Isolation barrier and CTR margin for a failing LED
The FOD2741BS: That 5kV rating means it holds off transient spikes common in motor-drive or PFC stages without tracking across the creepage gap — the 8-SMD Gull Wing package gives you the physical separation to meet reinforced insulation requirements in a single device. Sizing the input resistor to drive the LED at 10mA and expecting at least a 1:1 output current gives you margin before the board comes back with a dead channel.
Output drive and temperature range for industrial control
The output transistor can sink up to 50mA continuous per channel with a Vce saturation of 400mV max at rated current — enough to drive a small relay coil or a PLC input module directly. The 30V output max means it sits comfortably on a 24V industrial rail without an extra clamp. If your board lives near a heat sink or in a sealed enclosure that soaks above 85°C ambient, this part is not the right call — you need a wider-temp or high-CTR variant.