From OpenSCADAWiki
Variants of the constructive implementation of the PLC can be divided in mono-block and modular. Mono-block PLC provides the fixed configuration of the MOD, specialized for the limited range of tasks. Modular design provides an easy extension of configuration of MOD for the appropriate task. There are also the hybrid design which is the mono-block, able to expand its MOD by external MOD blocks connected to one of the standard interfaces such as RS-485.