PLC as the core of the actual control project, most of them can not be separated from the touch screen or the upper computer with, because we use PLC to do control, the main is to deal with some analog quantities, these analog quantities are what we want to monitor, the equipment on the pressure, temperature, flow rate and other such values, and then through these detected values, and then according to some conditions to control the equipment on the electric valve, fan, pump, etc.. But these values we directly from the PLC is not visible, want to see these values, we must use touch screen or industrial control machine and other human-machine interface.

About the principle and workflow of touch screen.

When we connect the touch screen to the PLC, we can see the data we want to see on the touch screen. In addition to data can also be controlled through the touch screen on the field equipment on a variety of controlled objects. For example, we configure a switch on the touch screen, as long as the touch screen click on the switch, you can let the field equipment on a motor start.

What is the principle of communication between the touch screen and PLC? Commonly used touch screen contains 3 interfaces.

1、Power supply interface: the power supply to the touch screen, the voltage is mostly DC24V. it is indicated on the parameter label of the touch screen.

2、Configuration download interface: that is, we provide the configuration software through the touch screen manufacturers, on the computer we want the screen and function are designed to complete, through this interface to download to the touch screen. Manufacturers of touch screen software is not common, but the principle is the same.

3、Communication interface: This interface is used to communicate with the PLC, the interface is mainly divided into the form of RS232, RS485, Ethernet, which are three kinds, it should be noted that the form of this interface must be the same as the PLC interface form. Figure 2 of the touch screen brand is Kunlun Tongtai, it is the interface is RS485, so when connected to the PLC, the PLC interface must also be RS485.

After looking at the exterior of the touch screen, the next step is to understand the interior of the touch screen.

The interior of the touch screen can be roughly divided into: communication interface unit, driver unit, memory variable unit, display unit. In addition to the display unit, the other three units in the interior of the touch screen we can not see.

1、Communication interface unit: this unit is mainly responsible for the driver unit packaged data packets, sent to the back of the touch screen communication interface, and then sent by the communication interface to the PLC, these processes we do not have to intervene, the touch screen will complete themselves. We have to do is to choose a driver unit in the driver, to put it bluntly, is to tell the touch screen which driver packet it wants to send.

2、Driver unit: this unit stores a lot of communication files and PLC connection, a file corresponds to a communication protocol (communication protocols do not know also see my previous article), we call these files drive. That is, a driver corresponds to a communication protocol, for example, Siemens S7-200PLC using PPI communication protocol, then the touch screen manufacturers will write a Siemens S7-200PLC PPI communication protocol, communication files, and then placed in the driver unit.

When our touch screen wants to connect with Siemens 200 PLC, we choose PPI driver in the touch screen. Want to connect with Siemens 300 PLC, select MPI (MPI is the communication protocol of Siemens 300 PLC) driver will be fine. These driver files are written in advance by the touch screen manufacturer, we can only choose not to change. It can be seen that the more drivers inside the touch screen, the wider the range of PLC brands or communication protocols we can choose. Now the mature touch screen manufacturers built-in driver, the basic can cover the common PLC and communication protocols on the market.

So when we have a touch screen in hand, want to choose a PLC to do communication, be sure to look at your touch screen there is no, and this PLC can communicate with the driver.

3，Memory variable unit: This unit is also built-in touch screen manufacturers, it is actually a storage area, which can store a variety of data, the type of data stored can be roughly divided into numeric, switch, character type, special type.

If we want to display a boiler water temperature on the touch screen, then we will be in the touch screen memory variable unit, create a new variable, the variable's name is "boiler water temperature", the data type select the numeric type. Then the touch screen will automatically in the memory unit, allocate a small area to the "boiler water temperature" this variable. When the touch screen and PLC communication, it will be read from the PLC to store the water temperature data to this small area, that is, "boiler water temperature" this variable. When we need to display multiple data, just create multiple variables.

4、Display unit: this unit is well understood, as long as we can see things on the touch screen screen are in the display unit. For example, we want to show the temperature of boiler water, just draw a display box on the display unit of the touch screen (every touch screen has this part, just drag it to the screen), and then connect this part with the new "boiler water temperature" variable above.
Above is the touch screen units, the most difficult to understand is the driver unit and the memory variable unit.

If a touch screen is connected to a Siemens S7-200 PLC, 1, a pressure value in the PLC can be displayed on the touch screen, and 2, a switch in the PLC can be controlled by a switch component on the touch screen.

The first step, we first select the driver in the touch screen, because it is connected to the Siemens S7-200PLC, so we have to select the "Siemens S7-200 PPI communication driver" in the driver unit. Driver selection is complete, the touch screen will be the selected driver and communication interface unit, and memory variable unit automatically connected. Let's look at the driver's internal, there are many addresses V0.0 VB10 VW10, etc. These addresses are one by one correspondence with the address in the PLC, the data and status inside is the same as the PLC. For example, if the data in VD10 of PLC is 123.5, then the data in VD10 of the touch screen driver is also 123.5. So there is data in the touch screen, but the data still can't be displayed because the data hasn't been transferred to the display unit. What do I have to do to make it work?

In the second step, we create a new variable called "tank pressure" in the memory variable unit, and we connect it to VD10 in the drive unit, so that the variable "tank pressure" will have the data 123.5. But now the data is still not displayed because it has not been transferred to the display unit.

In the last step, we place a display with a display function in the display unit and connect this display to the variable "tank pressure" in the memory variable unit. This will allow us to see the data 123.5. All we have to do is to connect these units through a new variable, and the touch screen will do everything else automatically.

Understanding the workflow of the touch screen value display, switching is also simple, the display unit put a switch components, and then create a new variable, the variable and the drive unit and display unit connected to the line. As for the touch screen is how to pass the data to the PLC, it does not care, these things will help you complete the communication interface unit.

By adding switch components to the touch screen, the use of actual switches can be reduced, thus reducing the impact on the control system caused by poor contact of the actual switch contacts, and also improving the stability of the system.