TABLE OF CONTENTS A PLC is continuously running through its program and updating it as a result of the input signals. Each such loop is termed a cycle. PLCs could be operated by each input being examined as it occurred in the program and its effect on the program determined and the output correspondingly changed. This mode of operation is termed continuous updating.
Because, with continuous updating, there is time spent interrogating each input in turn, the time taken to examine several hundred input/output points can become comparatively long. To allow a more rapid execution of a program, a specific area of RAM is used as a buffer store between the control logic and the input/output unit. Each input/output has an address in this memory. At the start of each program cycle the CPU scans all the inputs and copies their status into the input/output addresses in RAM. As the program is executed the stored input data is read, as required, from RAM and the logic operations carried out. The resulting output signals are stored in the reserved input/output section of RAM. At the end of each program cycle all the outputs are transferred from RAM to the appropriate output channels. The outputs then retain their status until the next updating. This method of operation is termed mass I/O copying. The sequence can be summarised as (Figure 4.30):
Scan all the inputs and copy into RAM
Fetch and decode and execute all program instructions in sequence,...
