Industrial Engineer · phase 1 of 16

CPU / Processor

Controller module that executes the user program, manages memory and communications, and coordinates I/O updates.

The Explain card

Plain English
The CPU is the module that actually thinks. It runs your program, keeps the tags in memory, talks to the network cards and tells the I/O when to update.
Example
On a ControlLogix rack the CPU is the slot with the key switch and the status lights. Pull it out and the rest of the rack is just powered metal.
Why it matters
Scan time, memory size and how many connections you can open all come from this one module. When a project outgrows its controller, this is the part you upgrade.
Hook
Everything else in the rack is waiting on this slot.

Word knowledge

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

On the S7-1500 rack the CPU in slot 1 reads the infeed photo-eye and writes the 24 V DC output to the motor starter coil.

Letter by letter
  • C Central the slot the rest of the rack takes its updates from
  • P Processing the work of solving the program, not a person at a desk
  • U Unit a removable module, not a chip soldered to a board
Where it came from
Origin
English computing compound from central, processing, and unit. Mainframe manuals of the early 1960s printed the three-word title; PLC catalogs later borrowed the initialism for the rack module.
Entered the language
early 1960s
What changed
First the arithmetic and control heart of a mainframe, then the chip inside any computer. Plant vendors kept the three letters for the module in the rack, and Processor stayed the Allen-Bradley catalog word for that same slot.
How it is spelled
Capitalisation
Written CPU in three capitals even mid-sentence. Processor, the other shop name, is capitalized as a part name on drawings, not as a job title.
Plural and possessive
The plural is CPUs; the possessive is CPU's. Never use CPU's for more than one.
Out loud
Spelled out letter by letter.

Same family

  • GPU
  • MPU
  • ALU
  • APU
Broken into chunks
  • central
    • center
    • concentric
    • decentralize
  • process
    • processor
    • processing
  • unit
    • unity
    • unite
    • unitary
What it sits beside

Same subject

Same shape

  • GPU
  • MPU
  • ALU
  • APU

Rack modules

Processing-unit initialisms

  • GPU
  • MPU
  • ALU

Where it sits in the deck

Phase 1: PLC Controller Hardware

The box itself, from processor and rack to the modules that plug into it, because nothing else in the deck lands until you know what a PLC physically is.

Neighbours in this phase