The PLC Processor

What the processor does

Every scan the processor performs the same sequence: it copies the state of the input modules into the input image, solves the program top to bottom against that image, writes the resulting output image to the output modules, and then services communications and housekeeping. The scan cycle page covers the order and its consequences in detail. The processor is also the module that talks to the programming software, to SCADA, and to other controllers, and it runs the diagnostics that decide whether the system is healthy.

Processing speed matters less than most people expect. Scan time is dominated by the size of the program and the volume of I/O and communication, and a controller that scans in ten milliseconds is not noticeably better than one that scans in twenty for a pump station. Determinism matters more: the scan must complete within a bounded time, every time, and the watchdog enforces it.

Memory

Program memory

Holds the logic, the routines, and the configuration. Sized in kilobytes or megabytes depending on the platform, and a full program memory stops further downloads.

Data memory

Holds the data table: input and output images, timers, counters, integers, floats, strings, and the user-defined structures. This is what the program reads and writes and what SCADA reads.

Retentive memory

The portion of data memory that survives a power cycle, backed by a battery, a capacitor, or nonvolatile storage. Setpoints, totalizers, and run hours live here. What is retentive and how it is protected is a platform property covered on the retentive memory page.

Nonvolatile program storage

A memory card or internal flash that holds a copy of the program so that a controller that has lost its memory can restore itself on power-up. Whether it loads automatically, and whether it overwrites recent online edits, is a setting worth knowing.

Modes

The processor is always in one of a few modes, selected by a keyswitch on the module, by the programming software, or both. The names differ by vendor, but the behaviors are the same everywhere.

Status lights

The lights on the front of the processor are the diagnostic that needs no software. Their exact set varies, but a technician arriving at a faulted panel should be able to read them before opening a laptop.

Run

Solid when the processor is executing logic. Off in program mode. Flashing usually means the processor is in run but a condition, such as a missing program, is preventing execution.

OK, or fault

Solid green when healthy. Flashing red usually means a recoverable fault; solid red an unrecoverable one. The manual for the platform gives the exact meanings.

I/O

Solid when every configured module and remote connection is present and healthy. Flashing when one or more is missing or faulted, which is the usual first sign of a remote rack losing its network.

Communication ports

Activity and link lights on the Ethernet and serial ports. A dark link light on a port that should be connected is a cable or switch problem, not a program problem.

Battery or energy storage

Lit when the retention backup is low or absent. It is the light that gets ignored until the day the power goes out and the setpoints go with it.

Faults

When the processor detects a condition it cannot safely continue through, it faults: it stops executing logic, drives outputs to their configured fault state, lights the fault indicator, and records a fault code with the routine and instruction where it happened. Faults are classified as recoverable, which can be cleared and the processor restarted, and unrecoverable, which need a power cycle or a download.

• Watchdog timeout, when a scan runs longer than the configured limit, usually from a loop in logic or a communication task that stalls.
• Program faults such as an array index out of range, a divide by zero on some platforms, or a jump to a missing label.
• I/O faults, when a configured module is missing, is the wrong type, or a remote connection has failed and the connection is set to fault the processor.
• Memory faults, from a failed battery, corrupted program storage, or a firmware mismatch after a module replacement.
• Power faults, from a brownout that dropped the backplane below its minimum, which can look like a random restart.

Firmware and replacement

The processor runs firmware, and the programming software, the project file, and the firmware have to be compatible with each other. A replacement processor from stock often carries a different firmware revision than the one it replaces, and the project will not download until they match. Recording the firmware revision on the rack drawing and keeping the matching software installed is what turns a processor replacement from a day into an hour.