Practical Guide to PLCs
Pocket Field Reference
A pocket field reference for architecture, scan cycles, hardware sizing, ladder logic, PID loops, and industrial networking. Maintained by Johnson Automation Group, Three Mile Creek Services, and Creekbound Services.
What is a PLC?
A Programmable Logic Controller (PLC) is a miniature industrial computer designed to automate electromechanical processes (assembly lines, packaging, food processing, amusement rides) in harsh environments with extended temperature ranges, electrical noise immunity, and vibration resistance.
Microprocessor + memory system. Operates in Programming Mode (accepts PC logic changes) or Run Mode (executes control loop in ~1/1000th sec).
Connects field sensors & actuators to the CPU via Discrete (ON/OFF), Analog (continuous range), or Specialty modules.
Interactive Scan Cycle Loop
Tap any step or press Step/Auto to inspect the repeating loop
Detects the physical ON/OFF or analog state of all field input devices (pushbuttons, proximity sensors, transmitters) connected to the PLC and snapshots them into the input image table.
History of the PLC
The Hardwired "Rat's Nest"
Factories relied on 50 to 100+ electromechanical power and control relays, pneumatic plunger timers, and rotary drum sequencers inside 6×4 ft enclosures. Worn contacts, coil failures, and point-to-point engineering changes created unreliable wiring "rat's nests."
GM Hydra-Matic & Dick Morley
General Motors issued a proposal for a "standard machine controller." Dick Morley (father of the PLC) and Bedford Associates responded with four core requirements:
- Solid-state system flexible like a computer, priced competitively with relays.
- Programmed and maintained in familiar Relay Ladder Logic.
- Ruggedized against factory dirt, moisture, EMI, and vibration.
- Modular form for easy component replacement and expandability.
From 125 Words to Commercial Breakthrough
An early prototype with 125 words of memory was too slow to match relay response times ($1/60\text{ s}$), prompting expansion to 1K and 4K words. Bedford Associates spun off Modicon (MOdular DIgital CONtroller) with prototype #84 (Modicon 084), followed by Michael Greenberg's Modicon 184, which made PLCs take off worldwide.
How to Choose a Controller
CPU Memory & Architecture Estimator
Based on Chapter 3 Step 6 Rule of Thumb (p. 13)
View All 9 Controller Selection Steps (pp. 10–13) ▼
PLC Hardware & I/O
The handbook compares PLC hardware to the human body: the Brain (CPU), the Spine (Base / Backplane data bus), and the Senses (I/O Modules).
CPU & fixed I/O in one housing. Lowest cost, compact footprint.
Interlocking I/O blocks. Removing a middle module breaks downstream bus.
Slotted rack backplane. Slide any card out without disturbing others.
DC Discrete I/O: Sinking vs. Sourcing
Tap to compare current path between Field Device & PLC
Sinking PLC Input: The PLC terminal provides the $0\text{ VDC}$ reference path (ground) to complete the circuit. Pair a Sinking PLC Input card with a Sourcing (PNP) field sensor.
PLC Software & Ladder Logic
5-3 Live Rung Simulator: Motor Seal-In
Tap the field switches below to test classic seal-in latch logic
How it works: Tapping START (X1) energizes coil Y1, which immediately closes the parallel seal-in contact MOTOR_Y1 so the motor keeps running after START is released.
Practical PLC Topics
6-1 Methods Behind PID Loop Control
p. 49Closed-loop control continuously calculates the error $e(t) = \text{SP} - \text{PV}$ between a desired Setpoint ($\text{SP}$) and measured Process Variable ($\text{PV}$) to drive the Control Output ($\text{CV}$):
Reacts to present error magnitude ($K_p \cdot e$).
Eliminates past steady-state offset ($\int e\,dt$).
Dampens overshoot via rate of change ($de/dt$).
6-2 & 6-3 Industrial Comms & EtherNet/IP
pp. 54–60Serial standards (RS-232 point-to-point up to ~50 ft; RS-485 multi-drop up to 4,000 ft) and protocols like Modbus RTU/TCP and EtherNet/IP connect PLCs to HMIs, drives, and SCADA.
Used for asynchronous, on-demand data, configuration, and diagnostics.
High-speed, scheduled multicast/unicast polling for time-critical I/O control.
6-4 Motion Control Explained
p. 61Typically open-loop using high-speed Pulse & Direction (PTO) commands. Cost-effective and high torque at low speeds.
Closed-loop with encoder feedback for dynamic loads, high speed, and tight positioning accuracy.
Collection of PLC Application Stories (p. 66) ▼
The handbook concludes with real-world automation stories illustrating how modern PLCs replace legacy relay panels across industries:
- Packaging & Material Handling: High-speed photoeye registration, conveyor indexing, and reject diverters.
- Water & Wastewater Treatment: Submersible level transmitters ($4\text{–}20\text{ mA}$) driving VFD pump staging and remote telemetry.
- Food, Craft Brewing & Batching: Automated PID mash/kettle temperature loops with C-more HMI recipe control.