The 1775-L1 Main Processor Module by Allen-Bradley is the main processor module for the Allen-Bradley PLC-3 programmable controller system, which occupies 2 processor slots on the chassis, and is used for program execution and memory management. The 1775-L1 is installed in the factory-recommended 2 leftmost slot of the processor chassis, and it draws power from the chassis backplane. The 1775-L1 main processor module has 4 AMD 2903's bit slice Microprocessors with a Level 1 Instruction Set for effective program execution, and the module draws a 9.4A backplane current at +5V DC input, and a 40mA backplane minimum current at +15V DC. To effectively execute the PLC-3 Controller programs, the 1775-L1 retains an onboard I/O image data table for the high-speed processing of the I/O points, and it uses the high-speed program scanning feature to quickly determine if the controller's outputs are True or False.

The 1775-L1 has inbuilt memory with dedicated expandable areas that include the system status, the system pointers, the data table, the user program, the system symbols, and the force table which allow users to handle a variety of applications, and to increase the memory size easily to meet application needs. The 1775-L1 features a Flexible and Expandable Data Table with an output image, an input image, a timer, a counter, a Decimal/Binary/ASCII system, and a pointer, and it features a real-time interrupt routine, which performs real-time events by requesting for subroutine on a real-time basis. The 1775-L1 also has a real-time internal clock/calendar time and date-stamped for programs and reports, and it has LED Diagnostic indicators which indicates pass or fail status. It is designed for use at a temperature of 32 to 140 °F at a relative humidity range of 5 to 95% without condensation, and it has a storage temperature range of -40 to 185 °F.