RS485-Link clarification

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Hi,

I wanted to know , when should one opt for an IsolatedRS485 link.

1. If the bus require 3rd party retrofit?.

2. Noise problems?

I intend to power 24VDC from the same wire to all my four nodes . Since all the Vcc and the Gnd are common what help would an galvanically isolated link would provide. Or I just doesnt require one. The application is automotive ans speed is slow 56Kbps or lower .

Thanks