Q

What is required to connect a field device to our control room?

We would like to implement a wireless Ethernet communication between a device in the field and a desktop computer in our plant control room. This is a point-to-point link that would eliminate the large amount of work involved in laying cables, etc. A link like this has extensive applications in the field of real time instrumentation and controls. The distance between the field device and the control room is 1.5 Km (1 mile.)

What devices would be required to implement this link and what are the precautions to be taken into consideration.

I can never stress enough the value of a good sight survey. Whether it's for a point-point bridge or a high-density LAN such in a corporate or educational organization, the effectiveness of your wireless link is dependant on your choice of equipment, and a good site survey will identify the exact equipment that should be used to achieve your business goals.

For 1.5 Km (1 mile) between the device in the plant room and a stationary link you are probably looking at a wireless bridge. Essentially the two computers remain cabled using standard category 5e cabling and the Cat5e connects to either end of the wireless bridge. For all intents and purposes the link acts exactly like a 1.5 Km cable except for the wonderful fact that its wireless.

In a city environment you would have to worry about constructive or destructive interference from other wireless LANs or multi-path interference within an enclosed area but for something like an open field environment that you're describing it should be pretty simple. However, saying that, I haven't seen the environment and there can be a lot of hidden snags when configuring a point-point (or point-multipoint bridge over more than a couple hundred meters.)

My advice, get a site survey from a local wireless authority.

This was first published in June 2003

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