Profibus-PA was designed as a substitute for HART and 4 to 20mA signal transmission in the process industries. It uses function blocks designed around process industry requirements and uses the IEC 61158-2 physical layer, making it compatible with intrinsic safety circuits.
Profibus-PA links to the control architecture via Profibus-DP with a segment coupler or link as shown. Segment couplers are signal converters that adapt the RS-485 signals to the 61158-2 signal level. They are transparent from the bus protocol point of view. If segment couplers are used, the baud rate on the DP (RS-485) segment must be restricted to 45 Kbits/sec. The segment coupler also injects power into the PA network for the segment instrumentation.