Description
- Model: Allen Bradley 1756-TBNH
- Brand: Allen Bradley (Rockwell Automation)
- Series: ControlLogix 1756 Series
- Core Function: 20-position NEMA screw clamp removable terminal block
- Product Type: Removable Terminal Block (RTB) / Module Housing
- Key Specs: 20 Pins | NEMA Clamp Style | Compatible with 1756 I/O
- Terminal Type: NEMA screw-clamp (heavy duty)
- Number of Positions: 20 pins
- Wire Size (Solid): 22 to 14 AWG (0.34 to 2.5 mm²)
- Wire Size (Stranded): 22 to 14 AWG (0.34 to 2.5 mm²)
- Torque: 0.8 N·m (7 lb·in)
- Module Compatibility: Most 1756 ControlLogix digital and analog I/O modules
- Housing Material: Industrial grade thermoplastic
- Operating Temperature: 0 to 60°C (32 to 140°F)
- Dimensions: Standard 1756 I/O profile
- Weight: 0.11 kg (0.25 lbs)
ALLEN BRADLEY 1756-TBNH
Application Scenarios & Pain Points
In a ControlLogix chassis, the I/O module does the “thinking,” but the 1756-TBNH does the “heavy lifting.” This RTB (Removable Terminal Block) is what actually holds your field wiring. The biggest pain point I see in the field isn’t the module failing—it’s the terminal block getting damaged during a rushed commissioning or a “blind” swap in a dark cabinet. If a screw is over-torqued and strips, or if a technician accidentally snaps the plastic retaining tabs, the entire I/O connection becomes unreliable.
Typical Application Scenarios:
- High-Vibration Environments: The NEMA clamp style is preferred in mining or heavy manufacturing because it holds the wire more securely than spring-clamp versions.
- Fast Module Replacement: Since it’s a “Removable” block, you can swap a faulty I/O module without disconnecting a single field wire. Just unplug the RTB, swap the module, and plug it back in.
- New System Assembly: Used as the standard interface for 1756-IA16, 1756-IB16, 1756-OB16E, and many other digital/analog cards.
- Retrofitting: Replacing older, worn-out terminal blocks where the plastic has become brittle over 10+ years of heat cycling.
Real-World Case: The Stripped Screw Disaster
Background: During a midnight shift at a regional water treatment plant, a digital output module failed. The on-site tech had the spare module but didn’t have a spare RTB.
Problem: While trying to move the wiring to the new module, one of the terminal screws on the old Allen Bradley 1756-TBNH stripped out. He couldn’t tighten the common wire, meaning half the station’s valves wouldn’t cycle. He tried to “jerry-rig” it with electrical tape, but the vibration from the pumps kept shaking it loose.
Solution: They called us at 2:00 AM. We had a bulk pack of 1756-TBNH in stock. We didn’t just ship one; we sent five so they’d have spares for the next “oops” moment. It arrived by 10:00 AM via hot-shot courier.
Result: The tech replaced the damaged RTB, secured the wires properly, and the station was back in “Auto” before the morning peak demand. Lesson learned: Always keep 2-3 spare TBNH blocks in the maintenance shop. They are cheap insurance against hours of downtime.
ALLEN BRADLEY 1756-TBNH
Compatible Replacement Models
The 1756 series has a few different terminal block styles. You need to make sure you’re picking the right “mouth” for your wires.
| Model | Style | Compatibility | Pros/Cons |
| 1756-TBNH | NEMA Screw | ✅ Direct Match | Pro: Most secure grip. Con: Slower to wire than spring style. |
| 1756-TBSH | Spring Clamp | ⚠️ Functional Only | Pro: Vibration resistant and fast. Con: Harder to troubleshoot with a probe. |
| 1756-TBCH | Extended Depth | ❌ Incompatible | Used for 36-pin high-density modules. Will not fit 20-pin cards. |
Engineer’s Note: Don’t mix NEMA (TBNH) and Spring (TBSH) blocks in the same rack if you can avoid it. It drives the maintenance guys crazy because they have to carry different tools (screwdriver vs. spring tool). Stick to the TBNH if that’s what your site standard is.
Troubleshooting Quick Reference
| Symptom | Possible Cause | Correlation | Quick Check | Action |
| I/O “Blown” Fuse LED | Short circuit at RTB | ⚠️ Medium | Look for stray wire strands touching adjacent terminals. | Trim and re-terminate wires. |
| Intermittent Input Signal | Loose Screw / Under-torque | ✅ High | Give every wire a “tug test.” If it moves, it’s not tight. | Torque to 7 lb-in. |
| RTB won’t seat in Module | Alignment Pins Broken | ✅ High | Inspect the plastic “fingers” at the top and bottom of the RTB. | Replace the 1756-TBNH. |
| Module Error (No Field Power) | Common/Power wire loose | ✅ High | Check voltage directly on the RTB screw heads. | Tighten power/common screws. |
Safety Warning: Never plug or unplug the RTB while field power is applied if the area is hazardous (Class 1, Div 2). Even though ControlLogix supports “RIUP” (Removal and Insertion Under Power), an arc at the terminal pins can cause an explosion in volatile environments.
ALLEN BRADLEY 1756-TBNH
Inventory List: Allen Bradley & ControlLogix
- 1756-L71 (ControlLogix 5570 CPU)
- 1756-IB16 (16-Point Digital Input)
- 1756-OB16E (16-Point Protected Output)
- 1756-IF16 (16-Point Analog Input)
- 1756-EN2T (EtherNet/IP Bridge)
- 1756-PA72 (Power Supply)
- 1756-TBNH (20-Pin NEMA RTB)
- 1756-TBSH (20-Pin Spring RTB)
- 1747-L541 (SLC 500 CPU)
- 1794-IB16 (Flex I/O Module)
