Product Overview

The Schneider Electric SR2B122BD is a Zelio Logic compact smart relay for small industrial machines, utility systems, and standalone control panels. It operates from a 24 V DC supply and combines eight digital inputs, four transistor outputs, an integrated LCD display, and a real-time clock. Four of the input channels can also be used for compatible analog measurements, making the controller suitable for applications that combine discrete control with basic process monitoring.

Unlike smart relays that use mechanical relay contacts for their outputs, the SR2B122BD uses semiconductor transistor outputs operating at 24 V DC. This arrangement supports faster switching and is useful for compatible DC control loads that require frequent operation. The outputs are intended for suitable control circuits rather than direct switching of AC loads or motor power circuits.

The eight input channels can receive signals from compatible switches, sensors, and auxiliary contacts. Four inputs can be configured for supported analog measurement, including compatible voltage signals and thermistor applications. This allows the controller to monitor selected process conditions while handling ordinary on/off signals through the same device.

For example, a small temperature-controlled ventilation unit can use a compatible thermistor input to monitor temperature, a digital input to receive a manual operating command, and transistor outputs to control suitable DC interface devices. The program can coordinate the operating conditions, apply time-based rules, and provide output signals for the wider control circuit.

The built-in real-time clock supports scheduled functions alongside ordinary input-driven logic. A small utility system can operate during programmed periods while retaining manual commands and equipment interlocks. The LCD display and navigation buttons provide a local interface for accessing supported device functions and monitoring the application.

Programming is supported through Zelio Soft, with ladder logic and function block diagram methods available for supported applications. Ladder logic suits straightforward interlock and sequencing tasks, while function block diagrams can help organize timing, comparison, counting, and other logic operations.

The transistor outputs are rated for 24 V DC operation and incorporate overload and short-circuit protection. Their load-current limits remain important when connecting solenoid valves, indicator devices, interface relays, or other DC loads. The available output current is not unlimited, and the permitted load must account for the individual output rating and the relevant common-terminal limits.

The SR2B122BD also retains its stored program in EEPROM memory for up to ten years at the specified reference temperature of 25°C. This helps preserve the application during a power interruption. Keeping an independent copy of the approved program remains useful when maintaining or replacing the controller.

The SR2B122BD is suited to compact automation applications that need 24 V DC control, a small number of analog-capable inputs, transistor switching, and clock-based functions. Applications requiring mechanical relay contacts, AC output switching, extensive I/O expansion, or advanced motion control may require a different controller or external interface equipment.

Technical Specifications

Product Features

• 24 V DC supply: Integrates with compatible low-voltage DC control systems.
• Eight digital inputs: Processes operating commands, sensor states, and equipment feedback.
• Four analog-capable inputs: Supports compatible voltage measurements and thermistor input configurations.
• Four transistor outputs: Provides semiconductor switching for compatible 24 V DC loads.
• Overload and short-circuit protection: Helps protect the transistor output circuits against specified output faults.
• Built-in real-time clock: Supports programmed schedules and time-dependent operating sequences.
• Integrated LCD display: Provides local access to supported device information and functions.
• Ladder logic and FBD programming: Supports different approaches to developing the control application.
• EEPROM program retention: Preserves stored program data for up to ten years at 25°C.
• DIN rail or screw mounting: Supports installation in suitable electrical control enclosures.

Working Principle

The SR2B122BD monitors its eight input channels, evaluates the stored program, and controls its four transistor outputs according to the programmed conditions. The input channels can handle discrete signals, while four channels can be configured for supported analog measurements. Its real-time clock also allows time-based conditions to be incorporated into the control sequence.

24 V DC Supply → SR2B122BD → Program Execution → Transistor Outputs → DC Control Loads

Digital input signals can come from push buttons, limit switches, proximity sensors, and auxiliary contacts. The controller evaluates these states as part of the application logic. For example, a start command may be accepted only when a machine interlock is satisfied and the relevant equipment is in the correct state.

When an analog-capable input is used, the controller reads a compatible measurement signal and makes the resulting value available to the supported program functions. A temperature-control application may use a compatible thermistor input to determine whether the measured temperature has crossed a configured threshold. The program can then change an output state or modify the operating sequence.

The analog inputs support an 8-bit conversion resolution. This is suitable for basic monitoring and threshold-based control, but it is not equivalent to a high-resolution process measurement system. Signal range, sensor characteristics, and the required measurement accuracy should be considered when designing the application.

The control program can be developed using ladder logic or function block diagram programming. Ladder logic represents operating conditions through contacts, coils, and supported logic elements. Function block diagrams use connected blocks to organize operations such as comparisons, timers, counters, and clock-based functions.

When an output is activated, the corresponding transistor switches its connected 24 V DC circuit. Unlike a mechanical relay, a transistor output does not use moving contacts. It is therefore suited to compatible DC loads where repeated switching is required, provided the current, polarity, and load characteristics remain within the specified limits.

The outputs incorporate overload and short-circuit protection, but these functions do not remove the need to select the load correctly. Inductive loads such as solenoids and relay coils can generate voltage transients. Appropriate suppression and interface components should be used where required by the load and wiring arrangement.

The real-time clock allows the program to combine scheduled operation with sensor inputs and other control conditions. For example, a ventilation unit can be enabled during programmed periods but prevented from starting if a required interlock is not satisfied. The final behavior depends on the program and the external control circuit.

During commissioning, each input should be tested using its intended field device. Analog readings should be compared with known signal values, and every transistor output should be checked against the connected load. Where a thermistor is used, the selected sensor type and the program’s interpretation must match the actual measurement arrangement.

EEPROM memory retains the program for up to ten years at 25°C. Following a power interruption, the restart behavior depends on the controller configuration, program logic, and external circuit states. The machine should be arranged to avoid unexpected or hazardous movement when power returns.

If a DC load does not operate, the diagnosis should distinguish between an unmet program condition, an inactive output, an incorrect polarity or connection, and an overloaded output circuit. For analog-related problems, check the sensor wiring, signal compatibility, and program comparison conditions before replacing the controller.

Role in the Industrial Control Architecture

The SR2B122BD acts as a local programmable controller between field devices and compatible DC control loads. It processes digital signals, evaluates supported analog measurements, applies clock-based logic, and operates its transistor outputs.

Field Sensors and Switches → SR2B122BD → Transistor Outputs → DC Interface Devices and Loads

The controller manages the programmed sequence, while external switching equipment handles loads that exceed its output capabilities or require different switching characteristics. Analog signal compatibility and the machine’s electrical protection requirements must be addressed in the overall design.

Industrial Applications

Installation and Maintenance

1. Verify the model reference: Confirm that the installed unit is SR2B122BD and that its transistor-output configuration suits the application.
2. Prepare the control enclosure: Provide suitable mounting, wiring access, and environmental conditions for the device.
3. Install the controller: Use a compatible 35 mm DIN rail or the supported screw-fixing arrangement.
4. Check the DC supply: Verify that the supply voltage remains within the specified 19.2–30 V DC range.
5. Identify the input channels: Label each digital input and identify which channels are assigned to analog measurement or thermistor functions.
6. Check sensor compatibility: Confirm that each connected sensor matches the supported signal type and input configuration.
7. Wire the transistor outputs: Follow the terminal diagram and verify output polarity, load current, and common-terminal limits.
8. Review inductive loads: Check whether solenoid valves, coils, or other inductive devices require suitable suppression components.
9. Develop the control program: Implement the operating sequence using supported ladder logic or function block diagram programming.
10. Configure clock functions: Set the real-time clock and required schedules for applications that use time-dependent operation.
11. Test inputs and outputs: Verify digital states, analog readings, output switching, and connected equipment responses under controlled conditions.
12. Maintain program records: Keep the approved application, I/O assignments, sensor settings, clock configuration, and wiring diagrams.

If an output fails to operate, check the program condition, output state, polarity, and connected load. If an analog value is incorrect, inspect the sensor wiring and input configuration. Isolate the appropriate electrical supply before carrying out wiring or maintenance work.

Related Components

Recommended Related Models

Key Advantages

• Combines eight digital inputs, four analog-capable inputs, and four transistor outputs.
• Operates from a 24 V DC supply.
• Supports compatible voltage measurements and thermistor input configurations.
• Provides semiconductor switching for compatible 24 V DC loads.
• Includes a real-time clock for scheduled control functions.
• Offers an integrated LCD display and supported programming tools.
• Retains the stored program in EEPROM under the specified conditions.

Technical FAQs

How does the SR2B122BD differ from the SR2B121BD?

Both models operate from a 24 V DC supply and provide 12 I/O points, a display, and a real-time clock. The SR2B122BD uses four transistor outputs, while the SR2B121BD uses four relay outputs. The transistor-output version is suited to compatible DC loads requiring semiconductor switching; the relay-output version offers mechanical contacts for compatible AC or DC control circuits.

Can the transistor outputs operate a 230 V AC solenoid valve directly?

No. The SR2B122BD outputs are intended for 24 V DC operation. A 230 V AC load requires an appropriately rated interface device and a separate AC switching circuit.

What types of measurements can the analog-capable inputs handle?

Four input channels support compatible analog configurations, including voltage measurement within the specified input range and supported thermistor applications. These inputs should not be treated as universal 4–20 mA current inputs; a suitable conversion arrangement is required for a current-output transmitter.

What should be checked if a transistor output stops switching a connected load?

Check the program conditions, output status, supply voltage, wiring polarity, and connected load current. Also inspect the load for a short circuit or excessive demand that could activate output protection. For inductive loads, check the suppression arrangement and the condition of the external wiring before returning the circuit to service.