ESD & PLC systems can be described as control systems, both of them are used to control process conditions under certain circumstances.
Both of them are programmables using the dedicated software, the programming language can varry from a simple ladder logic up to a detailed statement language depending on the manufacture and the application.
Difference between ESD & PLC
The main difference between both systems is the SIL rating for the application that will, for sure, the SIL rating of the hardware itself.
The ESD is used for SIL3 or higher rated loops requiring more safety requirements, this will affect the hardware structure, so the MTBF of the ESD hardware will be lower and you will find the hardware redundancy everywhere starting from the power passing by processor, communication and ending by I/O cards. Example for manufactures are: Triconex and Hima.
The PLC is used for simple applications that doesn’t require a high SIL rating, the hardware is much more simpler and it can be used on simple applications and normally the cost is much more less than the ESD one. Example for manufactures are: Siemens