After two hours of on-site rush, I found that the PLC program just wouldn’t download! This was a common predicament I faced when I first started in the industry. If you are about to get into PLCs, make sure to get the “environment setup” right to avoid unnecessary detours. Today, let’s discuss the configuration and use of PLC hardware to ensure your first steps are steady.
1
Hardware Selection and Understanding
There are many PLC brands, and for beginners, it is recommended to start with Siemens S7-200 SMART or Mitsubishi FX series. Why? These two models have a low entry barrier, abundant learning resources, and are similar to commonly used models in industrial settings. My first PLC was a second-hand Siemens S7-200, which cost me less than 400 yuan and was sufficient for my entry into the field.
After obtaining the PLC, first familiarize yourself with several components: power terminals, input terminals, output terminals, and communication ports. It may seem complex, but it is like a power strip at home combined with a brain that can understand commands. The input terminals are similar to the sockets on a power strip, receiving signals from buttons, sensors, etc.; the output terminals act as “switches” that can control lights and motors.
2
Wiring and Power Configuration
Wiring is a common area where beginners make mistakes. There is a basic principle for PLC wiring: the input side must provide power, and the output side must pay attention to load current and voltage. For example, the input side of the Siemens S7-200 requires a 24V DC connection; you cannot just connect the button directly.
Common wiring diagram: Input side: positive power → button/sensor → PLC input point → negative power Output side: power → PLC output point → load (e.g., indicator light) → power
Once, after wiring, there was no response at all, and after checking for a long time, I found that the power was not connected properly. Therefore, it is recommended to use wires with terminals to avoid “loose connections”. A little tip: before wiring, use a multimeter to confirm the continuity of each terminal; spending an extra 5 minutes can save you an hour of troubleshooting.
3
Programming Software Installation
Different brands of PLCs have dedicated software. Siemens uses STEP7 or TIA Portal, while Mitsubishi uses GX Works. When installing, pay attention to:
1. Ensure the software version is compatible with the PLC. I once had trouble connecting to an old PLC with a higher version of the software, only to find out later that the versions were mismatched.
2. Install the driver. Especially for USB to serial cables, if the driver is not installed, the software will not recognize the hardware.
3. Disable antivirus software during installation to avoid false positives on components. A colleague of mine had incomplete software functionality because the antivirus software blocked it.
After installation, do not rush to start programming; first, test the communication. Siemens PLCs commonly use PPI or MPI protocols, and communication parameters need to be set in the software. The baud rate, parity bit, and station number must match the PLC; otherwise, the connection will fail. If you cannot connect, first check these basic settings before suspecting hardware issues.
4
Hardware Connection Testing
Once the software is installed, write the simplest program to test: let the output point follow the input point’s status. This is like controlling a light bulb with a switch; when the switch (input) is pressed, the light bulb (output) lights up.
Taking the Siemens S7-200 as an example, the ladder diagram only requires two instructions: LD I0.0 (read the status of input point 0.0) = Q0.0 (pass the status to output point 0.0)
When downloading the program, remember to switch the PLC to STOP mode first, and switch back to RUN after downloading. The first time I connected, I forgot this step and received a “PLC protection level error” message, thinking the device was broken, but it was just that the mode was not switched.
5
Common Issues and Solutions
1. Connection failure: Check if the communication cable is securely connected, if the baud rate matches, and if the PLC operating mode is correct.
2. Program cannot be downloaded: Check if the PLC has password protection and if it is in running state. Most PLCs need to be in STOP state to download programs.
3. No response from input: Use a multimeter to measure the input terminal voltage; it should normally be 24V. Check if the common point is connected properly; some PLCs require a separate power connection for the input common terminal.
I once spent an entire afternoon troubleshooting why the button was unresponsive, only to find out I had forgotten to connect power to the input common terminal. Developing the habit of using a multimeter to measure voltage during debugging can solve 80% of the problems.
6
Safety Precautions
Controlling industrial equipment with PLCs carries risks, so safety must be a priority. During the initial learning phase, it is recommended to only connect small loads like LED lights and not to directly control large power devices like motors. Before operating, confirm that the wiring is correct to avoid short circuits. Remember one principle: wire first, then power on; stop first, then repair.
Although setting up the environment may seem cumbersome, once you master the steps, you can get started quickly. Overcoming this “threshold” will allow for smooth progress in your subsequent PLC programming studies. Next time, we will discuss the use of basic PLC instructions; feel free to leave comments about any issues you encounter while learning PLCs!
