Wired and Wireless Systems
Selecting the right communication technology can simplify life on the shop floor
By Jim Lorincz
There are a number of ways to connect machine tools to the network. They range from low to high, A to Z, inexpensive to expensive, simple to complex. Richard Hefner, chief network engineer of Refresh Your Memory Inc. (Santa Clara, CA) reveals the technical nuances and cost implications attached to each choice.
Manufacturing Engineering: What are the shop's options for connecting its CNC machine tools to its computers?
Richard Hefner: There are three different types of boxes commonly used to connect traditional RS232 CNCs. They all perform the same function–they provide a port which allows your computer to talk to a CNC. The difference is how they connect to your computer and where the box is placed.
- The first type of box is a USB-to-Serial adapter. This box plugs directly into the back of your computer and must be located within 15' (4.6 m) of your PC. The box may have anywhere from one to sixteen, or even more, ports to plug CNCs into. If you only have one CNC and your computer already has a serial port on the back, you may not even need the box. You would then run RS232 cables between your computer and the CNC. Generally people choose these boxes because they are the least expensive and easiest to set up.
- The second type of box is an Ethernet-to-Serial adapter. These look similar to the USB-to-Serial adapters, except that they would plug into your network instead of the USB port on the back of your PC. This gives you added flexibility in their placement. If you purchase a box that has eight ports and place it in the middle of your shop, you would have eight shorter RS232 cables run into the box and one long Ethernet cable run back to your network. This type of box is ideal if the computer running your DNC software is not located centrally to the CNCs it will control. In fact, the computer controlling that hub can be anywhere on your network—even in another building! Multiple hubs can be located throughout your shop to control different groups of CNCs, and generally these hubs are just as reliable as running a cable all the way back to your computer. You would probably choose this box if you had CNCs scattered across a large area or if you want to locate the computer that runs the system somewhere off the shop floor. The downside: Installing the devices will take at least a basic knowledge of networking, and the boxes cost slightly more than the USB-to-Serial adapters.
- The third type of box is what we call a Wireless-to-Serial adapter. This box mounts directly onto your CNC, and talks to your computer system through a standard 802.11b/g network (often called WiFi). You will need to mount wireless access points in your ceiling to provide a wireless signal to your shop, and if your shop is large enough you may need to mount multiple access points in different areas. Each of these access points will have an Ethernet line which runs back to your network. There is still a serial connection between the wireless adapter and your CNC. Contrary to what some vendors might imply, your CNC will have no drastically new functionality as a result of adding a wireless adapter because, in the end, it is still connected via the CNC's serial port. Wireless adapters are great for situations where running a wire is impractical such as if there are overhead cranes or the controller on the CNC moves during operation, which is a common situation on gantry-style CNCs. Also, some shops will install wireless in situations where the CNC machines are constantly moved. There are drawbacks, however. The first of which is price. The cost of these individual wireless boxes makes it substantially more expensive to run wirelessly than to run wires back to one of the traditional wired boxes. Also, if you need to drip-feed your CNC, that is, run a program directly from the computer when it will not fit in memory, you should use a wired box to make sure your program execution isn't interrupted in the event of a wireless network hiccup. While transferring into memory, it will not matter significantly if the transmission pauses for several seconds in the event of a problem, but when drip-feeding, this can leave a mark on your part. Lastly, the level of complexity is such that you will want someone who is savvy in both wired and wireless networks to help you sort through IP addresses and encryption settings. Keep in mind that if you have any future problems, it will take an experienced network technician to help you troubleshoot a wireless problem.
ME: What kind of software is needed?
Hefner: DNC software is the core component in a manufacturing system. It is not only responsible for the transfer of part programs, but additional modules can handle functions such as machine cycle monitoring, paging and e-mail alerts, in-process gaging integration, and paperless manufacturing. Regardless of which hardware solution you choose, you will need software to control it. There are a variety of different hardware and software vendors available. It's important that you find an open-architecture system. If you get a proprietary system today and the software vendor goes out of business, you may have to junk the whole thing the next time you get a new version of Windows. Or should a piece of hardware fail, you want to be able to plug in a similar product from another vendor if necessary.
ME: Isn't wireless the only good option when machines are moved around a lot?
Hefner: Wireless can be a good option, but it isn't the only option. If you are willing to spend the money for the wireless equipment and have the patience to sort through the added complexity of a wireless system, and you do not drip feed—this can't be emphasized enough—wire-less can be a good choice. It happens to be the most expensive method and the most complicated method, and some vendors push wireless based more on their bottom line than on the customer's needs. Actually, you can get most of the benefits of wireless using a one-port wired Ethernet-to-Serial adapter. This box will mount on your CNC and connect to the nearest network jack. If you have network jacks throughout your shop, each time you move the CNC just plug it into a new jack and it is up and running again, just like when you move a PC in your office. In this scenario, your network lines are not dedicated to a particular machine. They are just part of your infrastructure, the same as electrical and air lines. The benefits over wireless are: the boxes will cost less, they will be easier to configure and troubleshoot, and you will be able to drip-feed if you need to.
ME: How susceptible to interference is a wireless system?
Hefner: Interference can be a problem. That is not to say it will be a problem in your particular shop, but the truth is that nobody can say if interference will be a problem until your individual environment is tested. All 802.11b/g networks operate at an unlicensed frequency near 2.4GHz. This means that, within certain boundaries, anyone can create a device to use this spectrum. You may find that other wireless networks, wireless cameras, portable phones, and all sorts of other devices may be trying to use these frequencies. Also, the frequencies used here are similar to those used in a microwave oven, and users have experienced problems if devices are located too close to the break room. Probably more of a concern, though, is the uncontrolled noise out in your shop. Equipment like EDM machines can emit all sorts of electromagnetic interference that wreaks havoc with wireless devices. You should plan to thoroughly test any wireless system in the noisiest parts of your shop before rolling out the entire system.
ME: What is the difference between the two standards 802.11b and g that you refer to?
Hefner: Standards 802.11b and 802.11g are different versions of the standard for WiFi networks. You should not be too concerned with which flavor your DNC system uses. All 802.11g devices and access points are backwards-compatible with older 802.11b devices. The primary difference is speed; 802.11b is more than fast enough to handle any file transfer through your CNC's serial port.
ME: To avoid problems with cables, wouldn't it be best to just got rid of them?
Hefner: Most cable problems are the result of using the wrong cable or having it improperly terminated. A good cable will be good for decades of service in your shop when it is run out of harm's way. A common problem is that, advised by a network person, a shop will run CAT5 network cable where they really should be running a good RS232 cable. We recommend using a 24AWG, super low capacitance, shielded RS232 cable. It should have five or more lines and tinned strandedcopper conductors with either a braided or foil shield and drain wire. If you can't find it locally, turn to your DNC system suppliers. They should carry the correct type in bulk spools. When running the correct shielded cable, you should be able to achieve RS232 cable lengths of up to 275' (83.8 m) without problems. If that IT guy persists, remind him that running CAT5 network cable for an RS232 application is like using telco-wire to wire your network. As for the terminations, you want to make sure they are as solid as possible, with no chance of broken connections or shorting out. Avoid excess connections, such as having a cable with an RJ-45 style network connector on each end, and then using an adapter to turn it into the proper DB9 or DB25 connector. A good DNC cable will have the right connector on each end from the start, and you should not need any null modem or other adapters added on. If you aren't an expert at crimping or soldering a good connection, a professional can come into your shop and terminate cables you have already strung yourself. This can be a good cost- effective alternative to having a professional run the whole thing.
ME: What should I look for in a wired USB or Ethernet adapter?
Hefner: Your box should be open-architecture and backed by a good factory warranty. Open-architecture boxes will appear on your computer just like a standard COM port and be usable by any serial application. As for the warranty, replacements should be shipped to you immediately without having to wait for your existing box to go back and be repaired. Warranty periods of up to five years are increasingly common. A good box should also have the option of coming with surge suppression built in. Status lights can be helpful in telling you if a port is operating normally or not. For DNC applications, low latency is critical to ensure reliable communications. Look for boxes with adjustable buffer sizes or low-latency modes if possible.
ME: And what about a wireless adapter?
Hefner: All of the above, plus an 802.11b/g radio with encryption. If the box doesn't use 802.11b or g, it is not a standard wireless architecture. You may be locked into using the manufacturer's own access point equipment to talk with the box, and it won't talk to any existing network you have. As for encryption, all units should have at least 128-bit WEP encryption. On today's units, WPA encryption is more powerful and is becoming increasingly common. Make sure your wireless unit isn't really a wired unit with a separate bridge inside the box. These units are sometimes identified by a jumper cable that needs to be installed to operate in wireless mode. Such a box will generally take up twice as many addresses on your network and require two separate configuration routines for the wireless portion and the serial adapter. Historically they have been less reliable, because the serial adapter has no knowledge of or control over the wireless radio.
ME: Is an adapter needed for CNCs that have Ethernet built-in?
Hefner: You need a device called a wireless bridge. Because you don't need the additional logic required to provide an RS232 port on the machine, you would just install an off-the-shelf wired-to wireless bridge on your CNC. The wireless communication is just an extension of the wired Ethernet protocol, so all you need is this device to bridge the two together. Wireless bridges are generally less expensive than the wireless-to-serial devices we have been talking about.
This article was first published in the January 2007 edition of Manufacturing Engineering magazine.