Application Scenarios
A chemical processing plant operated several reactor vessels with temperature-critical exothermic reactions, monitored by 100-ohm platinum RTDs wired to a ControlLogix chassis. When a 1756-IR6I in the reactor control cabinet began exhibiting intermittent channel faults, maintenance engineers traced the problem to a failed isolation barrier on one input—a failure mode that could have propagated noise across adjacent channels in a non-isolated design. Because the module provides 250 V continuous isolation between inputs, the fault remained contained, and the remaining five channels continued reporting accurate temperatures until the replacement could be scheduled . For processes where a single temperature measurement failure can trigger a batch abort or safety interlock, this per-channel isolation is not a luxury—it is the difference between a contained fault and a cascading shutdown.
Parameter
| Main Parameters | Value/Description |
|---|---|
| Product Model | 1756-IR6I |
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Product Category | ControlLogix isolated RTD input module |
| Input Channels | 6, individually isolated |
| Isolation | 250 V continuous between inputs |
| Compatible RTD Types | Pt 100/200/500/1000 (α=385, 3916); Ni 100/120/200/500 (α=618); Ni 120 (α=672); Cu 10 |
| Input Ranges | 1–487 Ω, 2–1000 Ω, 4–2000 Ω, 8–4020 Ω |
| Resolution | 16-bit; 7.7 mΩ/bit at 1–487 Ω range, 60 mΩ/bit at 8–4020 Ω range |
| Data Formats | Integer (2’s complement) and IEEE 32-bit floating point |
| Backplane Current | 250 mA at 5 V; 125 mA at 24 V |
| Power Dissipation | 4.3 W maximum |
| Removable Terminal Block | 1756-TBNH (20-pin) or 1756-TBSH |
| Module Scan Time | 25 ms minimum (floating point, ohms); 50 ms minimum (floating point, temperature) |
| Lifecycle Status | Discontinued March 31, 2017; replacement: 1756-IRT8I |
Technical Principles and Innovative Values
- Per-Channel Isolation as a Fault-Containment Strategy: The 1756-IR6I isolates each of its six inputs to 250 V continuous, meaning a sensor wiring fault, ground loop, or induced surge on one RTD channel cannot corrupt measurements on its neighbors. In multi-point temperature monitoring where one bad thermowell can ruin an entire batch, this architecture converts a potential system-wide failure into a single-channel event .
- Dual Data Format Flexibility: The module operates in either integer mode (left-justified, 2’s complement) or IEEE 32-bit floating point. Integer mode suits applications where the controller handles scaling and linearization, while floating point mode enables the module to perform temperature linearization, process alarms, rate alarms, and digital filtering internally—reducing controller scan time for temperature-heavy applications .
- Configurable Notch Filter for Noisy Environments: The 1756-IR6I allows selection of a notch filter frequency to attenuate line noise and its harmonics, a feature that matters in plants where VFDs, contactors, or high-current heaters induce 50/60 Hz interference onto RTD cabling. Engineers can tune the filter to the dominant noise frequency rather than accepting degraded measurement stability .
- Real Time Sample Period Control: The RTS parameter defines how often the module samples all input channels, independent of the Requested Packet Interval (RPI) that governs data transmission to the controller. When RTS and RPI are matched appropriately, each reading is sent exactly once, avoiding redundant data transmission and optimizing backplane bandwidth .
