TRICONEX 3721 Analog Input Module 32 Points
The TRICONEX 3721 is a high-density analog input module designed for the Tricon triple modular redundant (TMR) safety instrumented system. It provides 32 independent differential analog input channels, each configurable for 4-20 mA current loop signals from field transmitters such as pressure, temperature, flow, and level sensors. The module is certified for safety integrity level (SIL) 3 applications per IEC 61508 and IEC 61511.
Input Specifications
Each of the 32 channels accepts a 4-20 mA analog signal (sink input). Input impedance is 250 Ω (typical), allowing direct connection to 2-wire or 4-wire transmitters. Overcurrent protection tolerates up to 40 mA without damage. The module provides loop power for 2-wire transmitters: each channel has an integrated 24 VDC supply with a current limit of 28 mA, derived from the backplane field power. Common mode voltage range is ±1 V between channel negative and chassis ground. Channel-to-channel isolation is not provided; all inputs share a common return. However, the module supports field wiring to isolated or non-isolated transmitters with appropriate grounding practices.
Resolution and Accuracy
The analog-to-digital converter (ADC) delivers 16-bit resolution (0.0015% of full scale). At the nominal 4-20 mA range, the effective resolution is approximately 0.5 µA per count. Calibrated accuracy at 25°C is ±0.05% of span, including linearity, hysteresis, and repeatability. Temperature drift is ±50 ppm per °C. Update rate for all 32 channels is 50 ms (20 Hz) when scanning sequentially. The module also supports fast update mode for selected channels, achieving 10 ms per channel (maximum 8 channels).
Triple Modular Redundancy Implementation
Inside the 3721 module, three independent analog input circuits operate in parallel. Each circuit digitizes the field signal independently. The Tricon processor reads all three values and performs median voting. If one circuit fails (e.g., out-of-range output, excessive deviation), its value is discarded, and the system continues using the two healthy values. This architecture eliminates single-point failures in the analog input path. The module also includes diagnostic injection circuitry that tests each channel with a precision current source during background tasks, detecting open circuits, short circuits, and ADC drift without interrupting process measurements.
Channel Configuration and Scaling
Each input channel can be individually configured through the TriStation software for measurement mode: 4-20 mA standard, 0-20 mA, or custom range. Scaling parameters (zero and span) convert the raw counts to engineering units such as PSI, °C, or m³/h. The module supports both linear and square root extraction for flow applications. Digital filtering is available with selectable time constants from 0.1 to 10 seconds. Burnout detection is configurable: high alarm (22 mA) or low alarm (3.6 mA) can be set to indicate transmitter failure.
Fault Detection and Diagnostics
The 3721 module continuously monitors each channel for overrange (21.5 mA), underrange (3.8 mA), open circuit (current < 3 mA), and short circuit to ground or power. A dedicated LED per channel (green/red) indicates normal or fault status. Two LEDs on the front panel show module health: PASS (green) indicates all three internal circuits are functional; FAULT (red) indicates an internal failure that requires module replacement. The module also reports diagnostic data including channel temperature, power supply voltage, and calibration checksum via the backplane to the processor, where it can be accessed through the TriStation software.
Termination and Field Wiring
Field wiring connects to the module via a removable terminal block (part number 7400209-100 or similar). The terminal block accepts 0.5 to 2.5 mm² (22–14 AWG) stranded wire. For 2-wire transmitters, the module supplies loop power; the positive transmitter wire connects to the channel input, and the negative connects to the return (common). For 4-wire transmitters (externally powered), the output positive connects to the channel input, and output negative connects to common; the external power supply must be isolated or referenced to the same ground as the module. Shielded twisted-pair cable is recommended for runs exceeding 30 meters, with the shield grounded at one end.
Power Requirements and Environmental Ratings
The module draws 0.6 A from the 5 VDC backplane logic supply and 1.2 A from the 24 VDC field supply (for loop power). Maximum power dissipation is 12 W. Operating temperature range is 0°C to 60°C, storage -25°C to 85°C. Humidity 5–95% non-condensing. Vibration resistance: 1 g at 5–100 Hz. Shock: 10 g, 11 ms half-sine. The module complies with CE, TÜV SIL 3, and FM Class I Division 2 hazardous location certifications.
Compatibility
The TRICONEX 3721 is compatible with all Tricon chassis (e.g., 3000602-100) and processors (3005, 3006, 3007, 3008). It can be installed in any I/O slot of the main chassis or remote expansion chassis. The module is also supported by legacy Tricon systems running TriStation 1131 version 4.0 or later. For SIL 2 applications, the module can be used in 1-out-of-2 (duplex) mode by installing two modules and configuring software voting.
Calibration and Maintenance
Factory calibration is stored in on-board EEPROM. Recalibration using external precision current sources (0.01% accuracy) is possible via the TriStation calibration utility. Recommended calibration interval is 24 months for SIL 3 applications. The module does not contain user-replaceable fuses or batteries; any internal failure requires module replacement. Regular inspection of terminal block tightness and visual check for corrosion or moisture ingress is advised during scheduled plant shutdowns.
The TRICONEX 3721 remains a standard analog input solution for Tricon safety systems, offering high channel density and proven TMR reliability for critical process measurements.
