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Key Technical Specifications
- Part Number: 1757-SRM/B
- Manufacturer: Allen-Bradley / Rockwell Automation
- Product Family: ControlLogix system redundancy module
- Module Type: Redundancy module
- Slot Width: 2 slots
- Supported Controllers: 1756-L61, 1756-L62, 1756-L63, 1756-L64
- Supported Communication Modules: 1756-CNB series D/E, 1756-CNBR series D/E, 1756-ENBT, 1756-EWEB
- Cable Type: 1757-SRCx fiber-optic cable
- Cable Connector: SC-type
- Fiber Type: 62.5/125 micron multimode
- Cable Length Options: 1 m, 3 m, 10 m, 50 m, 100 m
- Wavelength: 1300 nm
- Backplane Current @ 3.3 V DC: 0.75 A
- Backplane Current @ 5.1 V DC: 1.0 A
- Backplane Current @ 24 V DC: 0.160 A
- Power Dissipation: 11.28 W max
- Thermal Dissipation: 38.49 BTU/hr max
- Operating Temperature: 0-60 °C
- Weight: 0.452 kg
- Enclosure Rating: None, open-style
Product Introduction
The Allen-Bradley 1757-SRM/B is a ControlLogix and ProcessLogix system redundancy module used to keep primary and secondary controllers synchronized. It sits in a redundant chassis pair and uses a fiber-optic link to manage crossloading and failover behavior.
This module is chosen where uptime matters and a single controller cannot be the only path. The 2-slot design, SC fiber interface, and support for legacy ControlLogix 5560 controllers make it a key part of many older redundant PLC systems.
1757-SRM/B
1757-SRM/B
1757-SRM/B
Troubleshooting Quick Reference
| Symptom | Possible Cause | Relevance to this Part | Quick Check Method | Recommendation |
|---|---|---|---|---|
| Redundancy will not initialize | Missing cable, wrong cable type, or incompatible controller pair | ✅ High | Verify the 1757-SRC fiber cable and controller model numbers | Match hardware and cable before assuming module failure |
| Primary and secondary will not sync | Firmware mismatch or configuration mismatch between chassis | ✅ High | Compare controller firmware and redundancy settings | Align both sides to the same approved revision |
| Redundancy LEDs do not follow expected sequence | Module not seated, chassis issue, or failed SRM board | ✅ High | Reseat the module and watch the LED startup pattern | Check seating and chassis power first |
| Communication drops during switchover | Damaged fiber, dirty SC connector, or weak optical link | ✅ High | Inspect connector ends and test with a known-good cable | Clean or replace the fiber link before replacing the module |
| Module runs hot | Excessive chassis load or poor cabinet ventilation | ⚠️ Medium | Measure cabinet temperature and backplane load | Fix the thermal problem before changing hardware |
| Controller pair stays out of sync after power cycle | Wrong slot position or unsupported controller combination | ✅ High | Verify exact chassis slots and controller family | Recheck the system design against the installed hardware |
| Diagnostic buffer shows redundancy fault | Cable break, module fault, or configuration error | ✅ High | Read the controller diagnostics and inspect the fiber path | Use diagnostics to isolate cable versus module failure |
If you are stuck, send support photos of the LED state, fiber cable routing, and diagnostics logs.
Frequently Asked Questions
Q: What is the 1757-SRM/B used for?
A: It is a system redundancy module for ControlLogix and ProcessLogix controllers. Its job is to keep the standby controller synchronized so the system can recover from a primary-side failure.
Q: Is the 1757-SRM/B the same as the 1756-RM?
A: No. The 1757-SRM/B is the older 2-slot redundancy architecture, while the 1756-RM family is the newer style. They are not drop-in interchangeable.
Q: What cable does it use?
A: It uses 1757-SRCx fiber-optic redundancy cables with SC connectors and 62.5/125 micron multimode fiber. Cable length matters, so order the actual run length, not a guessed one.
Q: Can I mix controller revisions?
A: I would not. Redundancy systems are unforgiving when firmware or controller family does not match. Take the old configuration seriously and verify both sides before swap-out.
Q: Why is this part often sold as New Surplus?
A: Because the 1757-SRM/B is a legacy module. Most available stock comes from unused project leftovers, shelf inventory, or discontinued-system holdings.
Q: Does this module hold my logic?
A: No. It supports redundancy behavior; it does not store the control program. Your application stays in the controllers and the engineering project.
Q: What is the most common install mistake?
A: Using the wrong cable or putting the module in the wrong chassis slot. That is the classic failure mode. Take photos before removal and match the exact layout on re-install.
