EIA232: Interface between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) employing serial binary data interchange
This standard is referred to variously as RS232, EIA232 and EIA/TIA 232. All of these names are correct to one degree or another, though EIA/TIA232 is the most recent and most correct as the name changed over time as the standards body made revisions to the standard and the EIA merged with the TIA. Most folks know what you're talking about when you use any of those names. The EIA232 standard, originally called Recommended Standard 232 or RS-232 was developed in the 1960's to specify the electrical signalling between a communications device and the computer it is attached to. At that time, most data communication between computers was done over analog voice lines and modems were used to modulate the signal on the voice line to carry data and to demodulate it at the other end and deliver the data to the computer. For the sake of compatibility, standards were created so that the computers could be connected to the modems and the modems connected to the phone lines and communication would be possible using uniform cabling and interfaces.
The EIA232 standard specifies specific electrical and mechanical details of a serial connection such as
- Equipment Types
- Connectors
- Wire names and functions
- Voltage Levels and Bit signalling
- Communications speeds (bit rates)
EQUIPMENT TYPES: DCE AND DTE
- DCE
- DCE stands for Data Communications Equipment (more recently known as Data Circuit-terminating Equipment). This is the device that manages communication over a communications link to another device. A DCE device manages that connection on behalf of the DTE.
- DTE
- DTE stands for Data Terminating Equipment. The DTE is the device that is trying to communicate digital data. The DTE device uses a DCE device to communicate with another.
If we are talking about a computer and modem communicating over a phone line, then the DCE equipment is the modem and the DTE is the computer. Please note that it is possible to connect more than just modems to a computer through an port supporting EIA232. The PS2 port for the mouse and keyboard both use EIA232, but they are not modems. You can also connect two computers together using EIA232 over the DE9 port on the back of the computer (if you have one). The standard makes no mention of connecting anything other than a modem (DCE) to the computer (DTE) so these other connection types are outside the standard.
CONNECTOR SPECIFICATION
The RS232 specifies 14 possible communications configurations (simplex, half-duplex, duplex) of the 22 connectors. for the actual EIA232 connector utilizes a 25-pin D-shaped subminiature connector. For the DTE device (the computer) the connector is male (it has pins); for the DCE (modem) the connector is usually female (has holes for the pins), but this isn't in the standard and gender changers are sometimes necessary to connect two EIA232 devices. These connectors are often referred to as type DB25. One of the things that caused some confusion was that the old IBM PCs used a 9-pin D-shape connector (EB8) for EIA232. EIA232 was used by IBM for the keyboard and mouse as well, and that is why the PS2 ports on all PCs that have them use EIA232.
If you go shopping for a mainboard that has no legacy RS232/EIA232, then you
CONNECTION FUNCTIONS (DTE - 25-pin)
The standard specifies communications functions on various connectors to enable the computer (DTE) to control the modem (DCE) and to monitor and respond to the activity on the phone line while simultaneously sending and receiving data from the modem.
| CONNECTOR | NAME | FUNCTION |
| 1 | Chassis Ground | Electrically ground the chassis (also called Frame Ground) |
| 2 | Transmit Data (TxD) | Data channel DTE->DCE |
| 3 | Received Data (RxD) | Data channel DTE<-DCE |
| 4 | Request To Send (RTS) | Request to Send |
| 5 | Clear To Send (CTS) | Clear to Send |
| 6 | Data Set Ready (DSR) | Data Set Ready |
| 7 | Signal Ground (SG) | Common mode voltage connector; serves as second half of electrical loop for all of the other connections. |
| 8 | Data Carrier Detect (DCD) | A signal on this connection indicates that the modem has detected a carrier signal on the phone line. |
| 20 | Data Terminal Ready (DTR) | |
| 22 | Ring Indicator (RI) |
To connect DTE to DCE, a standard serial cable with the correct connector type is needed. DTE and DCE are designed to communicate with each other. To connect two computers (DTE to DTE) over a serial cable, you need a null-modem cable. A null-modem cable is NOT a crossover serial cable because:
- Pin 1 (FG) is not connected end to end
- Pin 6 (DSR) is wired straight through to Pin 6 on the other end
- Pin 8 (DCD) is wired to the electrical ground (pin 1 above) on the same end of the cable
- Pin 20 (RI) is not connected at all (no phone connection, therefore no ring indicator needed).
Click here to see the full DTE pinout for both 9-pin and 25-pin EIA232.
VOLTAGE LEVELS AND BIT SIGNALLING
The pinouts in the serial connection can be organized into six groups:
- Signal shield/ground
- Primary Channel
- Secondary Channel
- DCE Status and Control Signals (primarilly for modems)
- Transmission/Receiver timing
- Channel testing signals
EIA232 uses unbalanced voltage and control wires to enable asynchronous communication between DTE and DCE. Voltage on the wires can range from +/-3V to +/-15V, depending upon the implementation. Thare are even a few implementations that are not actually EIA232 that use voltages outside this range, mostly used in electronics and communication between components on computer mainboards.
Binary 1 is signalled using negative voltage. When there is nothing to transmit on the data lines, the voltage state reverts to that set up for binary 1. When no data is being transmitted, the receiving device sees what is electrically a steady stream of binary 1's. Any change from this steady stream indicates that the remote device is signalling.
Binary 0 is signalled using positive voltage.