Description
The 1756-PLS is a programmable limit switch module manufactured by Rockwell Automation under the Allen-Bradley brand for the ControlLogix 1756 platform. It monitors rotary machine position through a compatible resolver and automatically energizes or de-energizes outputs when predefined low-limit and high-limit positions are reached.
Rather than relying on multiple mechanical limit switches mounted along a machine travel path, the 1756-PLS uses resolver feedback and configurable setpoints to control actuators. This allows one module to manage several position-based events with greater flexibility and fewer mechanical switching components.
Application Scenarios
A rotary indexing machine needed to activate a clamp, start a fill operation, and release a discharge gate at different angles during each revolution. Mechanical limit switches had previously been adjusted by hand, creating setup variations and additional panel wiring. The maintenance team installed a resolver and configured the 1756-PLS to control the operations through programmable position windows.
Instead of adding more physical switches, the application used low-limit and high-limit setpoints to define when each output should turn on and off. The machine could then be reconfigured by changing setpoint values rather than moving hardware switches. This reduced mechanical adjustment time and made product-change records easier to manage.
The value of the 1756-PLS in this application was not simply the number of inputs and outputs. Its main advantage was the ability to convert a resolver position into multiple, repeatable limit-switch events inside one ControlLogix module.
Parameter
| Main Parameters | Value/Description |
|---|---|
| Product Model | 1756-PLS |
| Manufacturer | Rockwell Automation |
| Brand | Allen-Bradley |
| Product Category | Programmable limit switch module |
| Compatible Platform | ControlLogix 1756 |
| Intended Controller | Logix5550 controller |
| Inputs | 16 single-ended inputs, 10–31.2V DC |
| Outputs | 16 current-sourcing outputs, 10–31.2V DC |
| Output Current | 1 A maximum per output |
| Output Configuration | 4 groups of 4 outputs with separate power and common |
| Input Configuration | 4 groups of 4 inputs with separate common |
| Position Feedback | Compatible Allen-Bradley resolver |
| Resolver Excitation | 2V rms ±20% at 5 kHz ±20% |
| Angular Resolution | 0.088° per bit |
| Accuracy | ±0.0976% of full scale |
| Repeatability | ±0.0488% of full scale |
| Maximum Tracking Rate | ±1800 RPM |
| Backplane Current | 1 A at 5.1V DC; 125 mA at 24V DC |
| Power Dissipation | 25.7 W at 30 °C; 21.3 W at 60 °C |
| Isolation | 250V continuous between I/O groups and between the resolver and backplane |
| Chassis Requirement | Three contiguous slots |
| Terminal Blocks | Three removable terminal blocks; 1756-TBSH or 1756-TBNH |
| Operating Temperature | 0…60 °C (32…140 °F) |
| Humidity | 5…95% noncondensing |
| Agency Ratings | UL, CSA, CE, and Class I Division 2 |
| Lifecycle Status | Discontinued; no planned Rockwell Automation replacement |
