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?
As it turns out, it’s not a system fault or failure to perform at all. It’s the way Honeywell designed the modules.
For regulatory approval reasons, the Modbus remote reset function is disabled on all S7800 display modules. While digital communication remote reset is allowed by code in the United States, it’s not allowed in other parts of the world. To get worldwide agency approvals on the S7800 display module, the remote reset function has been disabled by default.
The solution to get Modbus remote reset is to put an S7810M1030 Modbus module directly on the RM7838C control. But that makes the device blind, because the S7810M Modbus module has no digital display:
If the digital display is needed, the S7800A1142 keyboard display module is wired to communicate with the S7810M Modbus module as shown in the wiring diagram below, where terminals 1, 2, and 3 use the Honeywell protocol. Note that the S7800 display module’s address must be set to 00 for it to communicate on the Honeywell network.
A single S7800 keyboard display can be powered from the S7801M Modbus module’s Pin 4 at distances up to about 10 feet. But, connecting multiple devices on the Honeywell protocol network requires the use of a 12 VDC power supply. (The diagram shows a 13 VDC supply, but a 12 VDC unit has been proven to work fine.)