The safety system runs in tandem with the production system. It must not impede the production system but run in parallel with it and oversee any hazards. The focus of the two systems is somewhat different. The production’s system focus is on throughput. The safety system is focused on protection. A safe system is not entirely possible and a risk is always present. The purpose of the safety system is to reduce the risk to acceptable levels. An absolutely safe system is not possible.
Ch 1 Introduction 14 How PLCs Solve Logic Programming a PLC requires a program that solves the same basic logic again and again guarding against the unexpected and keeping the machine running in an acceptable manner. There must be an orderly flow to the logic to control the machine. PLCs all follow the same general format utilizing the following four steps: 1) initialize from a safe state - usually off 2) sense inputs 3) solve logic in the program 4) outputs turned on or off to mechanical devices Steps 2, 3, and 4 are repeated again and again very rapidly to provide the orderly solving of logic and simulation of relays, timers, and counters. The process, while involving a number of complex actions, breaks down into the following: Initialize Program Sense Inputs Solve Logic Set Outputs Sense Inputs Solve Logic Set Outputs Sense Inputs Solve Logic Set Outputs Sense Inputs Solve Logic Set Outputs Fig. 1-2 Solving the PLC Program Scan by Scan This process repeats each one to 5 millisecond and is referred to as the scan of the PLC. Each repeated sensing of inputs, solving of logic and setting of outputs is a scan of the PLC. Siemens describes the cycle time of a processor as the entire time needed to read the inputs, execute the program one time and process the outputs. Their blocks are divided into networks and each network is divided into a number of statements. The time line is described as follows: