What Is Foundation Fieldbus?
Foundation Fieldbus (FF) is an all-digital, serial, two-way communication system that interconnects field equipment such as sensors, actuators, and controllers. Unlike traditional 4–20 mA analog signals, FF transmits multiple process variables and diagnostic information over a single twisted-pair cable — enabling true distributed control where control algorithms execute in the field device itself.
Key characteristics of Foundation Fieldbus H1:
- Digital bus architecture: Multiple devices share one cable segment
- Bus-powered: Devices draw power directly from the communication cable
- Function blocks on-device: PID control, arithmetic, and logic execute in the transmitter
- Intrinsically safe: Suitable for hazardous areas with proper barriers
- Interoperable: Devices from different vendors coexist on the same segment
For temperature measurement, FF transmitters like the NCS-TT108 FF can handle up to 8 sensor inputs while participating in control loops — a significant advantage over single-loop analog transmitters.
Foundation Fieldbus vs HART vs PROFIBUS PA
Understanding the differences between these protocols helps you select the right technology for your application:
| Feature | Foundation Fieldbus | HART | PROFIBUS PA |
|---|---|---|---|
| Communication | All-digital | 4–20 mA + digital overlay | All-digital |
| Control location | In the field device | In the controller/DCS | In the controller/PLC |
| Power supply | Bus-powered | Loop-powered | Bus-powered |
| Cable per device | Shared (multi-drop) | Dedicated (point-to-point) | Shared (multi-drop) |
| Data update rate | 1–10 Hz | Typically 1–4 updates/sec | 1–10 Hz |
| Diagnostic depth | Extensive (on-device) | Moderate | Extensive |
| Best for | Critical control loops | Monitoring, retrofit | Process industries, PLC-centric |
When to choose Foundation Fieldbus: If your application requires on-device control execution, deterministic communication, or you’re building a new plant with DCS-centric architecture, FF provides the most integrated solution.
Key Selection Criteria
1. Measurement Accuracy
Accuracy directly impacts process control quality. For the NCS-TT108 FF, key accuracy specifications include:
| Sensor Type | Accuracy |
|---|---|
| PT100 RTD | ±0.2°C |
| PT1000 RTD | ±0.1Ω |
| CU50 RTD | ±0.3°C |
| Type E, T Thermocouple | ±0.6°C |
| Type K, J Thermocouple | ±0.8°C |
| Type B, N, R, S Thermocouple | ±1.0°C |
Consider not just the base accuracy, but also long-term stability and temperature drift. The NCS-TT108 FF specifies temperature drift of ±0.001/°C, meaning measurement error remains stable across the operating range.
2. Channel Count and Density
Multi-channel transmitters reduce installed cost per measurement point. The NCS-TT108 FF offers 8 temperature channels in a single unit, compared to:
- Rosemount 644 FF: 1 channel
- E+H TMT82 FF: 2 channels
Higher channel density means fewer transmitters, fewer fieldbus addresses, less panel space, and simplified wiring — translating to lower total cost of ownership.
3. Sensor Compatibility
Verify the transmitter supports all sensor types used in your application. The NCS-TT108 FF covers:
- RTDs: PT100, PT1000, CU50, CU100, Ni100, Ni1000
- Thermocouples: Types E, T, J, K, B, N, R, S
- Resistance: 0–400Ω or 0–4000Ω range
Each channel can be individually configured, allowing mixed sensor types on the same transmitter.
4. Bus Power and Isolation
Foundation Fieldbus devices must operate within strict power budgets. Key NCS-TT108 FF specifications:
- Bus supply voltage: 9–32 VDC
- Quiescent current: ≤20 mA
- Bus isolation: 500 Vrms (channel-to-bus and module-to-module)
The 500 Vrms isolation rating protects against ground loops and transient voltages — critical for plants with variable frequency drives (VFDs) or other noise sources.
5. Certifications and Hazardous Area Approval
For installations in explosive atmospheres, verify intrinsic safety certifications. The NCS-TT108FC (intrinsically safe variant) carries:
- Ex ia IIC T4 Ga (ATEX/IECEx)
- FM/CSA Class I, Division 1 approval (verify specific model)
Always confirm the exact certification matches your region’s requirements before specifying.
6. Configuration and Interoperability
FF transmitters require Device Description (DD) files and/or Device Type Manager (DTM) software for configuration. The NCS-TT108 FF provides:
- FF-DD files: For DeltaV, Yokogawa CENTUM, Honeywell Experion, and other FF-compliant hosts
- FF-DTM: For FDT-based configuration tools (PACTware, FieldMate)
Before purchase, verify your host system supports the transmitter’s DD/DTM version — most major DCS platforms have pre-registered Microcyber FF devices.
NCS-TT108 FF Technical Specifications
| Parameter | Specification |
|---|---|
| Protocol | Foundation Fieldbus H1 (IEC 61158-2) |
| Channels | 8 (4 dual-channel isolated modules) |
| RTD Types | PT100, PT1000, CU50, CU100, Ni100, Ni1000 |
| Thermocouple Types | E, T, J, K, B, N, R, S |
| PT100 Accuracy | ±0.2°C |
| PT1000 Accuracy | ±0.1Ω |
| TC Accuracy (E, T) | ±0.6°C |
| TC Accuracy (K, J) | ±0.8°C |
| TC Accuracy (B, N, R, S) | ±1.0°C |
| Temperature Drift | ±0.001/°C |
| Update Rate | ≥1 Hz per channel |
| Bus Supply Voltage | 9–32 VDC |
| Quiescent Current | ≤20 mA |
| Bus Isolation | 500 Vrms |
| Operating Temperature | -40°C to +85°C |
| Protection Rating | IP20 (base unit) |
| Intrinsically Safe Model | NCS-TT108FC |
Competitor Comparison
| Feature | NCS-TT108 FF | Rosemount 644 FF | E+H TMT82 FF |
|---|---|---|---|
| Channels | 8 | 1 | 2 |
| RTD Accuracy | ±0.2°C | ±0.15°C | ±0.2°C |
| TC Accuracy | ±0.6–1.0°C | ±0.5–1.0°C | ±0.5–1.0°C |
| Bus Isolation | 500 Vrms | 500 Vrms | 500 Vrms |
| DD/DTM Support | Yes | Yes | Yes |
| Link Master | Configurable | Yes | Yes |
| Best For | High-density applications | Single-point critical loops | Dual-sensor redundancy |
Key differentiator: The NCS-TT108 FF’s 8-channel architecture significantly reduces installed cost for applications with many temperature points. However, for single-critical-loop control where redundancy through separate transmitters is required, single-channel devices like the Rosemount 644 may be more appropriate.
Installation Considerations
Bus Cable and Wiring Practices
Use dedicated fieldbus cable compliant with IEC 61158-2. Do not share wiring tubes with high-power cables. Shield wires should be grounded at both ends of the bus segment.
Termination
Both ends of the FF bus segment require terminators (typically 100Ω ±2Ω, 1µF in series). Missing or incorrect termination causes communication failures across the entire segment.
Outdoor Installation Requirements
The NCS-TT108 FF base unit carries an IP20 rating, designed for indoor panel or cabinet mounting only. For outdoor or harsh-environment deployments:
- Enclosure selection: Mount inside a field-rated enclosure with minimum IP67 protection.
- Cable glands: Use IP67-rated cable glands compatible with fieldbus cable diameter (6–12 mm).
- Condensation prevention: In humid environments, consider enclosures with built-in heaters or desiccant packets.
Channel Isolation and Mixed Sensors
The NCS-TT108 FF supports up to 8 temperature channels through 4 dual-channel isolated modules. Each module pair shares a common isolation barrier:
| Module | Channels | Isolation Group |
|---|---|---|
| Module 1 | CH1 + CH2 | Isolated from bus and other modules |
| Module 2 | CH3 + CH4 | Isolated from bus and other modules |
| Module 3 | CH5 + CH6 | Isolated from bus and other modules |
| Module 4 | CH7 + CH8 | Isolated from bus and other modules |
Implication: Channels within the same module pair share a common ground reference. If one sensor develops a ground fault, it may affect the other channel’s reading.
Power Budgeting on the FF Bus
Foundation Fieldbus devices are powered directly from the bus. Each device draws quiescent current:
- NCS-TT108 FF quiescent current: ≤20 mA per transmitter
- Operating voltage range: 9–32 VDC
Power budget calculation example:
Segment: 1 power supply, 8 FF devices including 2 × NCS-TT108 FF Current draw: 2 × 20 mA + 6 × 15 mA = 130 mA Power supply: Use 24V / 400 mA supply for safety margin Voltage drop: At 1900m max, expect ~2V drop
Rule of thumb: Never exceed 80% of the power supply’s rated capacity. For long bus segments, use a power conditioner or repeater.
Frequently Asked Questions
1. Does the NCS-TT108 FF support Link Active Scheduler (LAS) functionality?
The NCS-TT108 FF can operate as a Basic Device or Link Master. When configured as Link Master, it can assume LAS responsibilities if the primary LAS fails, providing redundancy for critical control loops.
2. Can I mix different sensor types on the same NCS-TT108 FF unit?
Yes. Each channel can be individually configured for RTD (PT100, PT1000, CU50) or thermocouple (types E, T, J, K, B, N, R, S). This allows one transmitter to handle diverse measurement points.
3. What’s the maximum bus length for an FF segment?
With standard FF cable (IEC 61158-2 Type A), the maximum segment length is 1900 meters without repeaters. With repeaters, you can extend to 10 km total.
4. How do I calibrate an FF temperature transmitter?
FF transmitters support remote calibration via the bus using standardized function blocks. For NCS-TT108 FF, use the manufacturer’s DTM in your host system to perform:
- Sensor trim (match to reference standard)
- Output trim (align digital value to physical measurement)
- Range configuration (set engineering units and span)
5. How does Foundation Fieldbus compare to WirelessHART?
| Factor | Foundation Fieldbus | WirelessHART |
|---|---|---|
| Power source | Bus-powered (continuous) | Battery or energy harvesting |
| Update rate | 1–10 Hz typical | 1 second to minutes |
| Control capability | Function blocks on-device | Limited; monitoring only |
| Installation cost | Higher (cable infrastructure) | Lower (no signal wiring) |
| Best for | Control loops, high-speed monitoring | Remote locations, retrofit |
Selecting the right Foundation Fieldbus temperature transmitter involves balancing protocol requirements, measurement performance, installation environment, and total cost of ownership. The NCS-TT108 FF stands out for:
- 8-channel capacity — reducing per-point cost and panel space
- Robust isolation — 500 Vrms between channels and bus
- IEC 61158-2 compliance — interoperability with major DCS platforms
- Intrinsically safe variant (NCS-TT108FC) — for hazardous areas
When comparing against Rosemount 644 FF or Endress+Hauser TMT82, evaluate not just initial price but channel density, configuration flexibility, and long-term support.
