The Allen-Bradley 1771-IXHR/C, also cataloged as the 1771-IXHR High Resolution Thermocouple/Millivolt Analog Input Module, samples 8 floating differential field sensors and moves converted words into the PLC-5 data table over the 1771 backplane.

Catalog Identity and Revision Cross-Reference

Hardware Specifications

Block Transfer and Backplane I/O Density Budget

The 1771-IXHR does not stream data through the normal discrete I/O image; it uses block transfer read/write blocks executed by the PLC-5 processor to move configuration words and converted analog results across the 1771 parallel backplane. Backplane bus communication velocity licences are not present on 1771 analog hardware; effective sample rate is bounded by the 25 ms/8-channel conversion cadence and the processor block-transfer queue occupancy. I/O density scaling in one chassis is limited by the 750 mA 5 VDC draw of this module added to the sum of all installed module backplane loads against the 1771 power supply rating. No field-flashable firmware resides in 1771-IXHR; revision A/B/C differences are handled by electronic keying through the I/O configuration file and the module ID resistor/keying comb. Replacement across revisions is accepted only when keying is set to compatible or disabled.

Ordering Information

• 1 x 1771-IXHR module (base unit, revision A/B/C subject to available NOS stock)
• 1 x 1771-WI removable field wiring arm, where not already panel-mounted
• 1 x Pre-seated chassis keying strip for 1771-IXHR slot acceptance
• 1 x Module installation data sheet / block-transfer register map insert

Rack and Sensor Wiring Practice

• Remove 1771 chassis 5 VDC backplane power before insertion or extraction; module is not hot-swappable
• Connect thermocouple extension wire of matching alloy to the 1771-WI terminal pairs; do not substitute copper for TC legs inside the cabinet
• Route mV-source signal pairs as twisted, shielded cable; terminate shield drain at the 1771-WI ground lug only
• Keep TC/mV input runs separated from 120 VAC, 24 VDC solenoid, and VFD output trays to preserve microvolt-level CJC accuracy
• Verify CJC sensor area on module face is not insulated by tape or painted panel door during commissioning

Technical Questions and Answers

Is 1771-IXHR hot-swappable in a live PLC-5 rack?

No. The module draws 750 mA at 5 VDC from the backplane and carries live differential input front-end circuitry. Insertion or removal under power can arc the backplane connector and force a chassis major fault. De-energize rack before swap.

What happens if cold junction compensation sensor drifts?

CJC error adds linearly to every converted TC channel while mV channels are less affected. Within 0-60 deg C ambient the stated CJC accuracy is +/-0.5 deg C; outside that band, TC readings retain 1 uV/mV conversion integrity but lose the published TC accuracy claim.

Can 1771-IXHR/C replace a failed 1771-IXHR/A without program edit?

If PLC-5 I/O configuration keying is exact, replacement with a different revision trips a fault on power-up. Set keying to compatible or disable it, then download; data-table mapping and block-transfer file numbers remain unchanged across A/B/C.

Are all 8 inputs galvanically isolated from each other?

No. The 8 inputs are floating differential pairs referred to a common analog-front-end bus inside the module, with isolation existing primarily between field side and 5 VDC backplane logic. Do not land potentials from separately grounded sensor networks without checking common-mode limits.

Why does the module need no external 24 VDC for TC input?

Thermocouple and low-level mV conversion is powered entirely from the 5 VDC 1771 backplane rail, which supplies the 750 mA load. Only the sensor loop continuity matters; no transmitter excitation is sourced by 1771-IXHR itself.

What is the consequence of a thermocouple open circuit in run?

The channel reports an open-circuit fault condition and the FLT LED illuminates if enabled in the configuration word. The remaining seven channels keep updating on their 25 ms scan; the faulted word does not hold last good value unless the application program latches it.

How many 1771-IXHR modules fit in one 1771-A2B 8-slot rack?

Backplane current, not slot count, governs loading. With a typical 8 A 5 VDC supply, allow 750 mA per 1771-IXHR plus the processor/adapter and any digital I/O; three 1771-IXHR modules already consume 2.25 A before any discrete modules are added.

Does loss of 5 VDC backplane freeze the last analog values?

No. Loss of backplane power removes the A/D supply and block-transfer handshake. All eight input words stop updating immediately and the RUN LED extinguishes; restored power reinitializes auto-calibration before valid conversions resume.