RS232 and USB are two different serial communication interfaces, so they need to be connected together via a converter. The following describes a commonly used RS232 to USB connection principle:
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1. Definition of RS232 end sequence: pin 1-TXD, 2-RXD, 3-DSR, 4-DTR, 5-GND, 6-DSR, 7-RTS, 8-CTS, 9-RI inside DB9 (female).
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2. Definition of the USB end sequence: The USB-A male port consists of four interfaces, namely red VCC, black GND, green D-, and white D. The USB-B male port also includes four interfaces, which are red VCC, black GND, green D-, and white D.
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3. The process of converting from RS232 to USB requires the use of an RS232 to USB converter, which usually has a DB9 female port and a USB interface. Since RS232 and USB belong to different communication protocols, then this converter needs to have the ability to convert protocols.
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4. Inside the converter, some circuits are needed to achieve signal transmission and conversion. It mainly includes the following steps:
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A. First, the RS232 signal is converted into a TTL level signal, that is, the signal of positive and negative levels is converted into 0 and 5V digital signals.
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b. Then convert the TTL level digital signal into the 5VDC level required by USB, and also convert the control signal in RS232 protocol (such as RTS, CTS, etc.) into the corresponding control signal in USB protocol.
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c. Finally, the converted signal is sent to the USB interface for communication with the computer.
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To sum up, the connection principle of RS232 to USB is to realize the interconnection between two different communication protocols through hardware conversion and protocol conversion.