Abstract This paper introduces the design and development of a remote temperature control system based on SIMATIC PLC and MCGS. The system's functions, hardware composition, communication, and software design are described in detail. After system simulation and assembly experiments, the system fully meets the design requirements. Keywords: PLC, MCGS, temperature control, touch screen 0 Introduction In the field of industrial automation, PLCs (Programmable Logic Controllers) are widely used in modern industrial automatic control due to their high reliability, strong anti-interference ability, simple programming, powerful functions, high cost-effectiveness, small size, and low energy consumption. Currently, PLCs are often chosen as field control devices for data acquisition and processing, logic judgment, and output control; while the host computer uses HMI/SCADA software to display industrial control status, processes, and parameters, realizing monitoring, management, analysis, and storage functions. This monitoring system fully utilizes the characteristics of both PLCs and computers and has been widely used. Based on this approach, a temperature control system was designed. A distributed control system was constructed by combining a PLC-based lower-level computer and a host computer that performs HMI/SCADA functions, realizing automatic temperature control. 1. System Working Principle The system temperature is controlled between 10°C and 100°C. When the temperature is below 10°C or above 100°C, it should automatically adjust. If the abnormal state persists after one minute of adjustment, the high-temperature alarm indicator should flash. The system has a start button to initiate the control program. Three indicator lights (green, red, and blue) indicate the temperature status. When the controlled temperature is within the required range, the green light illuminates, indicating normal system operation. When the controlled temperature exceeds the upper limit or the adjustment cannot return it to the normal range, the red light illuminates. When the controlled temperature is below the lower limit, the blue light illuminates. To convert the resistance of the PT100 temperature sensor, which changes with temperature, into a corresponding temperature change value, the following temperature formula is used: T℃ = (Digital temperature value - 0℃ bias value) / 1℃ digital value, where the digital temperature value is the value stored in AIWx (x=0,2,4), the 0℃ bias value is the digital value measured at 0℃, and the 1℃ digital value is the digital value generated per 1℃ increase in temperature. 2. The PLC control system uses a Siemens S7-200 PLC as the controller and should have analog input/output and calculation capabilities. Based on the requirements of the controlled system, a CPU222 DC/DC PLC basic unit is selected, and an EM235 analog input/output unit is configured. At the temperature measurement point in the controlled system, the temperature signal is converted into a 4-20mA current signal by a transmitter and sent to the second analog input channel AIW2 ​​of the EM235. After the PLC reads the temperature value, it takes the average value as the actual temperature value of the controlled system. If the measured temperature exceeds the allowable range, the high temperature alarm indicator lights up, and the system automatically adjusts. If the adjustment time is too long (more than one minute), the system immediately cuts off the power and stops heating. If the measured temperature is lower than the preset value, the low temperature alarm indicator lights up, and the system immediately starts the heater to heat until the set temperature range is reached. 3. Software Design and Implementation of the Temperature Control System The software design mainly includes the design of the upper-level monitoring software and the lower-level software. 3.1 Lower-Level Software Design The lower-level software design utilizes the STEP7 series programming software, STEP7 - Micro/Win4.0, at the monitoring level to complete the programming and debugging of the lower-level program. The debugged program is then downloaded to the PLC via a PPI programming cable. The lower-level software mainly realizes communication between the PLC and the monitoring level, automatic temperature control by the PLC, and fault diagnosis. Power-on initialization: When the system starts running, to ensure system safety, all output points are first set to a safe state. Initialization operations include clearing RAM and ROM, initializing control parameters, and resetting current analog sample values. The system requires temperature control within the range of 10°C to 100°C. For ease of control, a temperature comparison value (50°C) is set and used as the reference value for the ambient temperature. In this system, the PLC controls the green light to turn on/off via output port Q0.0, the red light to turn on/off via Q0.1, the blue light to turn on/off via Q0.2, the fan to start/stop via Q0.3, and the heater to start/stop via Q0.4. The system flowchart is shown in Figure 1. 3.2 Temperature Monitoring and Control Process 1) When the PLC is put into operation, it generates an initialization pulse through a special relay SM0.0 to initialize the system, including storing the temperature calibration value, set value, etc., into the relevant data registers and resetting the timer. 2) Pressing the start button puts the system into operation. 3) The Pt100 sensor sends the measured temperature to the temperature transmitter for standard quantity conversion (4-20mA). 4) The analog input channel AIW2 ​​reads a 4-2mA analog current and sends it to the PLC. 5) After program calculation, the actual measured temperature Q is obtained. 6) Q is compared with Qmax (the upper limit of the allowable temperature). If the upper limit is not exceeded, compare Q with Qmin (the lower limit of permissible temperature). If Qmin is also not lower than the lower limit, the temperature is normal, the green light illuminates, and the system waits for the next sampling. 7) If Q > Qmax, perform upper limit processing, calculate the temperature deviation between Q and the upper limit, calculate the adjustment amount based on the deviation, issue an adjustment command, and determine the adjustment time. If the adjustment time is too long, the over-temperature alarm light flashes. If the over-temperature is adjusted to the normal temperature range before the adjustment time is reached, the normal temperature indicator light illuminates. 8) When the sampled temperature is lower than the lower limit, i.e., Qmin > Qmax, the temperature is normal. 4. Conclusion A distributed temperature control system based on a PLC-based lower-level computer and an HMI/SCADA upper-level computer is a feasible and highly efficient system. In our simulation and assembly experiments, after adjustment, the temperature can be controlled within the target temperature range of -0.5 to +0.5 ℃, and after applying a disturbance, it can quickly achieve equilibrium. This meets the target requirements of our system. In conclusion, the PLC- and MCGS-based temperature control system fully considers the current situation and control requirements of small and medium-sized chemical enterprises and can be applied to a large number of similar working environments and occasions. Because the control system built using PLC and MCGS is reliable, flexible, and highly adaptable, it has strong viability in similar control system fields.