Industrial Internet of Things (IIoT) systems are actively and successfully implemented at enterprises. The variety of data transfer technologies and architectures used in IIoT, the large number of unreliable devices connected to a single Internet network, create risks of anomalies in the functioning of IIoT subsystems and the system as a whole due to their failures and successful cyber-attacks on them. One of the important hardware (HW) and software (SW) subsystems of IIoT is a large modular PLC, which can contain vulnerabilities in both SW and HW. This paper proposes a new method for ensuring the dependability (as reliability and cybersecurity) of a PLC with using of the large modular PLC architecture system analysis, an analysis of possible vulnerabilities of the PLC subsystem, the research of the PLC’s availability indices based on the mathematical apparatus of Markov processes, and the development of appropriate recommendations. This research is interdisciplinary and allows us to assess and ensure both cybersecurity and reliability of PLCs, and will enable manufacturers and operators to make correct and timely decisions.