Allen-Bradley PLC-2/20 Programmable Controller: Comprehensive Industrial Automation Solution

Where it sits in the PLC-2 family

The PLC-2 family was Allen-Bradley's step from the early fixed controllers into modular, rack-based systems. The processor itself carries a 1772-series part number, and the family scaled by processor model rather than by hardware redesign: the /15, /16, /20 and /30 shared the same rack, the same 1771 I/O family and the same programming model, with memory capacity and scan performance stepping up across the range. For a plant running one of these today, the practical meaning is simple — a /20 chassis accepts the same I/O as its siblings, so capacity grows by adding 1771 racks rather than by changing the control architecture.

The 1771 I/O system around it

The PLC-2/20 drives the 1771 I/O family: 1771 input and output modules in the familiar tall beige housings, digital and analog alike, plus specialty modules for counting, loop control and communications. Modules live in 1771 chassis, wired through a single I/O cable back to the processor. That separation is why the platform lasted so long in service — a processor failure meant swapping the processor, while a field-wiring change meant moving a module, and neither touched the other. Collectors of this ecosystem should note the numbering: 1772 is the processor, 1771 is the I/O, 1770 covers programming terminals and network hardware.

Programming and Data Highway

Ladder logic for the PLC-2/20 was written on the 1770-series programming terminals — the T2 and T3 industrial terminals being the common ones — which attached directly to the processor or through a language module. For plant-wide interconnection, the Data Highway token-passing network linked multiple processors and supervisory computers, with the 1770-KF2 module bridging a Data Highway to an RS-232 port on a PC. Programs were stored on tape or EPROM depending on the terminal generation, and any replacement processor has to be loaded before it will run the machine — there is no USB stick here.

Sourcing a legacy controller today

Rockwell ended this platform decades ago; official support channels now point at migration to the SLC 500, then onward to ControlLogix. But a large installed base kept the surplus market alive, and working PLC-2/20 processors, 1771 racks and modules still trade as tested spares. KOEED quotes this family from active and EOL stock — send the part numbers off the processor and I/O modules and get one quote covering stock and cross-references. For modern Allen-Bradley I/O in current production, the same catalogue carries items like the 1769-IQ32 CompactLogix input module.