Editing Programmable logic controller (section)

==Process of a scan cycle==
A PLC works in a program scan cycle, where it executes its program repeatedly. The simplest scan cycle consists of 3 steps:

# Read inputs.
# Execute the program.
# Write outputs.<ref name=":3">{{Harvnb|Parr|1998|p=446}}</ref>

The program follows the sequence of instructions. It typically takes a time span of tens of milliseconds for the processor to evaluate all the instructions and update the status of all outputs.<ref>Maher, Michael J. ''Real-Time Control and Communications''. 18th Annual ESD/SMI International Programmable Controllers Conference Proceedings, 1989, p. 431-436.</ref> If the system contains remote I/O—for example, an external rack with I/O modules—then that introduces additional uncertainty in the response time of the PLC system.<ref name=":3" />

As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented.<ref>Kinner, Russell H., P.E. ''Designing Programmable Controller Application Programs Using More than One Designer''. 14th Annual International Programmable Controllers Conference Proceedings, 1985, p. 97-110.</ref>

Special-purpose I/O modules may be used where the scan time of the PLC is too long to allow predictable performance. Precision timing modules, or counter modules for use with [[shaft encoder]]s, are used where the scan time would be too long to reliably count pulses or detect the sense of rotation of an encoder. This allows even a relatively slow PLC to still interpret the counted values to control a machine, as the accumulation of pulses is done by a dedicated module that is unaffected by the speed of program execution.<ref>{{Harvnb|Laughton|Warne|2002|loc=section 16.4.8}}</ref>