Description
Meta Description
Mitigate risk before sourcing an obsolete Molex SST-SR4-CLX-RLL 4-channel serial module. Evaluate backplane bandwidth, Direct vs. Paging modes, and long-term stocking strategies.
Technical Performance and Inventory Evaluation
Product Overview
The Molex Brad Woodhead SST-SR4-CLX-RLL is a 4-channel, isolated Modbus serial communication module designed to fit directly into an Allen-Bradley ControlLogix 1756 backplane. This specialty card functions as an industrial gateway, mapping Modbus RTU/ASCII data from field devices directly into the ControlLogix processor memory without requiring extensive custom messaging.
Core Specifications
Hardware configuration dictates how your field devices interface with the ControlLogix system. Ensure engineering matches these electrical parameters prior to hot-swapping a module.
| Parameter | Value |
| Brand & Model | Molex / Brad Woodhead SST-SR4-CLX-RLL (Series 112078) |
| Interface Style | 1756 ControlLogix Backplane Compatible |
| Serial Ports | 4 x 9-pin D-Sub Male |
| Electrical Isolation | Channels independently and electrically isolated |
| Communication Rate | 110 bps to 115.2 kbps Serial |
| Protocols Supported | Modbus RTU Master/Slave, Modbus ASCII Master/Slave |
| Direct Mode Capacity | 248 Input words + 248 Output words |
| Paging Mode Capacity | Up to 32,000 I/O words via Add-On Instruction (AOI) |
| Current Draw | 850 mA @ 5 V DC or 1.75 mA @ 24 V DC |
| Operating Temperature | 0 to +60 °C (+32 to +140 °F) |
Condition Classification
Because third-party communication cards frequently transition through long surplus storage cycles, clear pool definitions protect asset availability:
- New Original (New Surplus): Factory-sealed Molex box, unspent internal hardware life, pristine faceplate pins.
- Factory Sealed: Original packaging intact, containing standard diagnostic software utilities and configuration guides.
- Refurbished (Tested): Pre-owned unit run through full serial loopback port diagnostics and backplane read/write stress testing.
Scenario-Based Field Analysis
Integrating old oil-and-gas tank-farm transmitters into a modern ControlLogix rack reveals why this specific serial module remains heavily utilized. Using the SST-SR4-CLX-RLL allows the processing of four isolated legacy data loops simultaneously. That isolation prevents a single grounded field cable on an outdoor instrument from shorting out the other three ports—though your mileage may vary depending on the quality of your shield grounding at the junction box.
[Modbus Field Loop 1] --(Grounded Short)--x--> [Port 1 Isolated]
[Modbus Field Loop 2] ------------------------> [Port 2 Active] ===> [1756 Backplane]
[Modbus Field Loop 3] ------------------------> [Port 3 Active]
[Modbus Field Loop 4] ------------------------> [Port 4 Active] When upgrading your rack to Studio 5000 v32+, do not assume standard configuration profiles carry over seamlessly. Well, technically it supports integration using the custom Add-On Profile (AOP), but if your module is running firmware older than v2.12, the project may reject the generic I/O profile definition during runtime initialization. In that specific scenario, a flash update to the module or implementing the legacy Add-On Instruction (AOI) structure is required to restore backplane communication.
Inventory Management and Stocking Strategy
Spare Parts Stocking Matrix
Managing carrying cost as a percentage of inventory value requires rigorous tracking, especially since this part number is entering an advanced lifecycle phase. Use the matrix below to balance local buffer stock against supplier lead time variability.
| Part Classification | Criticality Level | Buffer Stock (Min/Max) | Replenishment Strategy |
| A (SST-SR4-CLX-RLL Card) | High (Total Loop Loss) | Min: 1 / Max: 2 | Strategic hoarding; keep 1-2 on-site as insurance |
| B (9-Pin D-Sub Cable Assemblies) | Low (Cable Damage) | Min: 4 / Max: 8 | Just-In-Time (JIT) replenishment |
| C (Configuration Manuals/Software) | Low (Information) | Min: 1 / Max: 1 | Digital archive on local plant network |
Total Cost of Ownership (TCO) and Obsolescence Risks
Holding a specialty network card like the SST-SR4-CLX-RLL on a warehouse shelf carries a real cost penalty of roughly 22% annually. However, because Molex has designated several legacy SST modules for discontinuation, the threat of obsolescence write-offs is eclipsed by the risk of extended downtime during a stock-out incident.
For plants running multiple processing lines with this module, avoid buying one spare per line. Use vendor consolidation and cross-site sharing instead. Stashing a single, tested unit in a climate-controlled central repository protects three or four connected plants, reducing excess holding costs while maintaining a rapid emergency response window.
Last-Time-Buy Planning
When managing legacy field migrations, watch for official product lifecycle changes from the vendor. If your infrastructure relies heavily on these units for multi-channel serial integration, allocate capital for a strategic last-time-buy. Secure these modules from verified surplus vendors before regional supplies dry up, ensuring you have enough hardware coverage to span the remaining lifecycle of the machine.
Technical Comparison: Direct Mode vs. Paging Mode
Data layout within the module affects how the ControlLogix processor scans data blocks.
- Logic Complexity: Direct Mode maps registers straight into the I/O table without writing extra ladder logic. Paging Mode requires an Add-On Instruction (AOI) block to sequence data packets sequentially over the backplane.
- Memory Footprint: Direct Mode tops out out at 248 words of input and 248 words of output data. Paging Mode handles up to 32,000 words, making it suitable for large networks of power meters or flow computers.
- Scan Efficiency: Direct Mode executes synchronously with the standard PLC I/O update cycle. Paging Mode splits large chunks of data across multiple asynchronous cycles, which can introduce latency on critical variables.
Quick Conclusions
- Secure 1-2 units on-site immediately if your field communications rely on all four isolated ports.
- Verify firmware versions against your Studio 5000 revision before deploying a replacement unit.
- Leverage cross-site sharing to offset the 22% annual warehouse holding cost of specialty legacy cards.
- Switch to Paging Mode only when your register count breaks the 248-word threshold.
Real-World Search FAQs
How does the electrical isolation work on individual SST-SR4-CLX-RLL ports?
Each of the four 9-pin serial ports contains independent optical isolation circuitry. This design decouples field loops from neighboring channels and protection circuitry on the 1756 backplane, preventing local surge loops from killing the entire communication rack.
Can this card run Modbus TCP/IP communication alongside serial streams?
No, it cannot. The SST-SR4-CLX-RLL handles serial protocols (Modbus RTU/ASCII) exclusively. If your application demands mixed serial and Ethernet architecture, you must look into the dual-purpose alternative module, model number SST-ESR2-CLX-RLL.
What is the primary cause of configuration mismatch faults during hotswaps?
A mismatch fault typically occurs when a replacement card features old firmware that does not recognize the modern Add-On Profile (AOP) slots inside the newer Studio 5000 environment. Always match the firmware revision level stamped on the side of the incoming module with the parameters configured inside your logic file.
Channel Adaptation Notes
LinkedIn Technical Post
Sourcing legacy networking components like the Molex SST-SR4-CLX-RLL presents an infrastructure challenge. With vendor support cycles shifting, automation teams face a balancing act: accept high warehouse carrying costs or risk extended stock-out incidents on legacy manufacturing lines.
In our plants, we treat these high-criticality serial cards as insurance policies. We keep limited backup units on-site, but we enforce strict firmware tracking across all assets. This steps around version conflicts during unexpected field replacements.
Are you proactively planning last-time-buys for your serial architectures, or are you migrating directly to native Ethernet devices? Let's discuss migration tracks below. B2B Marketplace Q&A
- Question: We need an exact replacement for a bad Woodhead module marked SST-SR4-CLX-RLL. Do you have these original factory sealed, and what is the standard lead time?
- Answer: We maintain original, factory-sealed surplus inventory of the SST-SR4-CLX-RLL (Series 112078) in our main warehouse. Because these units are vital for keeping legacy Modbus loops online, standard shipping clears within 1–3 business days from payment verification. Each unit carries a full 12-month technical performance warranty, and we provide tracking details directly upon carrier pickup to minimize your total downtime window. Let us know if you need us to check the internal firmware version before shipping out.
