Chapter 5 Understanding Common Devices Behavior of non-latched and latched devices 5.2.4. 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 5_ 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) For the PLC, the input numbers start from X0.0.
The number of inputs varies with the number of inputs on the digital input/output modules, and the inputs are numbered according to the order in which the digital input/output modules are 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.
5.2.5. Output Relays (Y) The function of the output The task of the output is sending the ON/OFF signal to drive the load connected to the output. The load can be an external signal lamp, a digital display, or an electromagnetic valve. There are three types of outputs. They are relays, 5-5 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