Chapter 2 Devices, Symbols and Instructions Behavior of non-latched and latched devices 2_ 2.1.4 Input Relays (X) The function of the input The input is connected to the input device (e.g. external devices such as button switches, rotary switches, number switches, and etc.), and the input signal is read into the PLC. Besides, contact A or contact B of the input can be used several times in the program, and the ON/OFF state of the input varies with the ON/OFF state of the input device. The input number (the decimal number) input/output modules, and the inputs are numbered according to the order in which the digital input/output modules are For the PLC, the input numbers start from X0.0.
The number of inputs varies with the number of inputs on the digital connected to the CPU module. The maximum number of inputs on the PLC can reach up to 8192, and the range is between X0.0 and X511.15. The input type The inputs are classified into two types. 1. Regenerated input: Before the program is executed, the data is fed into the PLC according to the states of the inputs which are regenerated. For example, LD X0.0. 2. Direct input: During the execution of the instructions, the data is fed into the PLC according to the states of the inputs. For example, LD DX0.0. 2-5 Device type Non-latched Latched Output relay PLC action Power:
OFF→ON Cleared Retained Cleared STOP ↓ RUN The output relay is cleared. Retained Retained Cleared The state of the output relay is retained. Retained Retained Retained The state of the output relay returns to that before the PLC’s stopping. Retained Retained The state of the output relay returns to that before the PLC’s stopping. The non-latched devices are cleared. Cleared Retained Cleared The state of latched devices is retained. Retained Retained Retained RUN→STOP Retained Retained Retained SM204 is ON. (All non-latched devices are cleared.) Cleared Retained Cleared SM205 is ON. (All latched devices are cleared.) Retained Cleared Retained Default value 0 0 0