The RS-232 Interface

Recommended Standard 232 (RS-232) emerged in the 1960s as a common interface standard for data communications equipment. This data communications was often an exchange of data between a mainframe computer and a remote terminal via an analogue telephone line, and a modem was required at each end of the connection to carry out the necessary signal conversion (digital-to-analogue and vice-versa). A standard was needed to ensure reliable communication and to enable equipment produced by different manufacturers to interoperate. The standard specified signal voltages, signal timing, the function of each circuit in the interface, and a protocol for the exchange of information. It also provided the specifications for physical connectors. In the four decades since the standard first appeared, the Electronic Industries Association has made a number of modifications to the standard. The most recent version, EIA232F, was introduced in 1997. It renames some of the signal lines, and introduces some new ones, including a shield conductor.

RS-232 defines the connection between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). Data terminal equipment is any end user device, such as a computer, that can be used to send data over a network. Data circuit-terminating equipment is a device that provides the interface between the DTE and the network, and is often a modem or terminal adapter. An RS-232-compatible interface was commonly used for computer serial communication (COM) ports, which were originally intended for connecting the computer to a modem. While the original RS232 standard specified 25-pin connections, many of the pins were not used in practice, and a 9-pin connection was implemented on most computers. Although the RS-232 port has now largely been superceded by USB for connecting peripheral devices to personal computers, it is still often provided to enable the connection of legacy devices. The illustration below shows the 25-pin and 9-pin DTE-to-DCE connections that would result if the EIA232 standard were strictly followed. The most commonly used signals are shown in red.

RS-232 allows data rates of up to 20 kbps, over cables of up to 15 metres. Control circuits are used to manage the connection between the DTE and DCE, and a special hardware circuit called a UART (Universal Asynchronous Receive/Transmit) or a USRT (Universal Synchronous Receive/Transmit) controls the serial port in a computer. Each data or control circuit operates in only one direction, and since the Transmit Data (TxD) and Receive Data (RxD) are separate circuits, the interface can operate in full duplex mode. The EIA232 standard uses negative, bipolar logic in which a negative voltage is used to represent a logic '1', and a positive voltage represents a logic '0'. A typical DTE-DCE interface setup is shown below.

Connectors and Signals

RS-232 devices are either DTE or DCE devices. Computer terminals are usually equipped with male connectors with DTE pin functions, while modems have female connectors with DCE pin functions. Although the standard specifies twenty different signal connections, most devices use only a few of these signals, enabling the smaller 9-pin (DB9) connectors to be used. The more commonly used signals are shown in the table below.

Commonly-used RS-232 signals

Signal	Description
TxD	Transmitted Data - data transmitted from the DTE to the DCE
RxD	Received Data - data transmitted from the DCE to the DTE
RTS	Request To Send - set to 0 (asserted) by the DTE to prepare the DCE to receive data
CTS	Clear To Send - set to 0 (asserted) by the DCE to acknowledge RTS and allow the DTE to transmit
DTR	Data Terminal Ready - set to 0 (asserted) by the DTE to indicate that it is ready to be connected
DSR	Data Set Ready - set to 0 (asserted) by the DCE to indicate an active connection
DCD	Data Carrier Detect - set to 0 (asserted) by the DCE when a connection has been established with a remote device
RI	Ring Indicator - set to 0 (asserted) by the DCE when it detects a ring signal from the telephone line

In a standard connection between a DCE device and a DTE device, the cable used will connect the same pin numbers in each connector (a "straight-through" cable). The standard pin assignments for DB25 and DB9 connectors are given in the following tables.

DB25 Pin Assignments

Pin	DTE (male connector)	DCE (female connector)
1	Shield	Shield
2	Transmitted Data	Received Data
3	Received Data	Transmitted Data
4	Request to Send	Clear to Send
5	Clear to send	Request to send
6	DCE Ready	DCE Ready
7	Signal Ground	Signal Ground
8	Received Line Signal Detect	Received Line Signal Detect
9	Reserved for testing	Reserved for testing
10	Reserved for testing	Reserved for testing
11	Unassigned	Unassigned
12	Second Received Line Signal Detect	Second Received Line Signal Detect
13	Second Clear to Send	Second Request to send
14	Second Transmitted Data	Second Received Data
15	Transmitter Signal Timing (DCE Source)	Transmitter Signal Timing (DCE Source)
16	Second Received Data	Second Transmitted Data
17	Receiver Signal Timing (DCE Source)	Receiver Signal Timing (DCE Source)
18	Local Loopback	Local Loopback
19	Second Request to Send	Second Clear to Send
20	DTE Ready	DTE Ready
21	Remote Loopback	Remote Loopback
22	Ring Indicator	Ring Indicator
23	Data Signal Rate Selector	Data Signal Rate Selector
24	Transmitter Signal Timing (DTE Source)	Transmitter Signal Timing (DTE Source)
25	Test Mode	Test Mode

DB9 Pin Assignments

Pin	DTE (male connector)	DCE (female connector)
1	Received Line Signal Detect	Received Line Signal Detect
2	Received Data	Transmitted Data
3	Transmitted Data	Received Data
4	DTE Ready	DTE Ready
5	Signal Ground	Signal Ground
6	DCE Ready	DCE Ready
7	Request to Send	Clear to Send
8	Clear to Send	Request to Send
9	DCE Ready	DCE Ready

A connection between two DTE devices (e.g. between two computers) requires a null modem that acts as a DCE between the two devices. This is essentially a crossover cable in which a number of signal lines are cross connected (i.e. TxD to RxD, DTR to DSR, and RTS to CTS). A 9-pin null modem connection is shown below.

A loopback connector can be used for testing, and is simply a DB9 female connector without a cable, internally wired to reroute signals back to the sender. The connector is attached to a DTE device such as a personal computer.