DNC Application Guide
This page reviews some of the issues to consider when choosing DNC systems and software including the various ways to connect CNC machine tools to computers.
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DNC System Overview
A DNC system stores computer numerical control (CNC) programs on a computer and distributes the programs to CNC machine tools on the shop floor (Distributed Numerical Control) or runs CNC programs directly from a computer rather than from the CNC's internal memory (Direct Numerical Control, or "drip feed"). Direct Numerical control is used when NC machines have no internal storage or when it is necessary to run large CNC programs that are too large to fit in the CNC's memory or would take too long to load into the memory.
A DNC system can use a single computer to service all CNC machine tools or use separate computers to service each individual CNC or group of CNCs (work cell). The following paragraphs explain various ways to connect CNC machine tools to a computer, how to select and install cable and connectors, how to integrate the DNC system with your office network and how to "drip feed" large files.
Connecting Multiple CNCs to a Computer
Dostek DNC works with any serial connecting device that uses a standard COM port or is installed as a Windows COM port. This means that any of the following devices can be used to connect multiple CNC machine tools to a single computer:
- Standard COM ports
- Mechanical switch box
- Electronic switch
- Multi-port serial board
- Ethernet/serial hub
- USB/serial hub
- Wireless communication devices
Standard COM ports and multi-port serial boards are installed inside the computer; all of the other devices are installed outside the computer. Hubs and wireless devices are connected to the computer's USB port or to the local area network (LAN).
Using Standard COM Ports
A standard COM port is one that works with Windows' built-in COM port device driver. Windows
computers typically allow for up to four standard COM ports, but most Pentium class computers have
only one or two standard COM ports installed. A plug-in circuit board can be installed in most
computers to increase the number of standard COM ports.
Using Manual and Electronic Switches
An inexpensive mechanical switch box can connect as many as 6 CNCs to each computer COM port, and multiple switches can be cascaded to connect additional CNCs. However, mechanical switches may become unreliable as the switch contacts wear, and they require the operator to correctly check and set the switch each time a file is transferred. An electronic switch is more reliable and eliminates the need for the operator to set the switch.When multiple CNCs are connected to the computer using a switch, whether mechanical or electronic, the computer can only communicate with one of the CNCs at a time. This prevents you from using the multi-tasking capabilities of the computer to load or drip feed to more than one CNC at a time. Multiport boards and hubs are a better choice than an electronic switch because they permit multi-tasking and are similar or only marginally higher in cost. To use the remote call-up feature provided by Dostek DNC MultiPort Edition, each CNC is normally connected to it's own dedicated COM port.
Using Multi-Port Serial Boards
Multi-port serial cards are available with 2, 4, 8, 16 or 32 ports per card in a
variety of configurations (ISA or PCI bus, 9-pin or 25-pin cable or panel connectors,
RS-232 or RS-422, surge protected, etc.).
Pictured are a RocketPort® ISA-Octa/DB25 8-port serial card for the ISA bus with with fan-out cable connections (lower left) and a RocketPort® PCI-32 32-port serial card for the PCI bus with two 16-port interface panel connections (upper right). The panel and card are connected by a cable (supplied) that is typically only about 6 feet in length. This means that the connector panel must be installed next to the computer and cables must be brought from each CNC back to the computer. Use an Ethernet/Serial hub if you wish to overcome this limitation.
Multi-port serial boards require installation of a device driver to work with Windows. Before purchasing a board, check that a device driver is available for the version of Windows you plan to use.
Using Ethernet/Serial Hubs
An Ethernet/Serial hub installed on the shop floor provides remote COM ports
via your local area network (LAN). There is no direct connection between the computer
and the Ethernet/Serial hub. Rather, the hub is connected anywhere on the LAN, and a software
driver installed on the computer causes the hub's remote COM ports to function as local
Windows COM ports. This means that Ethernet/Serial hubs are compatible with any software
that uses Windows COM ports, including Dostek DNC.
Quatech ThinQ® Serial Device Servers are pictured above. ThinQ device servers (hubs) are available with 1, 2, 4 or 8 ports with DE9 or RJ45 connectors. Dostek DNC also works with Comtrol DeviceMaster™, Digi PortServer™, Moxa NPort Server Pro™; and all other Ethernet/Serial hubs available with a virtual COM port driver.
Ethernet/Serial hubs should not be confused with the DNC hubs and Ethernet DNC adapters offered by some vendors. Ethernet/Serial hubs are general purpose devices that work with standard LAN hardware and standard software. There's no need to learn FTP, TCP/IP or Telnet to use Ethernet/Serial hubs. And general-purpose Ethernet/serial hubs provide CNC-to-Ethernet connectivity at lower cost than most special purpose terminals and devices.
An Ethernet/serial hub typically costs more per port than a multiple-port serial card, but provides greater flexibility and can reduce total installed cost by reducing RS-232 cable length and installation cost. There is no need to run numerous cables to a remote computer, and hubs can be installed where needed (several small hubs in different locations can be used instead of one large hub in a central location). Hubs can also simplify maintenance; a defective hub can be replaced without shutting down the computer, and a defective computer can be quickly replaced since there are no serial boards to remove and reinstall.
The Ethernet/serial hub provides an RJ45 network connector to connect to the LAN in the same way you would normally connect a computer. Typically, this means you will need one available port on an existing LAN hub and a standard category 5 cable up to 100 meters (328 feet) in length. See "Cables and Connectors" below for more information.
Using Wireless Communication
Several methods for wireless connection of CNCs are available, but products compatible with
the IEEE 802.11b standard (11 Mbps) or IEEE 802.11g standard (54 Mbps) are perhaps the most
popular and affordable. Both standards offer a typical indoor range of up to 150 feet, and
products are readily available from numerous vendors.
To build a wireless network, you can install a wireless device at each CNC, such as the Quatech ThinQ® Wireless Serial Device Server pictured above. Or, you can share one wireless connection among multiple CNC machines by connecting together a "wireless-to-Ethernet Bridge" and a multi-port Ethernet/Serial Hub. If you use a bridge, you may also connect computers or printers to the wireless connection.
A wireless system also requires one or more "access points" which provide the wireless connection to the LAN. Multiple access points may be used in different locations to provide a larger coverage area.
The wireless access points can also be used by other 802.11 compatible devices such as portable computers.
Wireless systems can be susceptible to unauthorized access, so be sure to choose products that offer encryption and other privacy features, and be sure to enable these features on your system.
Using USB/Serial Hubs
A USB/Serial hub is a relatively inexpensive way to add a small number of serial COM ports
to a USB-equipped computer. Because the USB/serial hub connects to the computer's USB port,
there is no need to open the computer to install an expansion card.
Many USB/Serial adapters and hubs are unsuitable for CNC communication.
The Quatech®
USB/serial hubs pictured here are designed with CNC communication in mind, and are currently
available in 1, 2, 4, 8 and 16-port models.
Using Dostek DNC With Multiple CNCs
When you use Dostek DNC with multiple CNC machine tools connected by a manual switch box, a user-defined message can be displayed to remind the operator which switch position to use.When you use multiple CNCs connected in any other way, a separate configuration file must be created for each CNC. This is because the COM port setting is different for each CNC (or, in the case of an electronic switch, the switch activation codes are different). When you send or receive a file using Dostek DNC Professional Edition or Lite Edition, you choose which CNC to connect by picking from a list. Or, Dostek DNC can automatically pick the CNC by matching it's name to the current folder. For more information about this feature, see the Professional Edition page.
Connecting Older NC/CNC Machines
Many older NC/CNC machines are not equipped with a serial communication port suitable for DNC. However, most machine controls equipped with an optical paper tape reader can be upgraded for DNC by installing the Dostek Model 440 Behind-the-Tape Reader (BTR) interface adapter. More information.
Cables and Connectors
Most CNC machine tools provide an RS-232 serial port to connect to the DNC system.
A standard "null modem" cable can be used to connect the computer to the
CNC, but these cables are not available "off-the-shelf" in lengths over 25 feet. You can
order custom-built cables from Dostek or you can assemble and install cables yourself
following the instructions provided in Dostek Application Note AN-07 titled "RS-232
Cable Installation" (available on the Documents page in the
Customer Download Area).
To build your own cables, Dostek recommends using 6-conductor 24 gauge stranded cable with a foil shield. Cable and solder-type connectors are available from Dostek.
The EIA RS-232C standard specifies a maximum cable length of 50 feet, but cable lengths of 200 feet or more are commonly used. Low-capacitance cable is recommended when using high serial data rates or long cables. A variety of options are available to extend distance up to several thousand feet, including RS-422 serial cards or adapters, short-haul modems, fiber-optic adapters or Ethernet/serial hubs.
Wiring With Category 5 Cable
Category 5 cable (widely used for 10/100-Base-T local area network wiring) can also be used to connect the computer to CNC serial ports. Standard RJ45 connectors can be installed on the Category 5 cable, and adapters can be used to connect these cables to the computer and CNC machines. Multi-port serial boards and Ethernet/serial hubs are available with RJ45 connector panels. Pre-wired and assembled RJ45/DE9 and RJ45/DB25 adpaters are available from Dostek.
Please note: The RJ45 connector on the back of your computer is an Ethernet local area network connector, not an RS-232 serial connector, so it can't be connected to a CNC RS-232 serial port.
Working With a Network
Using a local area network (LAN) provides several benefits. Storing files on a central "file server" simplifies file management (data backup, for example) and ensures that everyone uses the same up-to-date data. Dostek DNC can be configured to access files stored on the computer's own hard disk, on any Windows-compatible file server, or in any combination.Working With a Laptop/Notebook Computer
Dostek DNC runs on any Windows-based laptop or notebook computer, but needs an
RS232 serial COM port to communicate with the CNC machine tool. Although most older notebook
computers were equipped with at least one COM port, most newer models are not.
A COM port may be added to a notebook computer using a USB/Serial adapter or a
PCMCIA card (also called 'CardBus' or 'PC Card'). Single-port and multi-port cards and adapters
are available from Dostek.
Many USB/Serial adapters and hubs are unsuitable for CNC communication.
The Quatech®
USB/serial adapter pictured here (available from Dostek) is designed with CNC communication in mind.