Wu, Have they performed any second-order investigation concerning the loss of communication in multi-drop mode? It would seem that checking the HART FSK amplitude would be at the top of the list in terms of basic debug to see what might be causing the lapse in communication. Given the somewhat limited information we have right now, my guess is going to be that the root cause here is the frequency response of the input impedance of the system (across pins 1 and 2 of J1). FieldComm Group recommends an equivalent input impedance which is 100kOhm at DC with a 5nF capacitive roll-off. FCG makes this recommendation specifically to maintain signal integrity in multi-drop mode. My two recommendations for them would be: 1. Check the amplitude of the HART FSK signals as more devices are added to the bus, maybe check with 4, 8 and 15 nodes and see how it is decaying as more nodes are added 2. Measure the input impedance of the design across the J1 terminals We discuss the topic of receive impedance in our TI Design concerning HART. TIDA01504 on page 21.
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