All PLCs can be generally categorized into two types: Modular and Monolithic PLCs.
| Feature | Modular PLCs | Monolithic PLCs |
|---|---|---|
| Size | Large | Small |
| Construction | Consist of a rack into which circuit “cards” are plugged | Consist of a monolithic “brick” containing all processor, I/O, and communication functions |
| Cost | Expensive | Cheap |
| I/O Capability | Comprises of racks where ample number of input and output (I/O) points can be plugged as desired. | Limited in I/O capability |
| In Event of Failure | Easy to repair, since only failed card needs to be replaced. | Must be replaced as a whole in case of failure |
| Customization | I/O configuration can be customized | I/O configuration can’t be customized |
| Practical Application | Used for monitoring and controlling of large-scale operations | Used for small-scale processes |
| Example | Allen-Bradley SLC 500 series | General Electric Series One PLC |
| Number of Inputs | Can be increased as desired | Fixed |
| Fault detection and troubleshooting | Easy | Difficult |
Also see: Top 10 Questions on PLCs