Summary

RS-232 communication is commonly used to transfer NC programs between GibbsCAM and older CNC machine controls. Successful communication requires that the settings on the computer, communication software, and CNC control all match.

This article provides general setup recommendations and troubleshooting tips for establishing reliable RS-232 communication.

TABLE OF CONTENTS

Overview

RS-232 is a point-to-point serial communication standard commonly used by legacy CNC controls. Although many newer machines support Ethernet or USB program transfer, RS-232 remains widely used for older equipment.

Communication problems are almost always caused by one of the following:

  • Incorrect communication settings

  • Incorrect cable wiring

  • COM port configuration issues

  • Flow control mismatches

  • Machine control configuration errors

Successful communication requires that both the CNC control and the PC use identical serial communication settings.

Verify the Communication Settings

The following parameters must match exactly between the CNC machine and the communication software.

Setting

Typical Values

Baud Rate

2400, 4800, 9600, 19200, 38400

Data Bits

7 or 8

Parity

None, Even, Odd

Stop Bits

1 or 2

Flow Control

XON/XOFF, RTS/CTS, DTR/DSR

If even one setting differs, communication may fail.

Verify the COM Port

Confirm that the communication software is using the correct Windows COM port.

  1. Open Device Manager.

  2. Expand Ports (COM & LPT).

  3. Verify the assigned COM port number.

  4. Configure the same COM port in your communication software.

If using a USB-to-Serial adapter, Windows may assign a different COM port when the adapter is connected to another USB port.

Verify the Cable

A large percentage of RS-232 communication problems are caused by cable issues.

Verify:

  • The correct cable type is being used.

  • Pin assignments match the CNC manufacturer's requirements.

  • The cable is securely connected.

  • The cable is not damaged.

  • Cable length is within recommended limits.

Many CNC controls require a null modem cable, while others require a straight-through cable. Always consult the machine tool documentation before changing cables.

Verify Flow Control

The CNC control and communication software must use the same flow control method.

Common methods include:

  • XON/XOFF (Software Handshaking)

  • RTS/CTS (Hardware Handshaking)

  • DTR/DSR (Less common)

If one side uses hardware handshaking while the other uses software handshaking, communication may stop or fail.

Test with a Small Program

Before transferring a large NC program:

  1. Create a simple test program.

  2. Send the program to the CNC.

  3. Verify the machine receives the program successfully.

Using a short program makes it easier to identify communication issues before attempting larger program transfers.

Common Communication Problems

Nothing Is Received by the CNC

Possible causes:

  • Incorrect COM port

  • Incorrect cable

  • Wrong baud rate

  • Incorrect machine communication mode

  • Flow control mismatch


Garbled or Random Characters

Possible causes:

  • Incorrect baud rate

  • Incorrect parity

  • Incorrect data bits

  • Electrical interference

  • Faulty cable


Transfer Stops Before Completion

Possible causes:

  • Incorrect flow control

  • Buffer overflow

  • Faulty cable

  • USB-to-Serial adapter issues


One-Way Communication Works

If sending works but receiving does not (or vice versa), verify:

  • TX and RX wiring

  • Flow control settings

  • Cable pinout

  • Machine parameters

This often indicates a cable wiring or handshaking issue rather than a GibbsCAM configuration problem.

USB-to-Serial Adapters

Many modern computers no longer include a native RS-232 port.

When using a USB-to-Serial adapter:

  • Install the latest manufacturer driver.

  • Verify the assigned COM port.

  • Use the same USB port each time, if possible.

  • Industrial-grade adapters typically provide more reliable communication than low-cost consumer adapters.

Best Practices

  • Match all communication parameters exactly.

  • Use high-quality serial cables.

  • Label cables to avoid confusion.

  • Test communication with a small NC program first.

  • Document the machine's communication settings for future reference.

  • Consider Ethernet or USB transfer methods when supported by the CNC control.

Expected Result

After the communication settings, cable, and machine configuration are verified, GibbsCAM can reliably transfer NC programs using RS-232 communication.

Related Articles

Keywords

RS-232, Serial Communication, COM Port, Serial Port, DNC, Program Transfer, Baud Rate, Parity, Stop Bits, Flow Control, XON/XOFF, RTS/CTS, Null Modem, USB Serial Adapter, CNC Communication