RS-232

Recommended standard 232 also commonly called RS-232 is a standard that is used in the serial ports of the computer. The standard RS-232 defines the electrical signal characteristics such as voltage levels, signal rate and so on. It also defines interface mechanical characteristics, pluggable connectors and pin identifications. RS-232 is also set to interpret the functions of each circuit in the interface connector and subsets of the interface circuits for selected telecom applications.

RS-232 improves signal integrity in tests like microwave tests, simplifies the test setup and saves a lot of space. The other important advantage of the RS-232 interface is the remote programming capability it offers.

In RS-232 the data is sent in the form of time-series of bits and it supports both asynchronous and synchronous transmissions. RS-232 also defines the number of control circuits used to manage the connection between the DCE and DTE. Every circuit or control unit operates independently. The operation works in sending signals from DTE to DCE servers or in the reverse order. Since, the receiving circuits, and the transmitting circuits are different, RS-232 operates in the full duplex mode along with supporting concurrent data flow in both directions.

RS-232 standard also defines the voltage levels that correspond
to logical one and logical zero for both transmission and control signal lines.

The valid signals that the RS-232 standard supports the voltage range from ±3 to ±15 Volts. However, the negative signals near the zero range are not supported by the standard. It can specify a maximum of 25 volts in an open circuit. The general voltage levels are commonly seen depending on the power supplies available in the particular device. The devices of RS-232 should be capable of withstanding the fluctuations in the voltage levels and in controlling the slew or fast rates of the signal changes.

Logic one is defined as a negative voltage for the data transmission lines of TxD, RxD and the secondary equivalent channels and the signal condition is called as marking and has a functional significance. Logic zero is defined as positive voltage and the signal conditioning is termed spacing. The control signals are logically inverted which are different from the signals that are visible on the data transmission lines. When either logic zero or the logic one lines are active, the voltage would range between +3 to +15 volts and when the lines are inactive, it would be in the reverse range, -3 to -15 volts. Some of the examples of these control lines are RTS (Request to send), CTS (Clear to send), DTR (data terminal ready), and DSR (data set ready).

Usually, the voltage values in the integrated circuits are much higher than the logical levels and hence special circuits are required to translate the logic levels. The special circuits also help in protecting the device’s internal circuitry from short circuits or transients and hence provide sufficient current to comply with the slew rate requirements for data transmission.

Article published January 12th, 2010
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