What is the RS232 Interface Definition?

RS232 interface is formally known as the "technical standard for serial binary data exchange interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)."

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

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

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

(1) The interface signal levels are relatively high, which can easily damage the interface circuit chips. Additionally, since it is incompatible with TTL levels, level conversion circuits are required to connect with TTL circuits.

(2) The transmission rate is relatively low; in asynchronous transmission, the rate is 20 Kbps.

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

(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 (-), 1217 Undefined, 18 Data Receive (+), 19 Undefined, 20 DTR Data Terminal Ready, 21 Undefined, 22 RI Ring Indicator, 2324 Undefined, 25 Receive Return (-)

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