TRICONEX 3720 Analog Input Module 16 Channels

The TRICONEX 3720 is an analog input module for the Tricon triple modular redundant (TMR) safety system. It provides 16 differential analog input channels, each configurable for 4-20 mA current or 0-10 V voltage signals. The module is designed for SIL 2 (IEC 61508) applications but can be used in SIL 3 environments when paired with external voting or when redundancy is implemented at the system level. Part number 7400243-115 identifies a specific revision with enhanced EMC protection.

Channel Configurability

Each of the 16 channels can be individually configured via software (TriStation) for current mode (4-20 mA or 0-20 mA) or voltage mode (0-10 V, 0-5 V, ±10 V). Input impedance is 250 Ω in current mode and 100 kΩ in voltage mode. The module does not provide loop power; an external 24 VDC supply is required for 2-wire transmitters. Overvoltage protection clamps inputs to ±30 V DC. Common mode voltage range: ±1 V with respect to chassis ground.

Resolution and Accuracy

The analog-to-digital conversion uses a 12-bit successive approximation register (SAR) ADC, providing 4096 counts over the full input range. For 4-20 mA, the effective resolution is approximately 3.9 µA. Uncalibrated accuracy: ±0.25% of span at 25°C; calibrated accuracy: ±0.1% of span. Temperature drift: ±100 ppm per °C. Update rate for all 16 channels sequentially is 100 ms (10 Hz). A fast scan mode (8 channels) provides 50 ms update.

Triple Modular Redundancy Implementation

Inside the 3720, three independent measurement circuits exist, but unlike the 3721/3722 series, the 3720 is a single-width module that relies on external redundant modules for TMR. Typically, three 3720 modules are used for TMR analog input: one per channel trip. However, when used as a single module, the internal three circuits are voted by the Tricon processor, providing single-module redundancy. The module includes diagnostic self-test capability, injecting a reference voltage to verify ADC linearity every 10 seconds.

Input Ranges and Scaling

Supported input ranges:

• Current: 0–20 mA, 4–20 mA (default), 0–10 mA.

• Voltage: 0–5 V, 1–5 V, 0–10 V, ±10 V.

Scaling to engineering units (e.g., 0–100 PSI) is performed in the processor using user-defined slope and intercept values. The module also provides digital filtering options: exponential filter with time constants of 0.1, 0.2, 0.5, 1.0, or 2.0 seconds. Burnout detection can be configured to set a high or low alarm when the input exceeds 22 mA or falls below 3.6 mA (for 4-20 mA mode).

Termination Assembly

Field wiring connects via a removable 36-pin terminal block (part number 7400234-115). The terminal block accepts 0.5–2.5 mm² wire. Shielded cable recommended; the shield can be connected to chassis ground via a dedicated terminal. A green LED per channel indicates input activity (flashing for live signal). The module mounts directly in any I/O slot of the Tricon chassis and does not require an external termination panel.

Electrical and Environmental Ratings

Current draw: 0.5 A from 5 V backplane, 0.1 A from 24 V field supply. Power dissipation: 6 W. Operating temperature: 0°C to 60°C. Storage: -25°C to 85°C. Humidity: 5–95% non-condensing. Vibration: 1 g (5–100 Hz). Shock: 10 g, 11 ms half-sine. The module is CE marked and UL recognized for hazardous locations Class I Division 2. It is certified by TÜV for SIL 2 capability; for SIL 3, two additional modules (triple) are required.

Compatibility

The 3720 is compatible with all Tricon processors (3005, 3006, 3007, 3008) and chassis. It can be mixed with 3721, 3722, and 3722X modules in the same chassis. The 7400243-115 revision includes improved transient voltage suppression on each input, making it more robust in environments with frequent lightning or switching surges. This revision also has a different component layout for better heat dissipation.

Installation and Calibration

Insert the module into an available I/O slot; the backplane provides keying to prevent incorrect insertion. Apply power to the chassis; the module will start up and perform a self-test (PASS LED turns green after 2 seconds). Calibration is performed using TriStation software and an external precision signal source. The module stores calibration constants in on-board EEPROM. Recommended calibration interval is 12 months for SIL 2 applications. No field-replaceable fuses; internal failures require module replacement.

The TRICONEX 3720 offers a lower-cost analog input solution for applications that do not require the high channel density of the 3721. It remains widely used in legacy Tricon systems for SIL 2 safety functions such as pressure monitoring and temperature shutdown.