The Allen-Bradley 1775-L1B55B PLC-3 Main Processor Module is the main processor module for the Allen-Bradley PLC-3 Controller System installed on the slots of the processor and it is used for providing program execution and memory management. The processor module occupies the 2 leftmost slots on the processor chassis and it draws power from the chassis backplane. The processor features a highly efficient program scan time, using a high-speed program scan which quickly determines if an output is true or false, and it has a real-time internal clock, with stamps for critical programs. The 1775-L1B55B processor module comes with 4 AMD 2903's (bit slice) microprocessors, and it draws 9.4A back-plain current at +5V DC, and 40mA at +15V DC.

To effectively execute programs, the 1775-L1B55B processor module retains onboard I/O image table for high-speed processing of I/O points, and using high-speed programming scanning, the 1775-L1B55B can quickly determine if outputs are true or false. The processor module has dedicated and expandable memory segments, which allows users to effectively maintain a wide range of applications and at the same time increase memory capacity to meet application needs. The 1775-L1B55B offers a flexible data table with output/input image, timer, counter, integer, decimal, binary, and ASCII segments, and the processor performs self-diagnostics via LED Green and Red lights indicators which shows the pass/fail status of the module. The 1775-L1B55B’s operating temperature range is 0 to 60 °C (32 to 140 °F) at 5 to 95% relative humidity, and its storage temperature range is -40 to 85 °C (-40 to 185 °F).