The Allen-Bradley 1771-NBSC/A, also cataloged as the 1771-NBSC High Resolution Isolated Analog Combination Module, executes 6-channel 4-20 mA input acquisition and 2-channel 0-25 mA output drive across PLC-5 1771 backplane I/O image tables .

Catalog Number and Spare Cross-Reference

Electrical and Mechanical Data

Backplane Bus Transfer and I/O Density Scaling

The 1771-NBSC transfers converted analog words into the PLC-5 processor data table through the 1771 parallel backplane I/O scan, not through EtherNet/IP or DeviceNet. Backplane bus communication velocity licences are absent on 1771 N-series hardware; scan update is governed by the local chassis adapter and the processor sweep time. I/O density scaling is limited by the 3.0 A 5 VDC backplane draw of this module plus the sum of all co-installed 1771 modules against the rack power supply. No field-flashable firmware resides in 1771-NBSC; electronic keying is resolved by the processor I/O configuration file and the module hardware ID pins, so revision A replacement is accepted only when keying is set to compatible or disabled. Remote termination is performed via 1771-NC6 (1.82 m) or 1771-NC15 (4.56 m) cable to a 1771-RTP4 panel, which physically separates field wiring from the rack slot .

Ordering Information

• 1 x 1771-NBSC module (base unit, revision A or B subject to available NOS stock)
• 1 x 1771-NC6 6 ft / 1.82 m N-series analog remote termination cable, where remote panel is used
• 1 x 1771-RTP4 remote termination panel, where panel-mount field wiring is required
• 1 x Module-to-chassis keying strip, pre-seated on new-old-stock units

Rack and Field-Wiring Practice

• Insert module only after 1771 chassis 5 VDC backplane power and AC/dc auxiliary supply are removed
• Run 4-20 mA input twisted pair inside shielded cable; drain shield to 1771-RTP4 ground stud, not to both ends
• Feed two-wire transmitters from the N-series loop power terminals; do not parallel external 24 VDC onto sourced inputs
• Keep 0-25 mA output lines separated from input loops to preserve galvanic isolation boundary
• Confirm chassis frame ground impedance to panel before applying process-loop power

Frequently Asked Questions

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

No. 1771 analog modules are not RIUP-capable. Extracting or inserting the module with backplane power applied can arc the 5 VDC logic connector and force a chassis major fault. Remove rack power first.

Why do some catalogs show 2 x 4-20 mA while the nameplate shows 0-25 mA?

N-series combo modules support multiple output ranges through internal scaling and termination. Where the physical module legend reads 0-25 mA, honor that. Older Rockwell selection tables sometimes compressed the N-family into a generic 4-20 mA column .

What backs up the 12-bit input conversion claim?

12-bit conversion aligns with the N-series high-resolution analog class for 4-20 mA current inputs . Final scaling into engineering units is done in the PLC-5 data-table words via the application program, not inside the module.

How much backplane current does one 1771-NBSC consume?

The module draws up to 3.0 A from the 1771 chassis 5 VDC backplane rail . In an 8-slot or 16-slot rack, total installed-module backplane load must remain under the 1771 power supply rating before adding digital I/O.

Are input channels individually isolated from each other?

No. The N-series provides galvanic isolation between the field side and the 5 VDC backplane logic, and between analog field power and processor bus; intra-channel input grouping follows the 1771-NBSC internal barrier map, not full point-to-point isolation.

Can a 1771-NBSC/A replace a failed 1771-NBSC without program change?

If the PLC-5 I/O configuration keying is set to compatible, yes. If exact keying is enforced, the processor compares the returned module ID and faults on mismatch until the configuration is edited or keying relaxed.

What happens to outputs on loss of field 24 VDC but intact backplane 5 VDC?

Output drive amplifiers lose loop compliance voltage and both 0-25 mA outputs go to 0 mA. The module remains logically addressed on the backplane, but no current sinks or sources to the actuator until field power returns.

Is external surge protection required on 4-20 mA inputs?

Not internally provided beyond the N-series isolation barrier. In environments with long field runs past VFD panels or switchgear, fit external current-loop suppressors on the 1771-RTP4 entry terminals to limit transient coupling into the analog front end.