Getting Started with SINTRAN HDLC¶

A gentle introduction to the SINTRAN III HDLC subsystem for newcomers

What is SINTRAN HDLC?¶

SINTRAN III's HDLC subsystem provides reliable, frame-based serial communication over synchronous X.21 interfaces. It enables: - Terminal connections to remote systems - Network communication via X.25 packet switching - Reliable data transfer with error detection and recovery

Think of it as a sophisticated serial communication system that handles the complexities of framing, error checking, and flow control automatically.

System Architecture Overview¶

Key Components Explained¶

1. User Application¶

Your program that wants to send or receive data. It simply calls SINTRAN system functions - the complexity is hidden.

2. SINTRAN OS¶

The operating system software that manages HDLC operations: - Prepares data for transmission - Processes received data - Handles errors and retries - Manages buffers

3. DMA Controller¶

Hardware that transfers data between memory and the COM5025 chip without CPU intervention. This makes communication efficient.

4. COM5025 Chip¶

The heart of the HDLC system. This specialized chip: - Adds HDLC framing (flags, addresses, control) - Calculates and checks CRC (error detection) - Handles bit stuffing - Manages timing

5. X.21 Interface¶

The physical connection - wires and signals that connect to remote systems.

How Data Flows (Simple Example)¶

Sending Data¶

\\ Step 1: User Application "Send 'HELLO' to remote system" ↓ Step 2: SINTRAN Prepares - Validates request - Allocates buffer - Sets up DMA descriptor ↓ Step 3: DMA Transfer - Reads data from memory - Writes to COM5025 chip ↓ Step 4: COM5025 Processing - Adds opening FLAG (0x7E) - Adds address and control bytes - Transmits 'HELLO' - Calculates and adds CRC - Adds closing FLAG (0x7E) ↓ Step 5: Physical Transmission Frame sent over X.21 line to remote system \\

What Actually Goes on the Wire: \\ [FLAG][ADDRESS][CONTROL][H][E][L][L][O][CRC1][CRC2][FLAG] 0x7E 0x01 0x00 ... data ... computed 0x7E \\

Receiving Data¶

\\ Step 1: Physical Reception Frame arrives on X.21 line ↓ Step 2: COM5025 Processing - Detects opening FLAG - Strips flags and framing - Checks CRC - Signals "data ready" ↓ Step 3: DMA Transfer - COM5025 writes to memory via DMA - Interrupt generated when complete ↓ Step 4: SINTRAN Processing - Interrupt handler validates frame - Extracts user data - Delivers to application ↓ Step 5: User Application Receives 'HELLO' message \\

The Four Layers¶

SINTRAN HDLC is organized in layers, like a stack:

\\ ┌─────────────────────────────┐ │ Application Layer │ ← Your program ├─────────────────────────────┤ │ X.25 Packet Layer │ ← Network routing ├─────────────────────────────┤ │ LAPB (Link) Layer │ ← Reliable delivery ├─────────────────────────────┤ │ HDLC (Framing) Layer │ ← Frame structure ├─────────────────────────────┤ │ X.21 (Physical) Layer │ ← Wires and signals └─────────────────────────────┘ \\

Each layer has a specific job and talks only to the layers directly above and below it.

Key Terminology (Simple Definitions)¶

HDLC: The protocol that defines how data is framed for transmission
Frame: A complete unit of data with headers, data, and error checking
DMA: Hardware that moves data without bothering the CPU
Interrupt: Hardware signal that tells the CPU "something happened!"
Buffer: Memory area where data is stored temporarily
Register: Small, fast storage location in hardware for control/status
COM5025: The chip that does the actual HDLC work
X.21: The physical connection standard
LAPB: Higher-level protocol ensuring reliable delivery
X.25: Network protocol for routing packets

What Makes SINTRAN HDLC Special?¶

1. Direct Hardware Control¶

SINTRAN doesn't just configure the COM5025 chip - it directly embeds hardware control values in its DMA descriptors. This gives precise control over frame boundaries.

2. Efficient DMA Operation¶

Data transfers happen without CPU involvement, making the system fast and responsive.

3. Sophisticated Error Handling¶

Automatic retry mechanisms, timeout handling, and error recovery built into the system.

4. Interrupt-Driven¶

The CPU only gets involved when necessary (frame complete, error occurred), not for every byte.

Common Use Cases¶

1. Terminal Connections¶

Connecting a terminal to a remote NORD computer system via synchronous modem.

2. PAD (Packet Assembler/Disassembler)¶

Converting between asynchronous terminal traffic and synchronous X.25 packets.

3. Network Communications¶

Connecting NORD systems together in a network using X.25.

4. Point-to-Point Links¶

Direct synchronous connections between two systems.

What You'll Learn Next¶

Now that you understand the big picture, the next documents will explain:

Understanding Packets - What's in an HDLC frame and how it works
Hardware Overview - Details about COM5025, X.21, and registers
Software Flow - How SINTRAN software manages everything

Prerequisites¶

To understand this documentation, you should be familiar with: - Basic computer architecture (CPU, memory, I/O) - Hexadecimal and octal number systems - Basic networking concepts (though we'll explain specifics)

No prior HDLC knowledge required - we'll teach you everything!