The right PLC is chosen from the I/O count, the axis count, the cycle time and the communication it must carry. Most problems in the field, though, come from basics: PNP/NPN mismatches, retained data and scan-time overruns.
Basics 01
Choose a PLC with four questions: how many digital and analog I/O are needed, how many servo axes and which kind of motion (independent, synchronised, interpolated, CNC), what cycle time is targeted, and which networks it must carry (EtherCAT, CANopen, Modbus, Ethernet). Leave headroom for axes and I/O added later.
Basics 02
They can all live in the same project, and each suits a different job. Ladder (LD) for discrete interlock logic that maintenance staff can read, Structured Text (ST) for calculation, data handling and motion code, FBD for signal flow and control loops, and SFC for machine sequences that advance step by step. Instruction List (IL) was deprecated in the third edition of the standard.
CODESYSCommissioning 03
A PNP (sourcing) sensor puts +24 V on its output when active, so it goes to a sinking PLC input whose common is 0 V. An NPN (sinking) sensor pulls its output to 0 V when active and goes to a sourcing input whose common is +24 V. With a mismatch, the input never switches on, or never switches off.
Basics 04
Usually not. Standard digital inputs are read and filtered in the PLC scan cycle, so pulses shorter than the scan are missed. To read an encoder, count fast or generate pulses, use a high-speed counter (HSC) or pulse module; these count in hardware, independently of the scan.
MAT-LC I/O modulesTroubleshooting 05
A watchdog says a task did not finish within the time it was given. Typical causes: a loop over many elements, an endless WHILE loop, file or network work done inside the task, or newly added code pushing the task past its budget. Look at the maximum cycle time in the task monitor and move heavy work to a lower-priority task.
CODESYS task configurationDrive code, motor, controller and the alarm code you see: with those we reach the right answer faster.
