JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year INDUSTRIAL DATA COMMUNICATIONS Previous Question Paper (September - 2022 - R16)

The EIA-232 (formerly RS-232) standard defines the electrical and mechanical characteristics for serial binary data interchange between DTE (Data Terminal Equipment) and DCE (Data Circuit-terminating Equipment). It uses a single-ended signaling method where voltage levels represent binary states: logic 1 (MARK) is between -3V and -15V, and logic 0 (SPACE) is between +3V and +15V.

The interface facilitates communication through a defined set of control and data lines:

• Data Lines: Transmitted Data (TXD) carries serial bits from DTE to DCE, while Received Data (RXD) carries bits from DCE to DTE.
• Control Lines: Request to Send (RTS) and Clear to Send (CTS) manage flow control, while Data Terminal Ready (DTR) and Data Set Ready (DSR) indicate equipment status.
• Signal Ground: A common return path (Pin 7) is essential to establish a reference voltage for all signals.

The standard typically uses a DB-25 or DB-9 connector. Because it is a voltage-based, unbalanced interface, it is susceptible to noise and limited to short distances (typically under 15 meters) and lower data rates.

The 20 mA current loop is a robust communication method used to transmit data over long distances by representing binary states as the presence or absence of current in a loop, rather than voltage levels. This approach is highly immune to electrical noise and ground potential differences.

In a standard 20 mA loop configuration, a current source drives a loop consisting of the transmitter, the receiver, and the interconnecting wire. A logic 1 is represented by a current flow of 20 mA (MARK), and a logic 0 is represented by the absence of current (SPACE).

The system can be configured as:

• Active Mode: One device (usually the controller) provides the power supply to drive the loop.
• Passive Mode: The device relies on an external power supply provided by the loop.

Diagram description: The circuit consists of a DC voltage source connected in series with a current-limiting resistor, a switch (representing the transmitter), and an opto-isolator at the receiver end. The opto-isolator is critical as it provides galvanic isolation, protecting the sensitive receiver electronics from high-voltage spikes or ground loops. When the transmitter switch closes, 20 mA flows through the LED of the opto-isolator, causing the phototransistor to conduct and register a logic high at the receiver.