When an ABB BC810 module throws a CEX-Bus fault, maintenance teams often swap it out first. In my experience, repeated faults nearly always trace back to the bus termination. The TP857 and TB850 units are the most overlooked components in the AC800M system. This article covers how proper ABB BC810 CEX-Bus fault termination with these units eliminates communication errors, saving replacement modules and cutting downtime.
Understanding CEX-Bus Fault Symptoms and Termination
A CEX-Bus fault shows as a communication loss in the AC800M controller. The module’s status LED flashes red, and diagnostics may report a bus error. The cause is often missing or improper termination. The CEX-Bus, like any high-speed fieldbus, requires termination at both ends to stop signal reflections. Without it, even a new BC810 module will fail.
| LED State | Possible Cause |
|---|---|
| Steady red | Bus termination missing or wrong module type |
| Flashing red | Mismatched firmware or bus timeout |
| Off, but fault logged | Intermittent termination contact |
Diagnosing Termination vs. Module Faults
When a CEX-Bus fault appears, first check the TP857 termination unit. It plugs into the BC810’s front D-sub connector. If the unit is loose or missing, the bus will be unstable. Next, inspect the TB850 bus termination board. It is the last point where the bus cable is terminated. On many sites, the TB850 is not installed or set incorrectly. I recall a plant where the bus cable was simply cut and taped—no termination at all. The communication was erratic until we fitted a TB850.
To see if the BC810 module itself is faulty, swap it with a known-good one. If the fault moves with the module, replace it. If the fault stays at the same slot, the problem is the termination or backplane. This simple test can save hours.
Checking Termination Resistor Values
The TP857 includes a built-in resistor; its value must match the bus specification. If it is damaged, replace the entire unit. The TB850 also has a termination resistor that you can check with a multimeter—typically 120 Ω. A short or open here means the board needs replacement.
If swapping modules reveals a persistent slot fault despite verified termination, the backplane or module connector may be damaged. In such cases, a new BC810 module might still be required. Reach us at [email protected] for a tested replacement with fast shipping.
Setting Up TP857 and TB850 Termination
Correct installation is straightforward but often done wrong in the field. Follow these steps:
- Power down the BC810.
- Attach the TP857 to the BC810 front port and tighten its locking screws.
- Connect the bus cable to the TB850. The terminal block accepts the incoming cable and routes it to the resistor.
- Set the termination DIP switch on the TB850 to “ON” only if this is the last module on the segment.
- Power up and verify the bus error clears.
I have seen numerous sites where the TP857 screws were not tightened, leading to intermittent faults. This five‑second fix resolved what looked like a serious module failure.
TB850 Cable Wiring and Grounding
The TB850 also provides a grounding connection for the bus shield. Failure to connect the shield can introduce noise. Ensure the cable shield is properly landed on the TB850 terminal.
Implementing CEX-Bus Redundancy with Proper Termination
For high-availability systems, two BC810 modules are used. Each must have its own TP857 and connected bus. The termination rules are the same: each CEX-Bus segment needs its own termination at both ends. I recommend labelling each termination board and periodically testing failover. A termination failure on one bus can force a failover, hiding the root cause.
Sourcing Reliable ABB BC810 and Termination Parts
When a BC810, TP857, or TB850 fails, getting a genuine replacement quickly is critical. Joyoung International Trading Co., Limited carries stock of these ABB parts. We provide original products and can ship within 24 hours. Our clients in petrochemical, power, and steel rely on us for fast delivery of ABB automation spares.
Inventory Planning for Termination Units
For plants with multiple AC800M systems, keeping spare TP857 and TB850 units on hand reduces downtime. We can help you determine the right spare coverage based on your installed base. Contact us at [email protected] or call +86-181-5013-7565 to discuss your needs.
Resolving Recurring CEX-Bus Faults Quickly
Recurring CEX-Bus faults indicate a deeper issue—often termination or backplane. If you have verified the TP857 and TB850 and still see errors, the BC810 module or its slot may need replacement. We stock tested BC810, TP857, and TB850 units ready for immediate shipment. Email [email protected] or call +86-181-5013-7565 with your part numbers for a quote. Our team can also provide technical guidance.
Common Questions About ABB BC810 Termination
What is the most common cause of a CEX-Bus fault on ABB BC810?
The most common cause is missing or wrong bus termination. The TP857 and TB850 are often overlooked. Restoring proper termination fixes the fault in the majority of cases.
Can I use a generic termination resistor instead of TB850?
We do not recommend it. The ABB TB850 is matched to the CEX-Bus impedance and pinout. A generic resistor may cause reflections or low signal levels, leading to hard-to-trace intermittent errors.
How do I know if the BC810 module is faulty or the termination is wrong?
Swap the module. If the fault moves, replace the module. If it stays, check the TP857 and TB850. Most faults we see are termination, not module.
Do I need to terminate both ends of the CEX-Bus?
Yes. ABB’s CEX-Bus is a linear bus, so both ends need termination. Typically one end is at the BC810 and the other at a CI801 or CI627A.
How can I get BC810 termination units quickly if I need them urgently?
Joyoung International Trading keeps BC810, TP857, and TB850 in stock. In a critical downtime, you can reach us at [email protected] or call +86-181-5013-7565. We can provide a quote and shipping estimate, helping you restore your system fast.
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