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?

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