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Simple Wire Replacement - Tips and Fixes

12/11/2018

– WITH ZLINX™ RADIO MODEMS

Introduction 

A problem free wireless network installation is more than a wish – it is what most B+B SmartWorx customers experience. 

But, a little up front knowledge and planning makes succeeding on your first try more likely. B+B went to great lengths to make it simple enough to take your wireless modems out of the box, plug in power and data lines and, voilà, your network is wireless. 

There are few things more satisfying than seeing data roll in just like you’d run great distances of cable, yet quicker, easier, cheaper – and still reliable. But it is still possible you may run into interference. So, here is a short list of things to make your “out of the box” experience a good one. 

Environment 

The first thing to check is the environment where you want to install your radio modems. Is it outdoors or indoors? Will you send data “line-of-sight” or are there obstructions? What distance do you need to go? Are there wireless antennas on the same band nearby? 

Once these are established, getting the right equipment is simple. None of these factors are barriers to a successful system. But, they will determine the best placement for your radio modems, your antennas and whether you may need one or more repeaters. 

Latency 

The majority of industrial applications function completely well with wireless instead of hard wired links. But, with the convenience and flexibility of wireless, comes some tradeoffs. 

Latency or delay between sending and receiving data, is greater with wireless. Most networks handle a little added latency just fine. B+B has never seen a situation that was not solved by fine tuning response times. 

For example, a Modbus network may poll PLCs every 10ms, or have devices set to receive a response within 10ms. Even if it is just sending 200 bytes on a 115,200 baud connection, you are still not going to get it in 10ms on a wireless network. A simple change in polling rates and timeout timers can handle this with no impact to operations. 

Distance & Modem Selection 

One of the B+B SmartWorx “claims-to-fame” is a wide variety of radio modems. Long range, short range, 900 MHz and 2.4 GHz. 1mW to 1W outputs. 2.4 MHz works almost anywhere in the world; 900 MHz North America and South America. 

Now, you would think, with this wide variety to choose from, you could not go wrong. It turns out there are still a few technicians, who manage to get it wrong by not understanding the need for a wireless “performance buffer.” 

Way back in wireless pre-history, someone told an engineer, “Tell me the farthest mountain tops these things will transmit between.” So, he did some calculations, and the next thing he knew it was on every printed page and web page: “Maximum range of this fine equipment is 64km (40mi).” 

The true functional maximum range of any piece of wireless equipment, compared to the often advertised maximum range, is half that distance. An industrial wireless installation needs a 10:1 signal strength “performance buffer,” or a full 10 dBm signal strength buffer. Cutting the maximum possible distance in half makes up for most of that needed buffer. 

Do you really need 10X more power transmitted than a bare minimum, just as a buffer? Well, yes. When materials, machines, money and people are at stake, it is the safe, secure and sure thing to do. 

So, when our technical specifications state the maximum range with a directional antenna is 32km (20mi), you can count on your equipment to be capable of that range. But, keep in mind, there are still many factors that this performance will depend on.  

Other things being equal, it is better to go with 900 MHz modems instead of 2.4 GHz ones. The physics of RF make it so the 900 MHz frequency goes farther, penetrates nonmetal walls better, and has greater receive sensitivity. It is also less crowded with interference. 

But, if you have a really loaded 900 MHz wireless network nearby, you may want to choose the 2.4 GHz instead. 

Setting Up The Equipment 

When your new modems arrive, there is a natural inclination to just get them working and see what they will do. But, if the first thing you do is send them out as far as they need to be, plug them in and turn them on – and nothing happens – it is difficult to tell what to fix.

It is best to first conduct a round of bench testing. Either a loop back test or with equipment connected is good. If they do not work here, they will not work in the field. 

Other set-up issues range from non-matching channel numbers to RS-485 biasing termination. B+B SmartWorx quick start guides and manuals cover most everything. If you still have questions, call B+B SmartWorx Technical Support. 

Antennas 

Antennas are not difficult but, they can cause a lot of problems if you do not understand basics. If the signal does not get to the other end, your “wire” is, effectively “cut.” 

If there is a single best fix for antenna problems, it is to put the antenna higher. Sometimes, it may be the only option (for instance, to clear above a large grove of trees). Yet, naturally there are limits, like neighbors and airplanes. But, just a few added meters of height can make a big difference if you are competing with a nearby antenna on the same frequency. 

There are two basic antenna formats used for radio modems. An omni-directional antenna (“dipole” or “stick”) has shorter range but broadcasts 360°. “Omni” does not mean “all” in this case, as they do not broadcast or receive straight out from the tip. 

The directional (“high gain”) antenna is most often a “Yagi” multi-element. These have three or four (or more) dipoles lined up in parallel, getting progressively shorter toward the front. They direct the signal so it works in one direction, for both transmitting and receiving. 

For example, in a typical Modbus system, the Modbus master uses an omni-directional (or dipole) antenna that communicates with many devices. The slave devices, on the other hand, would most often be directional (or Yagi) antennas, as they only need to reach back to the master. 

Polarization refers to the horizontal vs. vertical orientation of the antenna, which is also the orientation of the radio signal that is sent or received. Both antennas must have identical polarization. 

This leads to interesting tricks with wireless setups. Four radio modems can share the same pathway, with two radios oriented horizontally and the other two vertically. This applies to directional Yagis, as well as omni-directional dipoles. 

Aiming a directional antenna is easy if you can see the other antenna. But, if it is too far to see, using a map or GPS and compass to point a more precise direction may be necessary to even start the process. 

An RF site survey can be done up front to establish signal strength patterns in challenging settings. Changing modem placement can make a big difference in signal strength around high-power metal equipment. Some devices even have signal strength meters to help with this. 

Cables 

The shorter, the better. Fewer connections are better. And, the longer the distance between your modem and antenna, the more you need cable that loses the least amount of signal strength possible along the way. 

Of course, antenna grade cable is more expensive than regular coaxial, but it is well worth it. 

Baud Rate & Receive Sensitivity

 Baud rate, or data rate, is another trade off. Many core industrial applications do not require much bandwidth, so this does not have to be a problem. 

However, baud rate can be more important as it affects the distance your wireless signal travels. As baud rate goes down, receive sensitivity goes up. And, receive sensitivity has as much to do with how far a wireless signal can be received as does transmit power. 

Zlinx LR (Long Range) radio modems are adjustable between 115,200 bps and 9,600 bps to maximize performance in longer distance situations. 

If the network is big enough, over-the-air data rate can also be an issue. If this is the case, such measures as fine tuning polling frequency and such are also effective here. 

A Wireless Disaster & Solution 

When all else fails, bring your modems back to your shop, reset all settings and options back to default, and do a bench test. A little parable shows how understanding a few basics can help after that: 

A customer called B+B SmartWorx with no signal between his two modems. After a bench test showed the modems were OK, a B+B technician reviewed one issue at a time. Come to find out, his dipole antennas were pointed antenna tip to antenna tip. No amount of signal strength could overcome that. 

The cable between his base station modem and the antenna was an incredible length. With 15 different connectors and many kinds of different cables linked together (most of it wasn’t antenna cable either). Once again, no wonder there was no signal.



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