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.
We’ve gotten feedback from several people who have asked what I carry around to deal with serial or Ethernet communications issues. So I dumped out my comm tool bag and here’s the list of all the stuff.
This one qualifies as a “Page the manufacturer left out of the manual”, and was brought to my attention by a customer who was having problems with remote reset on his flame safeguard system.
A typical multi-burner furnace has Honeywell 7800 flame safety controls on each burner as shown below. Modbus is used to fetch fault codes for the plant’s HMI system, and to allow the control system to remotely reset the flame safety controls.
The reported problem was that Modbus did everything as advertised except remote reset on the one control that had a S7800A keyboard display module attached (colored orange in the network image on the right, or as shown in the controller image below).
That unit would only do a reset when an operator pressed the reset button on the RM7838C controller. Modbus failed to get the controller to execute a remote reset.
The problem wasn’t communications – all Modbus read functions worked and Modbus writes to all the other controls worked as expected. But somehow, the write command to do a reset action was ignored by the S7800 keyboard display.
So, what’s causing the problem?
When you’re dealing with process recorder functions, some actions don’t like negative values, like a totalizer that sees a negative offset at zero flow as backflow, which impacts the total. But on the Honeywell X-Series paperless recorders, you can work around the issue, so they report only the positive values.