Schmersal AES 1235 Replacement: Upgrade to SRB-E-201ST

---

1. The Imperative for Upgrading Legacy Safety Systems

The industrial landscape is in constant motion, driven by evolving safety standards and the demand for higher machine uptime. Components that were once the industry standard, such as the Schmersal AES 1235 safety monitoring module, are now classified as phased-out or discontinued products. The AES 1235 was a workhorse for monitoring guard door position switches and E-Stop commands, typically achieving Safety Category 3 and Performance Level (PL) d according to the now-superseded EN 954-1, but its limitations in modern diagnostics and multi-functionality necessitate an upgrade.

The successor product family, the Schmersal SRB-E-201ST multi-functional safety relay, represents a significant leap forward in safety technology. The transition from the single-function AES 1235 to the highly configurable SRB-E-201ST is not just a component replacement; it is a critical migration to enhance machine safety integrity and streamline maintenance operations, ensuring compliance with the current global standard, EN ISO 13849-1 (up to PL e).

---

2. Operational Design Philosophy: Dedicated vs. Configurable Safety

2.1. Defining the Core Safety Function

The fundamental difference between the two modules lies in their operational design philosophy. The AES 1235 is a dedicated, single-function safety relay. Its primary, and almost exclusive, purpose is the evaluation of signals from magnetic safety sensors (BNS range) or limit switches, coupled with an obligatory feedback circuit monitoring external contactors. Its internal logic is fixed, making it highly reliable for its intended, specific application.

In contrast, the SRB-E-201ST is a multi-functional, configurable module. While it can seamlessly replace the AES 1235 in its core application (guard door monitoring with two-channel control), its internal logic can be set for up to 12 different applications, including two-hand control, safety mat evaluation, and light curtain monitoring, all via a simple rotary switch or jumper setting.

* Engineering Perspective: An engineer deciding on a replacement should not simply look at terminal equivalence. If the objective is to standardize safety components across various machine types and reduce spare parts inventory, the multi-functional SRB-E-201ST offers a distinct advantage. If the application is extremely basic and will never change, the simple logic of the AES 1235 might seem appealing, but this is often outweighed by the long-term obsolescence risk and the inability to meet modern diagnostic standards.

2.2. Safety Classification and Diagnostic Capabilities

A key driver for the upgrade is the ability to achieve higher and more transparent safety classifications.

Feature	Schmersal AES 1235 (Legacy)	Schmersal SRB-E-201ST (Modern)
Max. Performance Level (ISO 13849-1)	Typically PL d	Up to PL e
Safety Category (ISO 13849-1)	Category 3	Category 4
SIL (IEC 62061)	Suitable for SIL 2	Up to SIL 3
Diagnostic Coverage (DC)	Lower (Fixed, focused on main relay contacts)	Higher (Comprehensive internal diagnostics)
Status/Fault Indication	Single LED (Status/Fault coded by pulse sequence)	Five Dedicated LEDs (RUN, ERR, IN 1/2, OUT)
Cross-Circuit/Wire Break Monitoring	Yes	Yes (Enhanced, integral wire breakage detection)
Terminals	Screw Terminals (Fixed)	Plug-in Screw Terminals (Removable/Coding capability)

From a technical safety audit standpoint, the SRB-E-201ST is decisively superior because it is certified up to the highest level, PL e / Category 4 / SIL 3. This higher integrity level provides a more robust and dependable safety system, which is a non-negotiable requirement for new and significantly modified machinery.

---

3. Safety Standard Compliance Deep Dive: PL d to PL e Transition

The migration from Performance Level d (PL d) to Performance Level e (PL e) is the single most compelling reason for the upgrade, especially for new or substantially modified machinery in the EU and other regions adhering to ISO standards. PL is a discrete level used to specify the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions.

3.1. The Role of Diagnostic Coverage (DC)

The achievement of PL e requires a higher level of Diagnostic Coverage (DC) and a lower Probability of Dangerous Failure per Hour ($PFH_D$) compared to PL d.

• The AES 1235 typically relies on the switching mechanics of two force-guided relay contacts and the monitoring of the external feedback loop (X1-X2) for its diagnostic function, limiting its DC to a Medium level. Its diagnostics are primarily reactive, monitoring for external component failures (stuck contactors, short circuits).
• The SRB-E-201ST employs electronic safety outputs (PNP) and continuous internal monitoring of its redundant microprocessors and signal paths. This intrinsic redundancy and real-time self-testing elevate its DC to a High level, allowing it to detect a higher percentage of potential internal and external faults before they lead to a dangerous state.

* Impact on Legal Compliance: An engineer must consider the legal exposure. While the AES 1235 may have been compliant when installed, regulatory bodies increasingly demand the highest achievable safety integrity for systems interacting with high-risk machinery. The PL e capability of the SRB-E-201ST future-proofs the installation against more stringent safety audits and minimizes the liability associated with using phased-out components in critical applications.

3.2. Failure Mode Analysis: Mechanical vs. Electronic

The AES 1235 is fundamentally a relay-based module; its failure modes include:

• Welded Output Contacts: The most common failure in any relay, requiring the AES 1235’s feedback loop to detect this fault via external contactors.
• Mechanical Fatigue: The inherent limitation of mechanical components, affecting the specified mechanical life and requiring periodic replacement.
• Ambiguous LED Coding: Reliance on a single LED with pulse codes (e.g., three short flashes) to indicate a specific internal or external fault, leading to significant troubleshooting delays.

The SRB-E-201ST, by using semiconductor outputs, eliminates the primary mechanical failure mode of welded output contacts. It has no physical contacts to weld in the output stage. Its failure modes are governed by sophisticated electronic diagnostics, which are:

• Software/Microprocessor Faults: Detected immediately and redundantly monitored by dual CPUs, leading to a prompt safe shutdown (STOP 0).
• External Short Circuits: Detected by the internal overcurrent/short-circuit protection on the PNP outputs, preventing damage to the module itself.

This shift from mechanical to solid-state output technology is a decisive factor when judging reliability and longevity.

---

4. Real-World Deployment Scenario

The differences between the two relays become most apparent in real-world industrial settings, particularly within a high-speed automated packaging line.

The AES 1235 would typically be deployed at a single guard door accessing the robot cell. Its function is basic: if the magnetic sensor is closed, and the feedback from the load contactors (monitored by the feedback loop X1-X2) is confirmed, the machine is allowed to run. If a fault occurs (e.g., a short circuit in a sensor line, indicated by a coded LED flash), the line is stopped, and a maintenance technician must consult a paper or PDF manual to decode the single LED's flash sequence (e.g., two flashes mean a fault on inputs S2). This diagnosis can be time-consuming, leading to substantial downtime. Furthermore, the AES 1235's output is typically a simple relay, which requires external fusing or protection.

The SRB-E-201ST, in the same application, offers a much more streamlined experience. If a fault occurs, the dedicated ERR LED illuminates, and the separate IN 1/2 or OUT LEDs provide immediate visual feedback on the state of the inputs and outputs. The SRB-E-201ST also features safe semiconductor outputs (PNP) with a higher 5.5 A switching capacity and integrated electronic fuse/protection. This capability allows the module to be connected directly to the control system or a higher-level safety controller without the need for additional, bulky interposing relays or fuses, simplifying the control panel layout and reducing wiring complexity.

* Decision Flow for High-Speed Applications: If the primary objective is to minimize unplanned downtime in a production environment, the enhanced and immediate visual diagnostics of the SRB-E-201ST provide a compelling justification for the upgrade. It shifts the diagnostic process from "decoding a blink sequence" to "immediately identifying the affected channel."

---

5. I/O Integration and Compatibility: The Advantage of Safe PNP Outputs

The transition from traditional relay outputs (AES 1235) to safe semiconductor PNP outputs (SRB-E-201ST) fundamentally changes how the safety circuit integrates with the main control system (PLC).

5.1. Direct Integration with Modern Safety PLCs

The AES 1235 uses voltage-free contacts (dry contacts). To interface with a safety PLC, these contacts must feed into the safety PLC's digital inputs, often requiring an intermediate component to handle the coil current of the external contactors.

The SRB-E-201ST outputs are PNP (p-switching fail-safe performance semiconductor outputs). These are high-current (up to 5.5 A) digital outputs that can connect directly to the inputs of advanced safety controllers, such as a Schmersal PROTECT PSC or a modern safety PLC. This direct digital-to-digital connection eliminates the need for:

• Interposing Relays: Reducing panel space and wiring complexity.
• External Fusing: The SRB-E-201ST’s outputs are electronically protected.

* System Architecture Consideration: In larger, networked systems, the use of the SRB-E-201ST streamlines the safety architecture. Instead of wiring two safety relays for a single guard door into a complex harness, the SRB-E-201ST acts as a smart, multi-functional local controller that provides reliable status and safety signals directly to the network. This approach significantly reduces the potential for wiring errors, which are a leading cause of safety system malfunctions.

5.2. Signal Integrity and Cascade Connections

In applications requiring the cascading of multiple safety devices (e.g., a series of guard doors along a conveyor), the SRB-E-201ST excels. The solid-state output ensures a cleaner, faster switching signal compared to the inherent contact bounce and slower response time of mechanical relay outputs in the AES 1235. This improved signal integrity is crucial in meeting the stringent reaction time requirements defined in high-PL safety circuits.

---

6. Installation and Maintenance Notes

The experience of an electrician or maintenance engineer on the factory floor is fundamentally different when working with these two generations of relays.

6.1. Wiring and Terminal Management

The AES 1235 uses traditional, fixed screw terminals. This necessitates removing and re-wiring every connection point if the module needs to be replaced—a process that introduces human error, requires recalibration of torque, and increases replacement time.

The SRB-E-201ST utilizes plug-in screw terminals with coding. This is a major structural enhancement. An engineer can pre-wire the terminal blocks and simply plug them into the module. If the SRB-E-201ST needs replacement, the entire module can be swapped out quickly by un-plugging the terminal block, leaving the field wiring intact. The coding feature prevents the wrong terminal block from being plugged into the wrong module, eliminating a common and critical installation error.

6.2. Configuration and Setup

The AES 1235 is virtually unchangeable; its functionality is hard-wired. Any modification to the safety circuit's logic (e.g., adding an immediate start function) would require an entirely different AES model.

The SRB-E-201ST features a small, protected rotary switch or internal jumper settings. This allows the maintenance team to select one of its various functions (e.g., single-channel or dual-channel input, monitored manual start, or automatic start) without changing the hardware. This flexibility is invaluable during machine commissioning or minor modifications, significantly reducing the time required for system validation. For instance, converting a circuit from a manual start (with a Start push button connected) to an automatic start simply involves changing a setting on the SRB-E-201ST, instead of purchasing and installing a new, dedicated relay.

---

7. Operational Lifetime and Total Cost of Ownership (TCO) Analysis

To provide a comprehensive view for long-term operational planning, a comparative analysis based on component longevity, maintenance burden, and standards compliance is necessary. The initial cost of the SRB-E-201ST may be marginally higher than securing a legacy AES 1235 unit, but the TCO reveals a stark contrast.

Metric	Schmersal AES 1235	Schmersal SRB-E-201ST	Comparative Interpretation
Max. Switching Capacity (Safety Outputs)	3 A (AC-15) / 2 A (DC-13)	5.5 A (PNP Outputs)	Higher capacity minimizes the need for external interposing relays.
Replacement/Swap Time	High (Requires complete re-wiring of fixed terminals)	Low (Plug-in terminals for quick hot-swapping)	Direct impact on reducing unplanned downtime.
Mission Time (T10d for Safety)	Shorter, often unspecified for legacy units.	20 Years (Certified)	Provides long-term planning certainty for safety integrity.
Width	22.5 mm	22.5 mm	Direct form-factor compatibility for DIN rail mounting.

From this data, it is evident that while the physical size is identical, the modern SRB-E-201ST provides a significantly higher operational envelope (5.5 A switching capacity) and drastically superior ease of maintenance (plug-in terminals) compared to the older AES 1235. An investment in the SRB-E-201ST provides a certified 20-year mission time, making it a future-proof choice that aligns with modern machinery lifecycles and significantly lowers the risk associated with obsolescence.

---

8. Strategic Decision Framework for Safety Upgrades

When faced with a failed or failing AES 1235, the choice between finding a surplus legacy part and upgrading to the SRB-E-201ST is best approached by considering the long-term strategic value.

If a maintenance team's primary constraint is budget and minimum effort, they might opt for a direct, like-for-like replacement (AES 1235). This is a short-sighted approach that simply kicks the can down the road, as the unit is officially discontinued and compliance issues persist. This is a choice that prioritizes immediate, minimal expenditure over long-term risk and operational efficiency.

However, if the primary constraint is machine uptime, long-term safety compliance (PL e), and standardization, the SRB-E-201ST is the clear and superior choice. The initial time spent on re-wiring and verifying the new multi-functional module is quickly recouped through simplified diagnostics (five dedicated LEDs vs. one blinking LED), reduced spare parts complexity, and the elimination of a major future obsolescence risk. Furthermore, the higher current capacity and integrated diagnostics inherently provide a more resilient and dependable safety circuit. The upgrade transforms a legacy system into one fully compliant with the highest international safety standards.

---

Note to Readers: This article offers technical analysis for informational purposes only. Always consult the official product documentation and local safety regulations before implementing any changes to critical machine safety systems.

The author assumes no liability for any loss, damage, or malfunction resulting from the use or application of this information. Use is strictly at the reader's own risk.