight-through). Most DNC applications require null modem cable where pins 2 and 3 are crossed.

Reduce Transmission Speed

Lower the baud rate in both CNC RS-232C settings and DNC software to 4800 or 2400 baud to allow more processing time between data packets.

Slower is often more reliable for large programs over 100KB

Enable Flow Control

Set handshake to XON/XOFF in the RS-232C menu to allow the CNC to pause transmission when buffer is nearly full.

XON/XOFF prevents buffer overflow better than hardware handshaking for most Okuma systems

Split Large Programs

If program is extremely large (>500KB), consider splitting into smaller sections or using memory card transfer instead of DNC for initial program loading.

Check Program Format

Verify program starts with % and program number (e.g., %1001) and ends with % on separate line. Missing or incorrect format codes cause OSP to reject transfer.

Use text editor to view raw file and check for hidden characters

Validate Character Set

Ensure program contains only valid G-code characters. Remove any extended ASCII, Unicode, or special formatting characters that may have been introduced by word processors.

Never use word processors like MS Word to edit CNC programs

Test with Simple Program

Create minimal test program (%1000 G00 X0 Y0 M30 %) to verify communication works. If successful, issue is in original program content.