The ISO1211 and ISO1212 devices are fully-integrated, isolated digital-input receivers with IEC 61131-2 Type 1, 2, and 3 characteristics. The devices receive 24-V to 60-V digital-input signals and provide isolated digital outputs. No field-side power supply is required. An external resistor, RSENSE, on the input-signal path precisely sets the limit for the current drawn from the field input based on an internal feedback loop. The voltage transition thresholds are compliant with Type 1, 2, and 3 and can be increased further using an external resistor, RTHR. For more information on selecting the RSENSE and RTHR resistor values, see the Detailed Design Procedure section. The ISO121x devices use an ON-OFF keying (OOK) modulation scheme to transmit the digital data across a silicon-dioxide based isolation barrier. The transmitter sends a high frequency carrier across the barrier to represent one digital state and sends no signal to represent the other digital state. The receiver demodulates the signal after advanced signal conditioning and produces the output through a buffer stage. The conceptual block diagram of the ISO121x device is shown in the Functional Block Diagram section.