Archive for category Modbus
Written by: A.J. Piskor
Traditionally, combustion control panels have been living in a hard-wired world. More often than not, the status of a burner system is communicated by indicator lights on a Flame Safeguard (FSG) terminal.
With the increased use of Programmable Logic Controllers (PLCs) controlling all aspects of a combustion system, customers are demanding more information from the FSG, as many operators manage their systems from a centralized control room.
While some customers have traded in their indicator lights for relays with dry contacts that feed back into their PLC, other customers are looking to simplify the communication between the FSG and PLC, while extracting more information on the operation of their burner system.
Here, we will go over some examples of technologies that are available today, and how you can get the information you need to where you need it.
More and more people are using Modbus to get data from their instruments and controllers back into their control systems for reporting, alarming and troubleshooting.
And while I can’t be there to help you set up your Modbus master, I can give you 13 rules and some general practice advice for communicating to any Modbus RTU device.
RTDs are great temperature sensors – accurate and easy to install. But they are not friendly when it comes to trying to get a single RTD to go to two places, like when an RTD temperature measurement has to go to both a controller and a recorder. People call and ask, “How do I split an RTD signal?” The short answer is, “You can’t.”
An RTD cannot be wired in parallel or in series to a second device. Any RTD input supplies a known, regulated ‘excitation’ current to the RTD. Mixing RTD inputs would mix currents and that’s a Big No-No.
There’s also a lead wire compensation circuit for 3- or 4-wire RTDs that would create problems if a single RTD were connected to two different RTD inputs. There’s just no feasible means of making two RTD analog inputs play nice together.
But all is not lost. There are several ways to achieve your goal.