EIA-232 is a standard interface for data terminal equipment (DTE) which was first published by the Electronics Industry Alliance (EIA) in the early 1960s. It is originally known as RS-232 i.e. Recommended Standard 232. It addresses signal voltages, signal timing, signal function, a protocol for information exchange, and either 25-pin or 9-pin mechanical connectors. Most personal computers comprise an RS-232 serial port for connecting external modems, printers, scanners, and other peripheral devices. In telecommunications, RS-232 is a standard for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is generally used in computer serial ports. It is the most common asynchronous serial line standard. EIA-232 is the EIA equivalent of ITU-T V.24, and V.28 and it specifies only the gender and pin use of connectors, but not their physical type.
The standard classifies equipment as either Data Communications Equipment (DCE) or Data Terminal Equipment (DTE).
Data Communications Equipment (DCE): Equipment at the near end of the connection (the telephone line interface) is called the DCE device i.e. Data Circuit-terminating Equipment which is typically a modem. It has a female DB25 connector and utilizes 22 available pins for signals and ground. DCE receives data from DTE on pin 2 (TD) and sends that data out the analog line. Data received from the analog line is sent by the DCE on pin 3(RD).
Data Terminal Equipment (DTE): Equipment at the far end of the connection is named the DTE device i.e. Data Terminal Equipment which is typically a computer or terminal. It has a male DB25 connector and utilizes same 22 of the 25 available pins for signals or ground. DTE receives data on pin 3 and transmits on pin 2 (TD) The cable linking DTE and DCE devices is a parallel straight-through cable with no cross-overs or self-connects in the connector hoods.
Scope of the standard
The Electronics Industries Association (EIA) standard RS-232-C defines following elements:
- Electrical signal characteristics such as voltage levels, signaling rate, timing and slew-rate of signals, voltage withstand level; short-circuit behavior, and maximum load capacitance.
- Interface mechanical characteristics, pluggable connectors and pin identification.
- Functions of each circuit in the interface connector.
- Standard subsets of interface circuits for selected telecom applications.
However below mentioned elements are not defined by EIA-232 standard:
- Character encoding like ASCII, Baudot code or EBCDIC
- Framing of characters in the data stream, for example, bits per character, start/stop bits, parity
- protocols for error detection or algorithms for data compression
- Bit rates for transmission, although the standard says it is intended for bit rates lower than 20,000 bits per second. Many modern devices support speeds of 115,200 bit/s and above
- Power supply to external devices.
EIA232 interfacing may include at least one of the following problems:
- Absence or misconnection of flow control i.e. handshaking signals, resulting in buffer overflow or communications lock-up.
- Incorrect communications function (DTE versus DCE) for the cable in use, resulting in the reversal of the Transmit and Receive data lines as well as one or more handshaking lines.
- Incorrect connector gender or pin configuration, preventing cable connectors from mating properly.
Fortunately, EIA232 driver circuitry is extremely tolerant of misconnections, and will usually survive a drive signal being connected to ground, or two drive signals connected to each other. In any case, if the serial interface between two devices is not operating correctly, disconnect the cable joining this equipment until the problem is isolated.
Limitations of the standard
Following are the major limitations of EIA232 standard
- Large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.
- Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.
- Multi-drop connection among more than two devices is not defined whereas multi-drop "work-arounds" have been devised but they have limitations in speed and compatibility.
- Asymmetrical definitions of the two ends of the link make the assignment of the role of a newly developed device challenging hence the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.
- The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.
- No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low power devices such as mice.
- While the standard recommends a 25-way connector and its pinout, the connector is large by current standards.
Commonly used signals with EIA232 standard are listed below:
- Transmitted Data (TxD): It is the data sent from DTE to DCE.
- Received Data (RxD): It is the data sent from DCE to DTE.
- Request To Send (RTS): It is asserted (set to logic 0, positive voltage) by DTE to prepare DCE to receive data. This may require action on the part of the DCE, e.g. transmitting a carrier or reversing the direction of a half-duplex channel.
- Ready To Receive (RTR): It is asserted by DTE to indicate to DCE that DTE is ready to receive data. If in use, this signal appears on the pin that would otherwise be used for Request To Send, and the DCE assumes that RTS is always asserted.
- Clear To Send (CTS): It is asserted by DCE to acknowledge RTS and allow DTE to transmit. This signaling was originally used with half-duplex modems and by slave terminals on multidrop lines: The DTE would raise RTS to indicate that it had data to send, and the modem would raise CTS to indicate that transmission was possible.
- Data Terminal Ready (DTR): It is asserted by DTE to indicate that it is ready to be connected. If the DCE is a modem, this may "wake up" the modem, bringing it out of a power saving mode. This behavior is seen quite often in modern PSTN and GSM modems. When this signal is de-asserted, the modem may return to its standby mode, straight away hanging up any calls in progress.
- Data Set Ready (DSR): It is asserted by DCE to indicate the DCE is powered on and is ready to receive commands or data for transmission from the DTE. For example, if the DCE is a modem, DSR is asserted as soon as the modem is ready to receive dialing or other commands. DSR is not dependent on the connection to the remote DCE. If the DCE is not a modem then this signal should be permanently asserted (probably by a jumper to another signal).
- Data Carrier Detect (DCD): It is asserted by DCE when a connection has been established with remote equipment.
- Ring Indicator (RI): It is asserted by DCE when it detects a ring signal from the telephone line.