Archive for category Communications
The HART communication protocol has been firmly established as the standard means of configuring field instruments for some years. But talking to a field instrument needs a communicator.
There are the handheld communicators, Rosemount’s x75s and the “budget-priced” Meriam MFC 4150, but at a cost that’s more a capital appropriation than an MRO expense. Even the Meriam, with a 3-year field device description subscription starts at more than $4000.
People continue to ask me if there isn’t a more budget conscious approach to HART configuration.
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.
A customer has several UDC 3200 loop controllers with newly added Ethernet cards. He needed to configure each of the controllers’ IP addresses using Honeywell Process Instrument Explorer (PIE) software. Because the controllers are working in a 24×7 continuous process, he was concerned about how making those changes would affect each controller’s performance.
So he asked me: Would a PIE action of uploading config files from or downloading them back to a controller affect the controller’s performance?
In the past, I’d only ever changed a controller’s IP address when it was on my workbench, not when it was actively controlling a process. So I’d never paid attention to whether PIE communications would affect the controller’s output or its PID action. Since I couldn’t answer the question, I told the customer I’d run a test to find out for sure.