Dostek DNC™


Most CNC machine tools provide RS-232C serial ports for DNC communication. In Windows computers, RS-232C serial ports are called "COM" ports. Dostek DNC works with any COM port that includes a Windows device driver, including:

  • Standard built-in COM ports.
  • Serial boards.
  • Wired Ethernet/serial adapters and hubs.
  • Wireless Ethernet/serial adapters and hubs.
  • USB/serial adapters and hubs.
  • Cardbus and ExpressCard adapters.

This page explains how to choose ports and cable, and how to avoid some common problems. Topics include:


  • For detailed technical and troubleshooting information about RS-232C and COM ports, see your Dostek DNC's Communication Testing and Troubleshooting help file.

Understanding COM Ports

COM Port

This section helps you:

  • Identify your computer's built-in standard COM ports.
  • Avoid potential electrical problems.
  • Avoid potential COM port compatibility problems.

Standard COM Ports

Built-in standard COM ports were common on older computers, but are less common on new computers, especially laptop and netbook computers. You can identify a standard COM port by its 9-pin D-shaped connector, which may be labelled "10101."


  • Your computer may also have a 25-pin D-shaped connector. Although this connector looks like the RS-232C serial port found on CNC machines, it is actually the computer's parallel printer port, and is not suitable for CNC communication.

Avoiding Potential Electrical Problems

The RS-232C serial interface connect the computer's electrical ground to the CNC's electrical ground. When you use a multi-port RS-232C board or hub, the electrical ground of all machines are connected together. An electrical surge or grounding problem with one machine can potentially damage the board, hub or other machines.

To avoid electrical problems, you can:

  • Use single-port Ethernet/serial adapters to connect each CNC machine to your shop-floor network. Ethernet connections are magnetically-coupled, providing up to 1,500 volts of electrical isolation.
  • Use single-port wireless Ethernet/serial adapters to connect each CNC machine to your wireless shop-floor network. Wireless adapters are not electrically connected to the computer or to each other.
  • Install an optical isolator between each CNC and its serial cable. Optical isolation provides up to 2,500 volts of electrical isolation.

Avoiding COM Port Compatibility Problems

A variety of compatibility problems can occur when choosing COM ports for CNC communication:

  • The COM port's device driver software may not be compatible with your computer's Windows operating system. For example, some products do not include Windows 7 64-bit drivers. Plus, some older hardware may not have drivers to work with newer computers.
  • A plug-in serial card may not be compatible with your computer's expansion slots.
  • Some device driver software contains "bugs" that cause problems when used for CNC communication. For example, some Ethernet/serial adapters do not automatically reconnect when powered on.

For more information about compatibility problems, contact Dostek.

Recommended COM Ports

Dostek recommends Moxa single-port wired and wireless Ethernet/serial adapters. They provide electrical isolation and excellent device drivers at a reasonable cost.

Wired Ethernet/Serial Adapters


Single-port wired Ethernet/serial adapters are an affordable and very reliable way to connect CNC machines to your computer. Benefits include:

  • Electrical isolation. Magnetic coupling provides 1,500 volts isolation.
  • Reliable communication. Data is transferred between the computer and adapter using Ethernet's high-speed error-correcting protocol.
  • Distribute ports on the shop floor. Ethernet/serial adapters and hubs connect to your local area network (LAN) using standard network cables. There is no direct connection between the CNC and computer.
  • Less restrictive cabling. Use standard network cables up to 300 feet in length (versus 50 feet recommended for RS-232C).
  • Simplified maintenance. Ethernet/serial adapters are not installed inside your computer. Replacing computers or adapters is fast and easy.

Wireless Ethernet/Serial Adapters


Single-port wireless Ethernet/serial adapters are a convenient way to simplify your DNC system by eliminating cables. Wireless adapters provide all of the benefits of single-port wired adapters, plus:

  • Complete electrical isolation.
  • Eliminate cabling. Wireless adapters provide a range of up to 150 feet without cables. Installing additional access points extends the wireless range to cover the largest shop floor.
  • Simplified maintenance. Ethernet/serial adapters are not installed inside your computer. Replacing computers or adapters is fast and easy.

More COM Port Options

The following COM port options are suitable for CNC communication, although they do not offer the electrical isolation provided by the recommended single-port Ethernet/serial adapters:

  • Multi-port serial boards. An inexpensive way to add COM ports to a computer. Available with 2, 4 or 8 ports.
  • Ethernet/serial hubs. Slightly less expensive than single-port adapters. Can be a good choice when replacing a system and using existing wiring.
  • USB/serial adapters and hubs. Although USB/serial adapters and hubs are relatively low in cost, all of the tested devices include serious flaws that cause problems for CNC communication. For the latest information about USB/serial adapters, contact Dostek.

Choosing Communication Cables


A "cross-linked null modem" cable is required for most CNC machine tools. Standard cables are available from Dostek in lengths of 5, 10 and 25 feet. For longer cables or CNCs requiring non-standard wiring, contact Dostek.

You can also build your own cables. See your Dostek DNC manual and application notes for detailed instructions.

The EIA RS-232C standard specifies a maximum cable length of 50 feet. However, cable lengths of 200 feet or more are commonly used. Low-capacitance shielded cable is recommended for high serial data rates (9600 baud or faster) or long cables.

To avoid potential problems with long RS-232C cables, use single-port Ethernet/serial adapters.


Using Category 5 Cable for RS-232C Wiring

You can use Category 5 cable (widely used for local area network wiring) to simplify your RS-232C wiring. Category 5 cables use RJ45 plugs. Pre-wired 9-pin and 25-pin RJ45/D-sub wiring adapters are available from Dostek.

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