ControlLogix controller was first launched in 1997 as a replacement for Allen-Bradley's previous large-scale control platform, PLC-5. The ControlLogix platform includes a bulletin 1756 ControlLogix 5550 controller, bulletin 1756 ControlLogix I/O modules, and the RSLogix 5000 programming software platform (now referred to as Studio 5000 Logix Designer). ControlLogix represented a significant technological step forward that included a 32-bit ARM-6 RISC-core microprocessor and an ABrisc Boolean processor combined with a bus interface on the same silicon chip. At launch, the series 5 ControlLogix (also referred to as L5 and ControlLogix 5550) controllers were able to execute the code three times faster than PLC-5. The following diagram is an illustration of the original Logix L5 controller:

The L5 controller is considered to be a PAC rather than a traditional PLC due to its modern design, power, and capabilities beyond a traditional PLC (such as motion control, advanced networking, batching, and sequential control). The ControlLogix platform is built on the ControlBus backplane, which performs like a mini-network and allows devices to be Removed or Inserted Under Power (RIUP).

L5 has since been retired from the lineup, so we will focus on the newer L6 and L7 controllers in this book. Throughout this book, we will be referring to the ControlLogix controllers as PACs, which are the modern day equivalent of PLCs.