Usage of RS 232 in Modern Signalling in Railways

The most popular serial communication standard in use is certainly the EIA / TIA - 232-E specification. This standard which has been developed by electronic industry association and the telecommunication industry association (EIA / TIA) is more popularly referred to simply as RS 232 where RS stands for recommended standard.

RS 232 Specifications

RS 232 is a complete standard. This means that the standard sets out to ensure the compatibility between the host and the peripheral systems by specifying

• Common voltage and the signal levels
• Common Pin wiring configurations
• A minimal amount of control information between the host and the peripheral system

Unlike many standards which simply specify the electrical characteristics of the given interface RS 232, specifies electrical, functional and mechanical characteristics in order to meet the above three criteria. Each of these aspects of RS 232 standard is discussed below.

Electrical Characteristics

The electrical characteristics section of the RS 232 standard includes specifications on the voltage levels, rate of change of signal levels and line impedance. The original RS 232 standard was defined in 1962 as this was before the days of TL logic. It should not be surprising that the standard does not use 5V and ground logic levels. Instead, a high level of driver output is defined as being between -5 and -15 volts. The receiver logic levels were defined to provide a 2V noise margin. As such a high level of the receiver is defined as +3 to +15V and low level is -3 to -15V.

Functional characteristics

Since RS 232 is a complete standard it includes more than just specifications on the electrical characteristics. The second aspect of operation that is covered by the standard concerns the functional characteristics of the interface. These signals are divided into four different categories.

1) Common
2) Data
3) Control
4) Timing

Mechanical Interface characteristics

The third area covered by the RS 232 concerns the mechanical interface. In particular RS 232 specifies a 25 pin connector. This is the minimum connector size that can accommodate all the signals defined in the functional portion of the standard. The connector of the DCE equipment is male for connection housing and female for the connector pins. Likewise, the DTE connector is a female housing with male connection pins. Although RS 232 specifies a 25 position connector, it should be noted that often this connector is not used.

P Praveen