A host of input and output modules are connected to the PLC bus to exchange data with the
PLC Input Output Modules
These can be broadly categorized into Digital Input Modules, Digital Output
Modules, Analog Input Modules, Analog Output Modules and Special Purpose Modules.
Digital Input Modules
The digital inputs modules convert the external binary signals from the process to the internal
digital signal level of programmable controllers.
Digital Output Modules
The digital output modules convert the internal signal levels of the programmable controllers
into the binary signal levels required externally by the process.
Analog Input Modules
The analog input modules convert the analog signals from the process into digital values which are then processed by the programmable controller.
Analog Output Modules
The analog output modules convert digital values from the programmable controller into the
analog signals required by the process.
Special Purpose Modules
These may include special units for:
• High speed counting
• High accuracy positioning
• On-line self-optimizing control
• Multi axis synchronisation, interpolation
These modules contain additional processors, and are used to relieve the main CPU from the
high computational loads involved in the corresponding tasks.