Posts Tagged 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.
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?
I just got my hands on a cellular modem that can stream analog data from a remote site to a SCADA/HMI/PLC/DCS/PAC, send a text message or an email on alarm, or receive a text message that can control a relay output (yes, it’s all secure ! !).