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PLC's CPU works - Database & Sql Blog Articles
The CPU operates in different modes, which are indicated by LEDs on the front panel. A green LED indicates that the CPU is in the **RUN** mode, meaning it is actively executing the program. A red LED shows that the CPU is in the **STOP** mode, where the program is not running. If the **SF** (System Fault) LED is lit, it means there is a system error, and the PLC has stopped functioning.
1. **How the CPU Works**
- **STOP Mode**: When the CPU is in STOP mode, the program is not executed. This is the mode used for programming, downloading, or modifying the program. Any editing or configuration must be done while the CPU is in this mode.
- **RUN Mode**: In RUN mode, the CPU executes the user program according to its internal scan cycle. This is the normal operating state of the PLC when it is controlling the connected devices.
2. **Changing the Working Mode**
There are several ways to change the working mode of the CPU:
- **Using the Mode Switch**: The mode switch has three positions: **STOP**, **TERM (Terminal)**, and **RUN**.
- Setting the switch to **STOP** halts the execution of the program.
- Setting it to **RUN** starts the program execution.
- Setting it to **TERM** allows communication with the STEP7-Micro/WIN32 software to configure the CPU.
When power is applied, the CPU automatically enters the mode set by the switch. For example, if it's in STOP or TERM, it will start in STOP mode. If set to RUN, it will begin in RUN mode.
- **Using Programming Software**: You can also change the mode using the STEP7-Micro/WIN32 software. First, set the mode switch to **TERM**, then use the software to control the CPU’s operation mode.
- **Using Instructions in the Program**: You can insert a **STOP** instruction in your program. This will cause the CPU to transition from RUN to STOP mode during execution.
Below is a table summarizing the specifications of various CPU models:
| Model | CPU221 | CPU222 | CPU224 | CPU226 | CPU226MX |
|-------------|--------|--------|--------|--------|----------|
| User Data Storage Type | EEPROM | EEPROM | EEPROM | EEPROM | EEPROM |
| Program Space (Permanently Saved) | 2048 words | 2048 words | 4096 words | 4096 words | 8192 words |
| User Data Storage | 1024 words | 1024 words | 2560 words | 2560 words | 5120 words |
| Data Backup (Super Capacitor) | 50 H | 50 H | 190 H | 190 H | 190 H |
| Host I/O Points | 6/4 | 8/6 | 14/10 | 24/16 | 24/16 |
| Scalable Modules | No | 2 | 7 | 7 | 7 |
| 24V Sensor Power Supply Max Current / Limit | 180/600 mA | 180/600 mA | 280/600 mA | 400/1500 mA | 400/1500 mA |
| Maximum Analog Input/Output | No | 16/16 | 28/7 or 14 | 32/32 | 32/32 |
| 240V AC Power Supply Input Current / Max Load | 25/180 mA | 25/180 mA | 35/220 mA | 40/160 mA | 40/160 mA |
| 24V DC Power Supply Input Current / Max Load | 70/600 mA | 70/600 mA | 120/900 mA | 150/1050 mA | 150/1050 mA |
| DC5V Power Supply Output for Expansion Module | - | Max 340 mA | Max 660 mA | Max 1000 mA | Max 1000 mA |
| Built-in High Speed Counter | 4 (30kHz) | 4 (30kHz) | 6 (30kHz) | 6 (30kHz) | 6 (30kHz) |
| High Speed Pulse Output | 2 (20kHz) | 2 (20kHz) | 2 (20kHz) | 2 (20kHz) | 2 (20kHz) |
| Analog Adjustment Potentiometer | 1 | 1 | 2 | 2 | 2 |
| Real-Time Clock | Yes (Clock Card) | Yes (Clock Card) | Built-in | Built-in | Built-in |
| RS-485 Communication Port | 1 | 1 | 1 | 1 | 1 |
| Number of Input Points per Group | 4,2 | 4,4 | 8,6 | 13,11 | 13,11 |
| Output Points per Group | 4 (DC), 1,3 (AC) | 6 (DC), 3,3 (AC) | 5,5 (DC), 4,5,7 (AC) | 8,8 (DC), 4,5,7 (AC) | 8,8 (DC), 4,5,7 (AC) |