We have 2 flow meters on an equipment installation that are HART-capable. As they are installed today, configuring these devices in multi-drop mode would be much easier than running additional wire for a point-to-point configuration that seems to be more common in the HART realm of things. One requirement we have is to pull 4 parameters of information (PV/SP/TV/QV) from each meter, and I am combing through the protocol to try and understand 2 significant questions:

1.) Can PV/SP/TV/QV still be pulled if the device/network is in a multi-drop configuration?

2.) Does the 1200 bps / "roughly 2 parameters per second" apply here? Should I expect the real-time update of 4 values from a meter to take about 2 seconds? And if I have 2 meters, totaling 8 values, would the full sweep be expected to take about 4 seconds?

Thanks!

 

1. Yes, your HART master would need to send commands to the two device separately (initially based on their Poll Address values which must be different from each other).

2. Yes, your logic makes sense. If the HART master is capable of using Universal Command 3, that returns PV/SP/TV/QV in a single response. I would expect that using this command, both transmitters could be read in about 2 seconds.

 

I concur with Chris T

 

Thank you for clearing that up. I got the feeling this was the case but wasn’t 100% with the verbiage I’d found. I appreciate the fast responses. Will post results as soon as I have them.

 

Regarding your inquiry about configuring the flow meters in multi-drop mode, it is indeed a viable option for easier installation. As for your questions: 1.) In a multi-drop configuration, it is possible to pull PV/SP/TV/QV parameters from each meter. 2.) The 1200 bps rate typically allows for approximately 2 parameters per second. Therefore, retrieving real-time updates for 4 values from a meter may take around 2 seconds, and with 2 meters totaling 8 values, the full sweep could be expected to take approximately 4 seconds.