Field-to-controller input circuits · IEC 61131-2 Type 1/2/3 · current limit, thresholds, debounce, isolation
Between a 24 V proximity switch and the microcontroller or DSP that reads it sits a receiver circuit that converts the field signal into a logic level. Integrated receivers replace a resistor-and-optocoupler front end with a regulated input current, defined switching thresholds, a selectable debounce filter and, in isolated versions, a barrier between field and logic. Its current setting fixes which sensor types the design accepts and the heat each channel adds; its filter fixes the shortest pulse that gets through.
Integrated digital input receiver and serializer ICs for 24 V industrial inputs, isolated or used with separate isolators, configured to IEC 61131-2 Type 1, 2 or 3. Complete digital input cards and terminals are covered on the Digital input module page; 24 V output switch ICs on the Output driver circuits page; compact-PLC expansion modules on the I/O expansion module page. Discrete optocouplers are covered on the Optocoupler / isolator page and DIN-rail opto boards on the DIN-rail optocoupler isolation board page; RS-485 line drivers are outside this page.
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01
The receiver regulates how much current a closed sensor draws, which is what separates IEC 61131-2 Type 1 and 3 from Type 2 inputs.
Editorial calculation: at 24 V an ISO1212 input at its 2.47 mA maximum draws 24 × 2.47 = 59 mW from the field circuit, against 24 × 7 = 168 mW at the 7 mA maximum of the 200 Ω setting. Sixteen such channels come to about 0.95 W and 2.7 W respectively. Assumes all inputs on at 24 V; part of the power is dropped in the external resistor rather than the IC. [1]
TI states the ISO121x needs no field-side supply because it draws power from the input pins, and that it tolerates ±60 V with reverse-polarity protection. [1]
| Device | Current setting | Published value |
|---|---|---|
| TI ISO1212 | RSENSE = 562 Ω, 24 V, −40 to 125 °C | 2.2 mA min. to 2.47 mA max. across temperature, given on page 1 as the Type 3 current limit; adjustable up to 6.5 mA. [1] |
| TI ISO1212 | RSENSE = 200 Ω, VIL < VSENSE < 36 V | 5.5 mA min., 7 mA max. [1] |
| TI SN65HVS882 | RLIM = 25 kΩ | 3.15 mA min., 3.6 mA (typ.), 4 mA max.; range 0.2 to 5.2 mA by resistor. [2] |
| Infineon ISO1I811T | TS resistor selects Type 1/2/3 | Type 1 and 3 on all 8 channels; Type 2 by paralleling two inputs, leaving 4 channels. [3] |
| Beckhoff EL1008 terminal | EN 61131-2 type 1/3 | Input current 3 mA (typ.); signal "0" −3 to +5 V, signal "1" 11 to 30 V. [4] |
02
With RSENSE = 562 Ω and no threshold resistor, TI gives the ISO1212 module-input low threshold as 6.5 V min., 7 V (typ.) and the high threshold as 8.2 V (typ.), 8.55 V max., with 1 V min., 1.2 V (typ.) hysteresis. [1]
Adding RTHR raises both thresholds: with 4 kΩ the high threshold becomes 17 V (typ.), 18.25 V max. and the low threshold 15.2 V min., 15.8 V (typ.). TI also recommends RTHR = 1 kΩ with a 10 nF input capacitor for surge, ESD and EFT robustness. [1]
The SN65HVS882 limits current at its IP pins, where the high-level threshold is 5.2 V (typ.) and 5.5 V max. with RLIM = 25 kΩ; the module-terminal threshold depends on the series input resistor RIN, for which the datasheet uses 1.2 kΩ ± 5%. [2]
03
Contact bounce needs milliseconds of filtering; encoder or fast sensor pulses need microseconds. The three ICs offer fixed choices.
Infineon's ISO1I811T datasheet gives two scan-frequency references: its text gives the Table 10 filter times "for the typical frequency of 125 KHz", while the table footnote states they are valid for 100 kHz. Confirm the filter time at the ROSC value used. [3]
| Device | Filter options | Fast path |
|---|---|---|
| TI SN65HVS882 | 0 ms (bypassed), 1 ms or 3 ms via DB0/DB1. [2] | Max. data rate corresponds to 0 ms debounce. [2] |
| Infineon ISO1I811T | 1.0, 3.2 or 10.0 ms averaging; bypass 10 µs. [3] | Filter times shift with the ROSC-set scan frequency (50–150 kHz). [3] |
| TI ISO1212 | No digital filter; external CIN. | 150 ns minimum pulse width; 110 ns (typ.) low-to-high delay. [1] |
| Beckhoff EL1008 | 3 ms (typ.) input filter. [4] | — |
04
Isolated receivers carry the barrier inside the package; a serializer leaves it to external isolators but reduces how many isolation channels are needed.
| Device | Isolation | Logic interface |
|---|---|---|
| TI ISO1212 | UL 1577 2,500 VRMS; CMTI 25 kV/µs min., 70 kV/µs (typ.). [1] | Two parallel outputs, 2.25–5.5 V logic supply. [1] |
| Infineon ISO1I811T | 500 VAC coreless-transformer isolation (EN 60664-1, UL 1577). [3] | 3.3/5 V SPI or 8-bit parallel. [3] |
| TI SN65HVS882 | None in the IC; 5 V regulator output for external isolators. [2] | SPI-compatible shift register, cascadable in multiples of eight. [2] |
| Beckhoff EL1008 | 500 V E-bus to field. [4] | EtherCAT terminal. [4] |
05
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Sensors: [type; IEC 61131-2 Type 1/2/3] Voltage: [nominal; max.; reverse polarity] Current: [per channel; channels on together] Timing: [debounce; shortest pulse] Isolation: [channel-to-logic; channel-to-channel] Interface: [parallel; SPI; supply voltage]
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ISO1212 can be set to Type 2 current with RSENSE; Infineon's ISO1I811T needs two inputs paralleled, halving channels to four. [1] [3]
The limit fixes the on-state current: ISO1212 holds 2.2–2.47 mA at 24 V for Type 3, cutting heat per channel. [1]
Sources checked 2026-10-17. Receiver limits come from two TI datasheets and one Infineon datasheet. Beckhoff's EL1008 terminal is included as a finished-product reference for 3 mA type 3 inputs with a 3 ms filter.
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