JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV INDUSTRIAL DATA COMMUNICATIONS Previous Question Paper (September - 2020 - R16)

Data transmission in the OSI (Open Systems Interconnection) model occurs through a process of encapsulation at the sender and decapsulation at the receiver. As data moves down the protocol stack from the Application layer to the Physical layer, each layer adds its own control information, known as a header, to the data received from the layer above.

The process follows these steps:

• Application, Presentation, and Session Layers: These layers create the user data (often referred to as Data).
• Transport Layer: The data is segmented into smaller units called Segments. A header is added containing source and destination port numbers and sequence information.
• Network Layer: The segment is encapsulated into a Packet. A header containing logical source and destination IP addresses is added to facilitate routing.
• Data Link Layer: The packet is encapsulated into a Frame. This layer adds a header (containing physical MAC addresses) and a trailer (containing a Frame Check Sequence for error detection).
• Physical Layer: The frame is converted into a stream of Bits (electrical, optical, or radio signals) and transmitted over the physical medium.

Upon reaching the destination, the receiving device performs the reverse process. It receives the bits, reassembles them into frames, and strips off the headers at each corresponding layer (decapsulation). The Physical layer passes the frame to the Data Link layer, which verifies the MAC address and removes the header. This continues upward until the original data reaches the Application layer of the destination device. Throughout this process, each layer on the source machine communicates logically with its peer layer on the destination machine using Peer-to-Peer protocols.

The Data Highway Plus (DH+) is a proprietary local area network protocol developed by Allen-Bradley (now Rockwell Automation) for industrial communication between Programmable Logic Controllers (PLCs), operator interfaces, and programming terminals. It is designed to provide high-speed, reliable data exchange in harsh industrial environments.

Key features and technical characteristics of DH+ include:

• Topology and Physical Layer: DH+ typically utilizes a trunk-line/drop-line topology. It operates over a shielded twisted-pair cable, which provides excellent noise immunity. The network supports up to 64 nodes (addresses 0 to 77 octal).
• Transmission Speed: It supports three selectable baud rates: 57.6 kbps, 115.2 kbps, and 230.4 kbps. The choice of speed depends on the total cable length, with higher speeds requiring shorter cable runs to maintain signal integrity.
• Token Passing Access Method: DH+ employs a token-passing mechanism to manage bus access. A node must possess the 'token' to transmit data, which ensures deterministic communication and prevents data collisions. When a node finishes its transmission or its time slice expires, it passes the token to the next node in the logical ring.
• Communication Services: The protocol supports both peer-to-peer messaging and master-slave communication. It is primarily used for remote programming, data monitoring, and inter-PLC communication, allowing controllers to share status information and I/O data.
• Frame Structure: The protocol uses a specific frame format that includes a destination address, source address, command codes, and data fields, followed by a Cyclic Redundancy Check (CRC) to ensure error detection and data integrity.

In summary, DH+ remains a robust, field-proven protocol for legacy automation systems, offering a reliable balance between simplicity and performance for real-time control applications.