PLC Architecture

Three shapes of controller

Modular

A chassis with a backplane and slots. A power supply, a processor, and I/O or communication modules each occupy a slot. The chassis size sets the number of modules, and additional chassis can be added locally or remotely. This is the architecture of most plant and large station controllers.

Compact

The processor, power supply, and a fixed set of I/O in one housing, often with a few expansion modules that snap onto the side. Fewer parts, lower cost, and a hard ceiling on I/O. Most small lift stations, packaged systems, and machine controls use one.

Distributed

A processor with little or no local I/O, and I/O blocks placed around the plant on a network. The wiring runs a few feet to a nearby block instead of hundreds of feet to a central panel. The network becomes part of the control system, with everything that implies.

The backplane

The backplane is the printed circuit board at the back of the chassis that every module plugs into. It carries the regulated power from the power supply to the modules and it carries data between the processor and the modules. The processor reads every input module and writes every output module across the backplane on each scan, and the slot a module sits in is how the processor identifies it.

Two consequences follow. A module in the wrong slot is a different address, which is why moving modules around a chassis is a programming change and not just a hardware change. And the backplane has a power budget: each module draws a defined current from it, and a full chassis of high-draw modules can exceed what the power supply delivers, which shows up as intermittent module faults rather than a clear failure. The manufacturer publishes the draw of each module and the budget of each supply, and the sum is checked at design time.

What lives in the slots

• The power supply, which on most modular platforms is a module or bolts to one end of the chassis, converts line voltage or 24 VDC to the backplane voltages.
• The processor, in a fixed slot on some platforms and any slot on others, holds the program and drives the backplane.
• Discrete input and output modules, in groups of 8, 16, or 32 points, at the field voltage the devices use.
• Analog input and output modules, in groups of 4 to 16 channels, for 4 to 20 mA, voltage, RTD, and thermocouple signals.
• Communication modules for Ethernet, serial, and fieldbus connections to SCADA, drives, and other controllers.
• Specialty modules for high-speed counting, motion, weighing, and safety, where the timing or the certification exceeds what the processor does over the backplane.

Local, remote, and distributed I/O

When the I/O outgrows one chassis, or the equipment is far from the panel, the backplane is extended. Local expansion chassis connect with a short cable and behave like more slots. Remote racks connect over a network, EtherNet/IP, ControlNet, PROFINET, or a vendor scheme, and the processor exchanges I/O data with them on a schedule rather than on every scan.

Redundancy

Where a controller stopping is not acceptable, platforms offer redundant processors: two processors, usually in separate chassis, one running and one synchronized and ready to take over within a scan or two when the primary fails. The I/O is on remote racks reachable by both. Redundancy is expensive, it adds its own failure modes, and it does not protect against a program error, which both processors will execute faithfully. It belongs where a stopped controller costs more than the redundancy does, which in water and wastewater usually means a large plant, not a lift station.

Design decisions that follow from the architecture

• Spare slots. A chassis at capacity on day one has no room for the module that the first change order needs. Twenty percent spare slots and spare I/O points is a common rule.
• Heat. Modules dissipate power into the enclosure, and a chassis full of relay outputs and drives in the same enclosure needs a thermal calculation. The enclosure heat load calculator covers it.
• Grounding. The chassis is bonded to the panel ground, and analog signal shields land at a single point. The grounding pages cover the practice.
• Slot layout. Analog modules away from relay outputs and away from the power supply, and communication modules where their cables can be routed without crossing power wiring.
• Documentation. The rack layout drawing, with every slot, module part number, and address range, is the drawing a technician opens first at three in the morning.