10.2 The EIA-232 (RS-232) Standard

The EIA-232 is a standard that specifies the relation between the DTE and the DCE as specified in Figure 10.1.

EIA-232 was published by the Electronic Industry Association (EIA) in US.

It is also known as the RS-232 (Recommended Standard-232).

A number of RS-232 versions have been published including RS-232C, RS-232D and RS-232E.

The RS-232 standard includes four parts:

Mechanical:

		That is concerned with physical connection characteristics and the types of connection plugs including specifying male and female plugs.
		A 25-pin connector is specified with specific lead arrangement.
		DTE devices have male DB25 connectors while DCE have female connectors.
		A DB-25 connector is shown in Figure 10.2.
		In practice, not all the 25 pins are used in applications. The DB-9 is another RS-232 connector variation.

10.2 The EIA-232 (RS-232) Standard

Electrical:

That specifies the signaling between the DTE and the DCE including voltages, timing and encoding related issues.

RS-232 uses NRZ-L encoding which is described in Lecture 7.

Bit value 1 is represented by negative voltages less than -3V.

Positive voltages more than 3V are used to represent bit value 0.

Voltages can be as high as ± 25V.

Maximum transmitted rate that can be supported is 20Kpbs with a maximum distance of 15 meters.

10.2 The EIA-232 (RS-232) Standard

Functional:

That specifies the role of individual circuits and pins of the RS-232-compliant connectors such as the DB-25.

Figure 10.3 illustrates a possible connectivity setup between a DTE and a DCE which uses only 8 pins of DB-25 connectors.

The role of each interface pin is described in Table 10.1.

E.g., pin 2 handling TxD is used to transmit data from the DTE to the DCE.

The RxD signal handled by pin 3 is used to receive data sent by the DCE to the DTE.

The RS-232 circuits allow full duplex communication in both directions. Both synchronous and asynchronous communication is possible.

10.2 The EIA-232 (RS-232) Standard

Procedural:

		That specifies the sequence of events to transmit data.
		An example is connecting two computers over a short distance, known as the limited distance modem. In this scenario, the modem (the DCE) receives a digital signal from the sending computer (the DTE), converts it into an analog signal and transmits it over the transmission medium.
		On the receiving side, the DCE receives the analog signal, converts it into digital form and sends it to the receiving computer (DTE).
		Only few circuits are required to support this communication scenario which are listed in Table 10.2.
		The communication steps includes:

			The DCE on the sender side asserts the DCE Ready line (pin 6).
			When the DTE wants to send data, it asserts the Request to Send line (pin 4).
			The DCE asserts the Clear to Send line (pin 5) informing the DTE that it can start sending data.
			The DTE start sending data over the transmitted data line (pin 2).
			On the receiving side, when data arrives, the receiving DCE asserts the Received Line Signal Detector line (pin 8) which indicates the reception of a carrier signal.
			Data is then sent from the DCE to the DTE using the Received Data line (pin 3).

10.2 The EIA-232 (RS-232) Standard

Null Modem

When the distance between two DTEs (e.g. two computers) are very close, communication can be achieved directly without the need of DCEs on both sides to interface with the communication medium.

Such setup is possible using RS-232 using the Null Modem which bypasses the DCEs. Figure 10.4 illustrates the circuit connectivity for a Null Modem.

Figure 10.4: Null Modem Setup to Send Data Directly between two DTEs