A mechanical pressure switch is a diaphragm over a snap-action microswitch: cheap, zero-power, but the set point drifts as the diaphragm fatigues, the contacts arc, and the only way to see the pressure is to bolt on a separate gauge. The DS 351 collapses that into one device - a ceramic cell, two independently adjustable switch points, and a 4-digit LED that reads the live pressure.

What does IO-Link buy over a pair of PNP outputs on a smart switch?

Diagnostics, not just switching. IO-Link V1.1 at 38.4 kbit/s delivers process data, temperature, switch counter and alarm history back to the PLC, so a pump that has short-cycled 10 million times shows up as a maintenance warning before it fails. Two channels cover most machines: CH1 can be IO-Link or a PNP/NPN contact, and CH2 switches between 4-20 mA, 0-10 V and a second PNP/NPN. The ceramic cell reads from vacuum to 600 bar in one part number, the 316L housing survives washdown to IP67, and the optional PVDF port opens the door to aggressive media - acids, chlorides, and process fluids where a stainless wetted part would not last.