A programmable logic controller, or PLC, is the rugged computer that coordinates sensors, conveyors, scanners, sorters, and robots in a modern warehouse. Its CPU module reads inputs, runs the control program, updates outputs, and communicates with other automation devices. Understanding the CPU and memory is important because a warehouse system must respond quickly, reliably, and safely to thousands of changing conditions.
Good PLC design keeps packages moving while preventing jams, missed scans, and unsafe motion.
The PLC CPU works in repeated scan cycles: read input image, execute logic, update output image, and handle communications or diagnostics. Memory is divided into areas such as program memory, data memory, input and output tables, retentive tags, and firmware storage. In logistics systems, memory holds conveyor states, barcode data, queue counts, fault codes, recipe settings, and network parameters.
Choosing the right CPU speed and memory size helps the system meet timing requirements as warehouse traffic grows.
Key Facts
- PLC scan time is the time for one complete control cycle: input read, program execution, output update, and communications.
- CPU load percent = active processing time / total available cycle time x 100.
- Memory used percent = used memory / total memory x 100.
- A typical PLC scan sequence is: read inputs, execute program, update outputs, run diagnostics and communication tasks.
- Retentive memory keeps selected values after power loss, such as counters, recipes, and fault history.
- Required response time should be less than the maximum allowed process delay: scan time + input delay + output delay < allowed delay.
Vocabulary
- PLC CPU
- The processor module of a programmable logic controller that executes the control program and manages inputs, outputs, memory, and communications.
- Scan cycle
- One repeated sequence in which the PLC reads inputs, solves logic, updates outputs, and performs background tasks.
- Program memory
- The memory area that stores the control instructions, function blocks, routines, and configuration used by the PLC.
- Data memory
- The memory area that stores changing values such as sensor states, timers, counters, tag values, and calculated results.
- Retentive tag
- A stored value that is preserved through a power cycle or restart when the PLC is configured to keep it.
Common Mistakes to Avoid
- Ignoring scan time when adding more logic is wrong because extra routines, network messages, and calculations can make the PLC respond too slowly to fast-moving packages.
- Using retentive memory for every tag is wrong because unnecessary retained values can preserve stale states after a restart and make equipment behave unpredictably.
- Assuming PLC memory is only for the program is wrong because data tables, communication buffers, diagnostics, recipes, and historical values can also consume memory.
- Updating outputs directly without considering the output image is wrong because many PLCs apply output changes at a defined part of the scan cycle, not instantly at every instruction.
Practice Questions
- 1 A PLC has 4 MB of memory and its project uses 2.8 MB. What percent of the memory is used?
- 2 A conveyor diverter must respond within 60 ms. The input delay is 8 ms, the output delay is 12 ms, and communication adds 10 ms. What is the maximum allowed PLC scan time?
- 3 A warehouse PLC loses power and restarts. Explain why package counts, fault history, and conveyor run commands should not all be treated the same in retentive memory.