Component Snapshot At-a-Glance

• Model: TB850, full part number 3BSC950193R1
• Alt. P/N: TB851 (DB25 female CEX terminator, incompatible pinout); no functional substitute from third-party brands
• Product Series: ABB AC 800M / 800xA DCS & SIS, CEX (Controller Extension) Bus communication architecture
• Hardware Type: Passive unpowered bus termination plug, industrial plastic housing, screw-locked DB25P male connector
• Key Feature: Integrated factory-calibrated 120Ω termination resistor, eliminates signal reflection on daisy-chained CEX bus segments
• Primary Field Use: Must plug into the last DB25 port of every CEX bus daisy chain to match bus impedance and stabilize communication between PM8xx CPU, SM811 safety CPUs and communication interface modules.

Hard-Numbers: Technical Specifications

• Protocol Support: Passive termination only; matches proprietary differential CEX bus electrical impedance
• Port Count: Single 25-pin DB25 male connector with dual screw lock fasteners
• Termination Resistance: Fixed 120Ω differential line matching resistor built into housing
• Operating Temperature: 0°C to +55°C cabinet operational; -40°C to +85°C storage
• Isolation Rating: No external power required, fully passive component
• Power Draw: 0W, draws no supply voltage from CEX bus or rack rail
• Mechanical Spec: IP20 front face protection, DIN rail mountable clip lugs integrated into plastic shell
• Physical Dimensions: 50mm W × 16mm H × 55mm D
• Net Weight: 0.04kg (40 grams)
• Cable Compatibility: Matches TK850 DB25 CEX extension cables, TP868 SM811 baseplate CEX ports
• Vibration Compliance: IEC 60068-2-6 industrial cabinet vibration standard

The Real-World Problem It Solves

Unterminated CEX bus ends create severe signal reflection and ringing noise across the differential communication lines. The controller misreads data frames, triggering constant CEX BUS FAULT alarms, intermittent loss of SM811 safety CPU communication, and invalid SIS/ESD trip status points on HMI displays.Field crews often skip installing terminators to save spare parts, or leave loose DB25 jumpers on the final CEX port; without matched 120Ω impedance, bus scan jitter breaks deterministic safety logic execution on SIL3 SM811 modules.Generic off-brand DB25 plugs lack precise 120Ω resistors, cannot replicate factory bus matching and introduce persistent communication latency issues that degrade SIF SIL integrity.Where you’ll typically find it:

• Refinery SIS racks with daisy-chained SM811 safety CPUs on CEX bus segments
• Fossil power plant AC800M main controller racks with multiple PROFIBUS/Modbus communication modules
• Offshore FPSO 800xA control cabinets extending CEX bus via TK850 extension cables to remote I/O clustersThis low-cost passive termination plug eliminates bus reflection noise, stabilizes CEX communication timing, and prevents intermittent safety logic dropouts without extra rack power or wiring.

Hardware Architecture & Under-the-Hood Logic

This is a zero-power fully passive component with no semiconductors or processing chips; its sole function is electrical impedance matching for differential CEX bus pairs.

1. Dual screw-locked male pins make full contact with the final CEX bus device’s female port, eliminating loose vibration-induced pin disconnects.
2. Pre-soldered precision 120Ω carbon film resistor connects across the two differential signal pairs of the CEX bus internal wiring.
3. Industrial molded plastic housing shields the resistor from cabinet dust, humidity and stray EMI from nearby VFD power cables.
4. Integrated DIN rail clip lugs lock the terminator to the cabinet rail to stop cable weight from pulling the connector loose over time.
5. No LED indicators, no configuration software, no firmware—plug-and-play operation with zero setup required.
6. Small compact footprint fits tight cabinet cable duct zones without obstructing adjacent CEX bus cables or baseplate ports.

Field Service Pitfalls: What Rookies Get Wrong

Omitting TB850 Terminator On The Final CEX Bus Port

New techs leave the last port empty to conserve spare parts. Unterminated bus reflections flood CEX traffic with noise; SM811 safety CPU sync links drop out randomly, SIS interlock statuses flicker invalid on HMI.Field Rule: Every continuous CEX daisy chain must end with a TB850 3BSC950193R1; mark empty spare ports with blank dust caps only, never use as chain endpoints.

Mixing TB850 Male Terminator With TB851 Female Port Ends

Apprentices install TB851 female terminator on male device ports, creating pin misalignment and open-circuit bus signals. Full CEX bus communication drops, all attached communication/ modules report offline faults.Quick Fix: Match connector gender strictly: TB850 (male plug) for female baseplate/CPU ports; TB851 only for male cable ends.

Using Unlocked Plugs Instead Of Screw-Locked TB850

Crews substitute generic unlatched caps without screw fasteners. Cabinet blower vibration slowly loosens the connector, causing intermittent CEX bus timeouts during high plant load transients.Field Rule: Only use factory screw-locked terminators on all permanent CEX bus end points; generic unfastened caps are prohibited for permanent SIS/DCS installation.

Commercial Availability & Pricing Note

Please note: The listed price is for reference only and is not binding. Final pricing and terms are subject to negotiation based on current market conditions and availability.