The RS-232 interface complies with the standards for serial data communication set by the American Electronics Industry Association (EIA), with the original full designation being EIA-RS-232 (abbreviated as 232, RS232). It is widely used for connecting peripherals to computers via serial interfaces. The connection cables and the mechanical, electrical characteristics, signal functions, and transmission processes are specified.
This standard specifies the use of a 25-pin DB25 connector, detailing the signal content for each pin of the connector, and also stipulates the voltage levels for various signals. With the continuous improvement of devices, the DB9 interface has emerged to replace the DB25, and now the RS232 interface is commonly referred to as DB9.
RS-232 is one of the mainstream serial communication interfaces today. Due to the early emergence of the RS232 interface standard, it inevitably has some shortcomings, mainly including the following four points:
(1) The signal voltage levels of the interface are relatively high, which can damage the chips in the interface circuit, and since it is not compatible with TTL levels, a level conversion circuit is required to connect with TTL circuits.
(2) The transmission rate is relatively low; in asynchronous transmission, the baud rate is 20Kbps. Therefore, in the “Southern Old Tree 51 CPLD Development Board,” the baud rate for the comprehensive program can only be set to 19200, which is the reason for this limitation.
(3) The interface uses one signal line and one signal return line to form a common ground transmission method, which is prone to common-mode interference, making it weak against noise interference.
(4) The transmission distance is limited, with a maximum standard transmission distance of 50 feet, and in practice, it can only be used for about 50 meters.
Interface Definition
RS232 (DB9)
DB9
Pin Definition
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1 DCD Carrier Detect
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2 RXD Receive Data
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3 TXD Transmit Data
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4 DTR Data Terminal Ready
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5 SG Signal Ground
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6 DSR Data Set Ready
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7 RTS Request To Send
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8 CTS Clear To Send
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9 RI Ring Indicator
Interface Levels
RS232 uses negative logic levels:
-15~-3: Logic 1;
+15~+3: Logic 0;
The voltage values are typically around 7V.
Pin Diagram
The RS-232 interface is a commonly used industrial communication interface, serving as an efficient and fast duplex serial communication method that has found widespread application.
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