Archive for category Modbus
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.