The Allen-Bradley 1756-IF8IH, also cataloged as the 1756-IF8IH HART Isolated Analog Input Module, converts eight galvanically isolated 4-20 mA or 0-20 mA field loops into ControlLogix producer-consumer tags over the backplane .

Model Family And Suffix Logic

The base catalog 1756-IF8IH splits as follows: "1756" = ControlLogix I/O platform; "IF" = analog input; "8" = eight channels; "I" = individually isolated (channel-to-channel, channel-to-backplane, channel-to-frame ground); "H" = HART modem present on every channel. It is not a voltage-input device: 1756-IF8I carries voltage ranges, while 1756-IF8IH is current-only with HART overlay. No user-selectable voltage range exists on 1756-IF8IH regardless of Studio 5000 configuration .

Hardware Specifications

Backplane current figures for 1756-IF8IH specifically are inconsistently republished; a directly attributed source cites 210 mA @ 5.1 VDC and 110 mA @ 24 VDC . Where a project margin is tight, confirm the exact value on the module label or 1756-TDxxx page before sizing a 1756-PA75 / PB75 supply.

ControlLogix Backplane, EtherNet/IP Ownership And Firmware Match

The 1756-IF8IH occupies one single-width slot in a 1756 chassis and transfers sampled 32-bit floating-point channel data to the owning controller through the ControlLogix backplane using scheduled or rack-optimized I/O connections. When the module sits in a remote chassis behind an EtherNet/IP or ControlNet bridge, HART pass-through is still handled as CIP-unconnected or explicit messaging from Studio 5000 Logix Designer, not as a transparent HART master on the control network. Electronic keying enforces catalog number, major revision and input-type match at insertion; a replacement 1756-IF8IH with a higher major firmware revision than the project's configured module profile is rejected until the ACD project is upgraded and re-downloaded, even if the hardware seats mechanically .

Ordering Information

• 1 x 1756-IF8IH HART isolated analog input module, series as shipped
• 1 x product identification label / catalog tag sleeve
• 1 x 1756-TBCH screw-clamp RTB or 1756-TBS6H spring-clamp RTB, ordered separately
• 1 x module installation / replacement sheet covering 36-pin RTB wiring and HART loop loading

Local Loop Wiring And Panel Rules

• Run each 2-wire or 4-wire 4-20 mA transmitter on its own twisted-pair shield; do not parallel multiple transmitters onto one isolated channel return, since channel-to-channel isolation is broken by a shared 0 V reference.
• Terminate the cable shield to the DIN-rail / panel ground at one end only for each loop, unless the 1756 system ground study calls for a driven shield; floating shields couple 50/60 Hz common-mode into the sigma-delta front end.
• Allow the 250 V AC rms isolation barrier to remain unbridged: do not land intrinsically safe barrier outputs and non-IS loops on adjacent 1756-IF8IH channels unless the barrier approval explicitly permits mixed energy zones on one isolated bank.
• After hot insertion (RIUP), wait for the module OK and I/O timing LEDs to clear fault before enabling HART write commands; the per-channel modem reinitializes its carrier detect after backplane power restoration .

Technical Q&A

• Does 1756-IF8IH read voltage signals if wired to a 0-10 V transmitter?

No. The "I" current-only front end of 1756-IF8IH has no voltage sense resistor bank switched in by software. A 0-10 V source on a current input either reads open-loop full scale or overloads the input. Use 1756-IF8I for mixed current/voltage, or 1756-IF8H for HART + voltage .

• Is the module replaceable under power in a running rack?

Yes, within ControlLogix RIUP rules. Insert the replacement 1756-IF8IH, reseat the RTB, and the controller re-establishes the I/O connection after electronic keying passes. The affected eight loops drop to their configured fault value for the re-scan interval; it is not a bumpless transfer at the loop level .

• Can all eight HART modems run simultaneously at 1200 bps?

Practically yes for PV polling. All eight channels carry independent HART modems and can address eight field instruments in parallel. Continuous HART read of dynamic variables, status and long ASCII tags across all eight channels increases the module's CIP explicit-message load and lengthens the controller scan contribution versus analog-only mode .

• What happens if a replacement has a different series letter?

Series / major firmware mismatch triggers a minor fault or connection rejection under strict electronic keying. Under compatible keying, an equal-or-higher series 1756-IF8IH may run, but HART behavior and noise-filter timing can differ from the as-built loop commissioning record; re-verify zero and span after swap.

• How is channel-to-channel crosstalk suppressed between isolated inputs?

Each channel uses a separate sigma-dDelta ADC and a separate galvanic barrier to backplane and frame, giving 250 V AC rms continuous isolation. Crosstalk is bounded by the isolated barrier capacitance and common-mode rejection (>90 dB typical with 20 Hz filter), not by software averaging alone .

• Does 1756-IF8IH require an external 24 VDC loop supply?

For 2-wire HART transmitters, yes. The module senses the 4-20 mA loop current but does not source the field excitation on most 2-wire instruments; the loop power must come from an external 24 VDC barrier / power conditioner sized for the HART ripple allowance.

• What data formats are available for the controller tag?

Four formats are selectable per channel: analog only; analog + HART PV; analog + HART configured-device data (no); analog + HART configured-device data (yes). Choosing full HART device objects expands the produced tag size and the connection instance count on the owning controller .

• Is 1756-IF8IH usable in a 1756 redundant chassis pair?

Yes, if the redundant chassis carries matched 1756-RM / 1756-RM2 synchronization and the module is present in the same slot of both primary and standby racks with identical major revision. On primary failure, the standby already holds the last valid analog image and HART PV, so the I/O ownership transfer does not force an 8-channel re-initialization gap.