Allen Bradley 1775-MEF Error Correcting Memory Module for PLC-5 Processors

The Allen Bradley 1775-MEF is a 256K error correcting memory module designed for PLC-5 series programmable logic controllers. This module uses ECC (Error Correction Code) technology to detect and correct single-bit errors in real time, significantly enhancing data integrity and system reliability. It is intended for critical control applications where memory corruption could lead to machine malfunction or production loss. The 1775-MEF is compatible with PLC-5/15, /25, /40, and /60 processors as well as some 1775 series backplanes.

Technical Specifications

The 1775-MEF provides 256 kilobytes (256K x 16 bits) of static RAM with ECC. The module operates at 5 VDC supplied from the backplane, consuming 1.2 A typical. Access time is 70 ns, fully compatible with PLC-5 scan cycles. The error correction logic can detect and correct single-bit errors and detect double-bit errors, triggering a non-recoverable fault if multiple bit errors occur. The module includes a status LED (green/red) to indicate power and error conditions.

Functionality and Error Handling

When installed, the 1775-MEF automatically takes over the memory function from the processor’s onboard RAM. The ECC logic continuously monitors the memory bus. If a single-bit error is detected, the module corrects it on the fly and logs the event in a status register that can be read by the PLC program. Double-bit errors are not correctable but will generate a hardware fault, causing the processor to go into fault mode. This proactive error detection prevents silent data corruption, which is crucial for safety-related applications.

Installation and Compatibility

The 1775-MEF plugs into a dedicated memory slot on the PLC-5 processor (usually the leftmost slot of the processor chassis). Before installation, the processor must be powered off. The module is keyed to prevent incorrect insertion. It is compatible with PLC-5 processors that have firmware revision B or later. The module requires no additional configuration; the processor automatically recognizes its presence and size. However, the user program must not exceed the available memory (256K minus system overhead).

Environmental Ratings

Operating temperature: 0°C to 60°C (32°F to 140°F), storage -40°C to 85°C. Humidity: 5% to 95% non-condensing. The module is rated for industrial environments with vibration resistance of 2G at 10-500 Hz (IEC 60068-2-6). It meets UL 508, CSA C22.2 No. 142, and CE (EMC directive) standards. The 1775-MEF is RoHS compliant and carries a Class 1 static sensitive device rating; proper ESD precautions must be observed during handling.

Typical Applications

This memory module is used in PLC-5 based systems that handle critical processes such as chemical batch control, power generation, and high-speed packaging. It is particularly valuable in environments with high electrical noise or radiation where memory corruption is more likely. The error correcting feature reduces unexpected processor halts, improving overall equipment effectiveness (OEE). Many system integrators choose the 1775-MEF for legacy PLC-5 upgrades where reliability is paramount.

Maintenance and Lifecycle

The 1775-MEF has no user-serviceable parts and does not require battery backup because it is non-volatile? Actually, SRAM is volatile; the PLC-5 processor provides battery backup for the entire memory system. Therefore, the processor’s backup battery must be maintained as per standard practice. The module has a typical MTBF (Mean Time Between Failures) of over 200,000 hours. Allen Bradley (Rockwell Automation) has discontinued the 1775 series, but this module remains available through the aftermarket. For new designs, consider CompactLogix or ControlLogix systems.

Ordering and Documentation

Order number: 1775-MEF. The module is supplied in an anti-static bag. Installation instructions are provided in publication 1775-5.1. For programming, no special instructions are required; standard PLC-5 addressing applies. When replacing a standard memory module (non-ECC) with the 1775-MEF, ensure the total program and data size does not exceed 256K. The ECC function is transparent to the user program and can be monitored via status file S:33 and S:34 bits.