HART communication

The limitation is a HART 5 limitation, according to the HART Foundation's HART Communication Application Guide, my copy dated March 2010.

"The multidrop mode of operation requires only a single pair of wires and, if applicable, safety barriers and an auxiliary power supply for up to 15 field devices (HART 5) or 62 field devices (HART 7)"

The limit might be due to legacy limitations of memory (HART goes back a ways) and backwards compatability, or it could be due to perceived intrinsic safety issues with the required electrical current to run a multidrop network.

"MULTIDROP IS NETWORKS
HART multidrop networks are particularly suitable for intrinsically safe installations. With a multidrop configuration, fewer barriers or isolators are required. In addition, because each field device takes only 4mA (for a total of 16mA in a four‐device loop), plain zener barriers can be used. With a 250 Ω load, 25 V – (340 + 250 Ω × 16mA = 15.5 V, which is well above the transmitter lift‐off voltage and leaves a margin for cable resistance."
 
Thanks for your answer. I'm still confused abt one thing, if 62 devices can be connected as per Hart 7 then how the intrinsic safety issue is addressed.
 
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