ABB TU844 3BSE021445R1 – S800 I/O Module Termination Unit for reliable field wiring and clean signal routing

The ABB TU844 (part no. 3BSE021445R1) is a Module Termination Unit for the ABB S800 I/O family. From my experience, it’s the go-to base when you want neat, serviceable wiring between field devices and S800 I/O modules like AI845/AI845A or AO845A. It handles the field-side terminations, provides channel-by-channel access for testing, and connects the I/O module to the ModuleBus backplane—no fuss, no external accessories. One thing I appreciate is how it keeps panel wiring straightforward, which typically cuts installation time and avoids rework during FAT or shutdowns.

Order Placement Process and Guarantees

• Warranty: 365 days
• Lead time: 1 week if in stock; no more than 1 month at the latest
• Payment: 50% advance payment; full payment before delivery
• Express options: FedEx, UPS, DHL

Key Features

• Purpose-built for S800 I/O – Designed to host ABB S800 I/O modules, with a robust backplane connector to the ModuleBus.
• Clean field wiring – Screw-terminal layout (5.08 mm pitch in many cases) for quick termination, inspection, and re-termination during maintenance.
• HART-friendly channel mapping – Commonly paired with HART-capable AI/AO modules (e.g., AI845/AO845A), helping maintain signal integrity and diagnostics.
• DIN-rail mounting – Standard 35 mm DIN-rail fit for compact cabinet builds and easy module replacement.
• No separate power needed – The MTU itself doesn’t require a dedicated supply; it distributes the field power and relies on the I/O module for internal needs.
• Service-friendly design – You might notice that channel access and labeling reduce wiring errors and make loop checks faster.

Technical Specifications

Related or Supporting Products

• AI845 / AI845A – 8-channel analog input, HART capable; typically paired with TU844 for clean HART pass-through and channel access.
• AO845A – 8-channel analog output, HART; commonly used with TU844 when HART write-back is needed.
• TU842 / TU843 – Alternative S800 MTUs with different terminal styles or wiring density; choose based on cable type and termination preference.
• S800 power feed modules and Segment Modules – For distributing 24 V field power and ModuleBus segments if your rack grows.

A maintenance lead at a chemical plant told us their team swapped older bases for TU844 to standardize on screw terminals—loop checks went smoother and they cut a day off commissioning. That seems to be a common theme in brownfield jobs.

Installation & Maintenance

• Cabinet & rail – Mount on 35 mm DIN-rail within a clean, ventilated control cabinet. Keep wiring bend radius and segregation (AI/AO vs DI/DO vs power) per plant standards.
• Wiring practice – Use ferrules on stranded conductors; torque screw terminals to spec and label channels clearly. Shielded cable for analog/HART is strongly recommended.
• Grounding & shields – Terminate shields consistently at the designated points to avoid ground loops; don’t mix shield drain with functional earth inadvertently.
• Ventilation – Maintain spacing for airflow; avoid mounting next to heat sources. In most cases, ambient 45–50 °C cabinet design gives ample margin.
• Routine checks – Re-torque terminals during shutdowns, inspect for discoloration or loosened conductors, and verify HART communication during loop checks.
• Firmware – The MTU has no firmware, but keep the connected I/O module firmware aligned with your control system revision.

Quality & Certifications

• CE compliant
• UL/cUL (series dependent) and RoHS
• Manufactured under ISO 9001 quality systems
• Manufacturer’s standard warranty applies; we provide a 365-day warranty on supply

If you’re replacing an existing base, send a photo of your current wiring card or the I/O module model. Matching the TU version to the channel map usually saves surprises during switchover.