Fieldservers and protocol converters

A FieldServer is a type of communications bridge or gateway that is designed to either convert one protocol to another as the messages are sent or map tags from different devices and handle communications.
By Frank Lamb, Automation Primer March 5, 2016

While programmable logic controllers (PLCs) are common in the industrial world, when it comes to controlling buildings and campuses in the commercial space the landscape looks a bit different. Instead of production machinery, the equipment and facilities involve environmental controls; boiler rooms, chilling plants, lighting controllers, security systems etc. HVAC systems, computer uninterruptible power supply (UPS) systems and gas monitoring are also tied into the network.

There are many different manufacturers and vendors in this space that may use different protocols for communications between systems and devices. Some may be the same as those in industry; Profibus and Modbus are common. Others, such as BACnet, are specific to the building automation and control world.

Back in the 1990s I did a few projects where data had to be exchanged and converted. It involved working with things like Allen-Bradley’s SLC based BASIC module and writing computer code on a dedicated PC. This was very time-consuming and expensive. Now, the cost and time involved has been reduced significantly due to devices like FieldServers. 

A FieldServer is a type of communications bridge or gateway. Some are designed to simply convert one protocol to another as the messages are sent. Others actually map tags from different devices to one another and handle communications in both directions. This involves onboard memory and some configuration.

A cool thing about this device is that it uses three different sets of registers; the server side mapped to the PLC and specific to it’s names and structures, an intermediate internal register which can be manipulated by doing math or swapping bytes with each tag or group, and the client side mapped via BACnet to the building control system. This allows the naming conventions and structures to be dictated by both parties involved.

The FieldServer can only read "flat" tags, which means that the tags all have to be mapped from their structured form into basic BOOLs, DINTs and REALs. While this is pretty easy, tags still have to be entered manually into the code by browsing for their location in the software. They also have to be typed into the FieldServer configuration file. That means a lot of copy and paste from Excel.

Frank Lamb is the founder of Automation Consulting Services Inc. This article originally appeared on the Automation Primer blog. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

ONLINE extra

For more articles on PLCs and programming, see related articles below.