The control of the PLC itself is completed by changing the state variables such as control register D, input port X, soft relay state M and time relay T, so as long as the state of these variables can be modified, the PLC control can be completed. The touch screen can complete these operations through the PLC's 232 or 485 port communication, so the touch screen can control the PLC.

The touch screen can be understood as a "soft dashboard and button tray". The buttons on the touch screen actually work the same as the buttons in the real world. It is also the two states of the switch. They all have an address. The actual button may be I0.0, and the touch screen button may be M0.0, such a match, representing a two-state switch variable, the actual button directly into the PLC through the external port, the essence is to determine a fixed address, The touch screen is the same, the determined address is the same as the PLC, and the state inside is directly modified.

 

Many times, the PLC manufacturer does not open these addresses and variables in the communication port, but there is a program editing download window between the PLC and the computer. It is said that the touch screen manufacturer has cracked this window and mastered its communication format, so it can directly modify the PLC status. It is.