Understanding the EIA-232 Standard: The Foundation of Serial Communication

The EIA-232 interface standard was originally created with the specific purpose of connecting data terminal equipment (DTE) and data circuit-terminating equipment (DCE) that use serial binary data communication. In essence, EIA-232 was designed for linking data terminals to modems. It was first issued in the USA in 1969 by the engineering department of the EIA.

The standard specifies how two devices – a DTE and a DCE – should be connected. DTE refers to devices like computers or printers, which communicate with a DCE device. DCE refers to data communications equipment such as modems and is also referred to as data circuit-terminating equipment in EIA/TIA-232E. A DCE receives data from the DTE and forwards it to another DCE over a communications link such as a telephone line.

Major elements of EIA-232

The EIA-232 standard is made up of three key sections that define:

The electrical characteristics

The mechanical properties of the interface

A functional description of the interchange circuits

Electrical signal characteristics

EIA-232 specifies signal characteristics such as voltage levels and grounding requirements for an unbalanced communication system.

The transmitter must output voltages between ±15 V and ±25 V as follows:

Logic 1: –5 V to –25 V

Logic 0: +5 V to +25 V

Undefined logic level: –5 V to +5 V

At the receiver side, the voltage ranges are defined as:

Logic 1: –3 V to –25 V

Logic 0: +3 V to +25 V

Undefined logic level: –3 V to +3 V

For microprocessors, the voltage levels typically used are 0 V to +5 V for TTL (transistor–transistor logic). A line driver is therefore required at the transmitting end to adjust the voltage levels for the communication link. Similarly, a line receiver is used at the receiving end to convert the voltage back to TTL levels for microprocessor use. Even though the input voltage is bipolar, TTL-compatible EIA-232 receivers operate with only a single +5 V power supply. Modern PC power supplies generally provide a standard +12 V output suitable for the line driver.

The control (handshaking) lines follow the same voltage range as the data signals for logic 0 and logic 1, except that they have opposite polarity:

An asserted/active control line has a voltage of +5 V to +25 V (receiver allows +3 V to +25 V).

An inactive/inhibited control line has a voltage of –5 V to –25 V (receiver allows –3 V to –25 V).

 Each data and control line at the receiver end requires a line receiver to lower the voltage to the 0 V and +5 V logic levels expected by internal electronics.

The EIA-232 interface defines 25 electrical connections, grouped as:

Data lines

Control lines

Timing lines

Special secondary functions

Data lines carry the actual data signals, viewed from the perspective of the DTE.

Control lines are used for interactive control between devices (hardware handshaking). They manage how data flows over the interface. The four most common control signals are:

RTS: Request to Send

CTS: Clear to Send

DSR: Data Set Ready (called DCE Ready in EIA-232D/E)

DTR: Data Terminal Ready (called DTE Ready in EIA-232D/E)

Hardware handshaking is often the main source of interfacing problems. Some manufacturers omit control lines or use them in nonstandard ways. For this reason, many applications avoid hardware handshaking and rely only on the three main data lines (transmit, receive, and signal ground), implementing software handshaking instead. Here, data flow control is handled by the application software itself. Most instrumentation and control communication systems use software-based protocols rather than hardware handshaking.

There is a direct connection between data transmission speed and cable length. As transmission speeds increase, signal quality (voltage transitions from –25 V to +25 V) becomes more sensitive to cable capacitance and inductance. Voltage slew rate mainly depends on cable capacitance, which increases with length. The maximum cable length is limited by the acceptable error rate. The EIA-232 D/E specification limits the total cable capacitance to 2500 pF. With modern cables having improved capacitance (from ~160 pF/m to ~50 pF/m), the maximum cable length has increased from ~15 m (50 ft) to ~50 m (166 ft).

Typical baud rates for EIA-232 include 110, 300, 600, 1200, 2400, 4800, 9600, and 19200 bps. For shorter cables, higher rates like 38 400, 57 600, and 115 200 bps are also achievable. Table 3.1 illustrates the relationship between baud rate and cable length:

Baud rate	Cable length (meters)
110	850
300	800
600	700
1200	500
2400	200
4800	100
9600	70
19200	50
115K	20

Mechanical characteristics of the interface

The EIA-232 standard also defines the mechanical properties of the DTE–DCE interface. It specifies that the interface must have a plug and socket, with the socket usually placed on the DCE.

Although the DB-25 connector (25-pin D-type) was not originally defined in EIA-232C, it became closely associated with the standard and was officially specified in revision D. Revision E introduced a physically smaller 26-pin ALT A connector, which supports all 25 signals and suits modern equipment. Pin 26 is unused.

For devices with minimal or no handshaking needs, the DB-9 connector (9-pin D-type) is widely used. This originated with IBM’s combined serial/parallel adapter for AT&T PCs, where a compact connector was necessary to fit multiple ports on a standard ISA card. Over time, the DB-9 became an industry standard to reduce unused pins. The table below shows pin assignments for DB-9, DB-25 (EIA-232), and DB-25 (EIA-530):

Pin no. (DTE)	DB-9 connector (IBM)	DB-25 connector (EIA-232)	DB-25 connector (EIA-530)
1	Received line signal	Shield	Shield
2	Received data	Transmitted data	Transmitted data (A)
3	Transmitted data	Received data	Received data (A)
4	DTE ready	Request to send	Request to send (A)
5	Signal/Common ground	Clear to send	Clear to send (A)
6	DCE ready	DCE ready	DCE ready (A)
7	Request to send	Signal/Common ground	Signal/Common ground
8	Clear to send	Received line signal	Received line signal (A)
9	Ring indicator	+Voltage (testing)	DTE element timing (B)
10		-Voltage (testing)	Received line (B)
11		Unassigned	Transmitter signal (B)
12		Sec received line detector	DTE element timing (B)
13		Sec clear to send	Clear to send (B)
14		Sec transmitted data	Transmitted data (B)
15		Transmitter signal	Transmitter signal (A)
16		DCE element timing	DCE element timing (A)
17		Sec received data	Received data (B)
18		Receiver signal	Receiver signal (A)
19		DCE element timing	DCE element timing (B)
20		Local loopback	Local loopback
21		Sec request to send	Request to send (B)
22		DTE ready	DTE ready (A)
23		Remote loopback/signal quality	Remote loopback
24		Ring indicator	DCE ready (B)
25		Data signal rate	Transmitter signal (B)
		Test mode	Test mode

Functional description of the interchange circuits

The EIA-232 standard also defines the purpose of each data, timing, and control signal at the DTE–DCE interface. Only a few are typically relevant for instrumentation and control applications. The signals, defined relative to the DTE, include:

Protective ground (shield): Ensures the DTE and DCE chassis are at the same potential (can sometimes cause circulating earth currents).

Transmitted data (TxD): Serial data from the DTE to the DCE; held negative during idle.

Received data (RxD): Serial data from the DCE to the DTE.

Request to send (RTS): Activated (+V) by the DTE when it wants to send; the DCE responds by asserting CTS.

Clear to send (CTS): Activated (+V) by the modem to indicate the DTE may transmit.

DCE ready (DSR): Indicates the modem/DCE is operational.

Signal ground: Common return path for all signals; always connected.

Data carrier detect (DCD): Asserted by the modem upon receiving a remote carrier; remains so for the duration of the connection.

DTE ready (DTR): Enables modem operation (auto-dial or auto-answer), formerly called Data Terminal Ready.

Ring indicator: Asserted when a ring signal is detected on the line.

Data signal rate selector (DSRS): Used to select a higher data rate when two rates are available; rarely used today.

Conclusion

The EIA-232 standard remains one of the most important and widely recognized serial communication interfaces. By clearly defining the electrical, mechanical, and functional characteristics of data exchange between DTE and DCE devices, it provides a reliable foundation for connecting computers, modems, and other communication equipment. While modern interfaces may offer higher speeds and different signaling methods, EIA-232’s principles of signal definition, grounding, and flow control still influence contemporary standards. Understanding its operation helps engineers design compatible, robust, and efficient communication systems.