The Allen-Bradley 1775-MS4 is an obsolete memory module that is used with the Programmable Logic Controllers, or PLC, series. To be precise, this error correcting memory module is one of the memory module options required by the following PLC-3 controller processor chassis: the 1775-A1, 1775-A2, and the 1775-A3. It is equipped with a 16,384-word or 16K RAM and it has backplane requirements of +5V DC with 2.5 A, +15V DC with 150 MA, and -15V DC (with 0 A required). Note that the 1775-MS4 memory module has alternate part numbers of 96670801 and 96670802.

Primarily used for automating various industrial processes and machinery, from individual machines to plant automation and integrated manufacturing lines, the PLC-3 series offers not only the traditional features of a controller but also various advanced capabilities that result in better efficiency and improved production operations. This modular, field expandable controller also comes with multiple processors.

Each controller of this series has the following basic system components on average: a main processor chassis with 6 module slots available, power supply, expansion system, front panel comprised of a 16-character alphanumeric display where users can see various diagnostic and status messages, 4 system status indicators, memory modules such as the 1775-MS4 memory module that is inserted on one of the module slots of the processor chassis, a main processor module, a keyswitch, an industrial terminal for editing, loading and monitoring program instructions and report generation messages, and I/O scanner modules. Note that users do not necessarily need all of these system components in order for the system to work.