RS232 Interface Definition

RS232 Interface The full name is the "serial binary data exchange interface technical standard between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)."

The standard specifies the use of a 25-pin DB25 connector, defining the signal content of each pin of the connector, as well as the signal levels. With the continuous improvement of equipment, the DB9 interface has emerged as a replacement for DB25. Nowadays, the RS232 interface is commonly referred to as DB9.

RS-232 is one of the mainstream serial communication interfaces.

Since the RS232 interface standard was developed early, it has the following four main characteristics:

(1) The interface signal level is relatively high, which can easily damage the chips of the interface circuit. Additionally, since it is incompatible 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 transmission rate is 20Kbps.

(3) The interface uses one signal line and one signal return line to form a common-ground transmission. This common-ground transmission is prone to common-mode interference, resulting in weak anti-noise interference capability.

(4) The transmission distance is limited. The maximum standard transmission distance is 50 feet, and in practice, it can only be used within approximately 50 meters.

Interface Definition
RS232 (DB9)

1 DCD Carrier Detect, 2 RXD Receive Data, 3 TXD Transmit Data, 4 DTR Data Terminal Ready

5 SG Signal Ground, 6 DSR Data Set Ready, 7 RTS Request to Send, 8 CTS Clear to Send, 9 RI Ring Indicator

RS232 (DB25)
1 Shield Ground, 2 TXD Transmit Data, 3 RXD Receive Data, 4 RTS Request to Send

5 CTS Clear to Send, 6 DSR Data Set Ready, 7 SG Signal Ground, 8 DCD Carrier Detect

9 Transmit Return (+), 10 Undefined, 11 Data Transmit (-), 12~17 Undefined, 18 Data Receive (+)

19 Undefined, 20 Data Terminal Ready DTR, 21 Undefined, 22 Ring Indicator RI, 23~24 Undefined

25 Receive Return (-)

Interface Level
RS232 uses negative logic levels: -15~-3V: Logic 1; +15~+3V: Logic 0; The voltage value is typically around 7V.