Allen-Bradley 1756-TBNH Retrofit-Ready Terminal Block for ControlLogix

Allen-Bradley 1756-TBNH Retrofit-Ready Terminal Block for ControlLogix Control Systems

The Allen-Bradley 1756-TBNH is a 20-pin screw-type terminal block designed for direct field-wiring connection to ControlLogix 1756-series I/O modules. As legacy control panels age and OEM support windows close, the 1756-TBNH has become a critical retrofit component for engineers tasked with modernizing existing automation infrastructure without full system replacement. Whether you are upgrading a discontinued wiring harness, replacing a damaged terminal assembly, or migrating field wiring from an older SLC 500 or PLC-5 platform to a ControlLogix backbone, the 1756-TBNH provides a reliable, code-compliant interface that preserves your existing field cable runs and minimizes re-termination labor.

In retrofit and upgrade projects, the 1756-TBNH is typically installed alongside ControlLogix digital and analog I/O modules such as the 1756-IB16, 1756-OB16E, 1756-IF8, and 1756-OF8. These modules mount into a 1756-A7 or 1756-A13 chassis and are powered by a 1756-PA72 or 1756-PB72 power supply. The terminal block connects directly to the I/O module face via a 20-pin RTB connector, eliminating the need for intermediate marshalling panels in many retrofit scenarios. Engineers should verify that the existing field cable gauge and insulation rating are compatible with the 1756-TBNH screw terminal torque specification (0.5–0.6 N·m) before re-terminating wires from legacy systems.

When planning a retrofit from a legacy platform, the following verification steps are essential: confirm the I/O module type (sink or source) matches the field device wiring polarity; verify that the 1756-TBNH variant (NH = standard screw, IFM = interface module style) matches the target I/O module’s RTB connector type; check that the ControlLogix chassis backplane slot addressing is correctly configured in Studio 5000 Logix Designer to avoid I/O tree conflicts; and validate that any existing HMI screens built on FactoryTalk View SE or ME reference the correct tag database paths after the module is re-addressed. Communication links via EtherNet/IP using a 1756-EN2T or 1756-EN3TR bridge module should be tested end-to-end before returning the panel to live production.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

A successful retrofit using the 1756-TBNH begins well before the maintenance window opens. During the pre-engineering phase, document every field terminal point currently wired to the legacy I/O rack — including wire color, signal type, device address, and PLC rung reference. This documentation becomes the wiring migration map that technicians follow during the cutover. In SLC 500 to ControlLogix migrations, the SLC 5/05 processor and 1746-series I/O modules are replaced with a 1756-L73 or 1756-L83E ControlLogix controller seated in a 1756-A13 chassis. The 1756-TBNH terminal blocks are pre-wired to the new 1756-IB16 and 1756-OB16E digital I/O modules in the shop before the panel arrives on site, dramatically reducing field termination time.

Power budget verification is a mandatory step. The 1756-PA72 power supply delivers 10 A at 5 VDC backplane power; adding multiple I/O modules, a 1756-EN2T EtherNet/IP bridge, and a 1756-RM2 redundancy module can approach this limit quickly. Use Rockwell’s Chassis Sizing Tool to confirm the power budget before finalizing the BOM. For analog signal migration, the 1756-IF8 input module and 1756-OF8 output module use the same 1756-TBNH RTB interface, allowing field 4–20 mA loops to be re-terminated without signal conditioning changes in most cases.

Where legacy systems used a 1784-U2DHP USB-to-DH+ programming cable to connect to PLC-5 processors, the new ControlLogix platform uses a standard USB-A to USB-B cable connected to the 1756-L8x controller’s USB port, or an Ethernet crossover to the 1756-EN3TR module. Ensure the programming workstation has Studio 5000 Logix Designer installed and that the converted program — typically migrated using Rockwell’s Migration Conversion Tool — has been validated offline before the live cutover begins.

Downtime Control During System Migration

Minimizing unplanned downtime is the primary commercial driver behind most ControlLogix retrofit projects. The 1756-TBNH’s removable terminal block design is central to this strategy: field wiring can be pre-terminated to the RTB on the bench, and the entire wired assembly can be plugged into the I/O module in seconds during the live cutover window, rather than terminating individual wires under time pressure in the field.

For critical production lines, a parallel-run strategy is recommended. The new ControlLogix rack — fully wired with 1756-TBNH terminal blocks, powered by a 1756-PA72 supply, and communicating via a 1756-EN2T bridge — is installed alongside the legacy system. The converted program is loaded and verified in offline simulation using Studio 5000. On cutover day, field devices are switched from the old terminal strips to the pre-wired 1756-TBNH blocks one zone at a time, allowing partial production continuity. HMI screens on FactoryTalk View are updated to reflect the new tag structure before operators are transitioned to the new interface. This phased approach typically reduces total cutover downtime from 8–16 hours to under 4 hours for a standard 128-point I/O system.

All units supplied by SMARTNEXMSK are function-tested prior to shipment and covered by a 12-month warranty against manufacturing defects. Inventory is maintained in stock for immediate dispatch, supporting emergency breakdown scenarios where lead time from the OEM channel is unacceptable.

Retrofit Support FAQ

Q1: Is the 1756-TBNH a direct drop-in replacement for the original Allen-Bradley RTB on my existing 1756-IB16 module?
Yes. The 1756-TBNH is the OEM-specified 20-pin screw-type removable terminal block for the 1756-IB16 and compatible 1756-series I/O modules. It plugs directly into the module face connector without any adapter or modification. Verify the wire gauge of your existing field cables falls within the 14–22 AWG specification before re-terminating.

Q2: Can I pre-wire the 1756-TBNH off-site and then plug it in during the cutover window?
Yes, and this is the recommended practice for minimizing downtime. The removable terminal block design allows complete field wiring to be completed on the bench. Label each terminal clearly against your wiring migration map. During the cutover, power down the I/O module, remove the old RTB, insert the pre-wired 1756-TBNH, and restore power. The module will re-scan and report I/O status within one controller scan cycle.

Q3: What commissioning checks should I perform after installing the 1756-TBNH in a retrofit application?
After installation, verify the following in Studio 5000 Logix Designer: (1) the I/O module appears online with no faults in the I/O tree; (2) each input channel reflects the correct field device state; (3) each output channel can be forced on/off and the field device responds correctly; (4) no wiring short or open-circuit faults are reported on analog channels. For EtherNet/IP-connected racks, confirm the 1756-EN2T or 1756-EN3TR module shows a green status LED and the controller’s connection count is within the module’s supported limit.

Q4: What warranty and testing does SMARTNEXMSK provide with the 1756-TBNH?
Every 1756-TBNH unit shipped by SMARTNEXMSK is inspected and function-tested before dispatch. All units carry a 12-month warranty covering manufacturing defects and functional failures under normal operating conditions. In the event of a warranty claim, contact sales@smartnexmsk.com with your order number and a description of the fault. Replacement units are dispatched within 2 business days of claim approval.

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