Definition of RS-232 in the Network Encyclopedia.

What is RS-232?

More properly known as RS-232C, is a widely implemented serial transmission interface developed by the Electronic Industries Alliance (EIA) that is used for connecting data terminal equipment (DTE) such as computers or terminals to data communications equipment (DCE) such as modems, packet assembler/disassemblers (PADs), or serial printers. The RS-232 specification uses unbalanced lines to provide full-duplex serial communication using baseband transmission.

RS-232 pinout
RS-232 pinout

RS-232 provides a typical data rate of 19.2 Kbps over a maximum distance of 15 meters, but the maximum data transfer rate is 115.2 Kbps. Up to two devices can be connected using RS-232. Devices cannot be daisy-chained together using RS-232. (Use RS-422/423 instead.)

RS-232 specifies the types of wires and connectors, the pinning of the connectors and the function of each wire, the voltage levels and their meanings, and control procedures such as handshaking. RS-232 cables (cables designed to use the RS-232 serial interface specification) are typically 25-wire unshielded twisted-pair (UTP) cables with DB25 type connectors or 9-wire cables with DB9 connectors. The pin assignments are shown in the following table. Note that only pins 1 through 8 and pin 20 are required for most basic RS-232 functions, which means that 9-pin DB9 connectors can be used on RS-232 serial cables for most applications.

Pin Assignments of RS-232

Pin NumberFunction
1Equipment ground (for protection)
2DTE transmit data
3DTE receive data
4Request-to-send (RTS), controlled by the DTE
5Clear-to-send (CTS), controlled by the DCE
6Data-set-ready (DSR), controlled by the DTE
7Signal ground (common return path)
8Carrier-detect (CD)
9+ Voltage
10– Voltage
11Not used
12Secondary received line signal indicator
13Secondary CTS
14Secondary DTE transmit data
15DCE transmitter signal timing
16Secondary DTE receive data
17Receiver signal timing
18Local loopback
19Secondary RTS
20Data-terminal-ready (DTR), controlled by the DTE
21Signal quality detector
22Ring indicator
23Data signal rate selector
24DTE transmitter signal timing
25Test mode

V.24 and V.28

In a PC, RS-232 is typically implemented in a universal asynchronous receiver-transmitter (UART) chip, which converts the internal parallel bus signal to a serial bit stream and vice versa, enabling communication between your system bus and serial devices.

RS-232 is compatible with the V.24 and V.28 standards from the International Telecommunication Union (ITU).

Null Modem

You can use RS-232 with a null modem cable to connect two pieces of DTE (for example, to transfer files). If you are having trouble with an RS-232 connection, be sure that you’re using a regular cable for DTE–DCE communication or a null modem cable for DTE–DTE communication – they look the same but are incompatible.

Although the maximum speed of RS-232 is 115.2 Kbps, older PCs support rates of up to only 56 or 64 Kbps. Newer PCs have a 16550 UART chip that supports serial throughput rates of up to 460.8 Kbps.

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