Serial Communication, USART, RS-232 & Asynchronous Data Transfer

TAKEAWAYS:

• Serial communication transmits data one bit at a time over a single wire (vs. parallel’s multiple wires), reducing cost and complexity in long-distance or multi-device links.
• The 8251 USART handles both synchronous (clock-based) and asynchronous (start/stop-bit) data transfer, with programmable baud rates, parity, and word lengths.
• RS-232 defines electrical signals (±3V to ±15V), connectors (DB-25/DB-9), and handshake lines (RTS/CTS) for reliable serial communication between devices like printers and modems.
• Asynchronous communication uses start/stop bits to synchronize sender/receiver clocks, while synchronous uses a shared clock signal for precise timing.
• USART advantages over parallel I/O include longer cable distances, fewer wires, and easier interfacing with external peripherals (e.g., GPS modules, sensors).
• Real-world examples: eSewa terminals use USART for secure serial communication with payment gateways, Ncell modems rely on RS-232 for AT command-based configuration, and industrial PLCs use serial links for remote monitoring.

1. Serial vs. Parallel Communication: Why Serial Wins in Most Cases

Serial communication transmits data bit-by-bit over a single wire (or pair), while parallel sends multiple bits simultaneously over multiple wires. While parallel is faster for short distances (e.g., CPU-memory bus), serial dominates in:

• Long-distance links (e.g., modems, GPS, sensors).
• Multi-device networks (e.g., USB, UART, I²C).
• Cost-sensitive applications (fewer wires = cheaper cables/connections).

Comparison Table: Serial vs. Parallel

2. Asynchronous Serial Communication: Start/Stop Bits and Baud Rate

Asynchronous communication (e.g., UART) does not use a clock signal. Instead, it relies on:

• Start bit (always 0): Signals the receiver that data is coming.
• Data bits (5–9 bits): The actual payload (e.g., ASCII character).
• Parity bit (optional): Simple error-checking (even/odd).
• Stop bit (always 1): Marks the end of transmission.

How It Works: Bit Timing in Asynchronous Mode

timeline
    title Asynchronous Data Frame (8N1: 8 data bits, No parity, 1 stop bit)
    Start Bit: 0 (1 bit)
    Data Bits: 1 0 0 0 0 1 1 0 (8 bits, e.g., 'A' in ASCII)
    Parity: (None)
    Stop Bit: 1 (1 bit)

• Baud rate: Bits per second (e.g., 9600 baud = 9600 bits/sec).
• No clock signal: Sender/receiver must agree on baud rate beforehand.

Worked Example: Sending 'A' (ASCII 65) at 9600 Baud

• Time per bit: $\frac{1}{9600}\approx 104.2\mu s$.
• Total frame time: 10 bits × 104.2 µs = 1.042 ms.
• If sender transmits 'A' every 2 ms, the receiver sees:

  0 (start) | 0 1 0 0 0 1 1 0 (data) | 1 (stop) | [idle: 958 µs]
  

3. The 8251 USART: Architecture and Registers

The 8251 USART (Universal Synchronous/Asynchronous Receiver/Transmitter) is a programmable chip that handles both synchronous (clock-based) and asynchronous (start/stop-bit) serial communication. It is widely used in embedded systems (e.g., industrial controllers, modems).

Block Diagram of 8251 USART

Key Components:

1. Control Register: Configures mode (sync/async), word length, parity, baud rate.
2. Status Register: Reports errors (overrun, framing, parity) and buffer status (empty/full).
3. Transmit/Receive Buffer: Holds data temporarily.
4. Baud Rate Generator: Derives clock from input frequency (e.g., 1.8432 MHz → 9600 baud).

Register Fields: Mode Command Register (Asynchronous Mode)

4. RS-232 Standard: Electrical Levels and Handshaking

RS-232 (Recommended Standard 232) defines:

• Electrical levels: -3V to -15V = 1 (logic high), +3V to +15V = 0 (logic low).
• Connectors: DB-25 (older) or DB-9 (modern).
• Handshake lines: Ensures reliable data transfer between DTE (Data Terminal Equipment, e.g., PC) and DTE (Data Circuit-terminating Equipment, e.g., modem).

RS-232 Pinout (DB-9 Connector)

Key Signals:

• TXD/RXD: Transmit/Receive data.
• RTS/CTS: Hardware flow control (sender waits for CTS before transmitting).
• DTR/DSR: Indicates if devices are ready.

Real-World Example: Ncell Modem Configuration via RS-232

Ncell’s 3G/4G modems use RS-232 for AT command-based configuration:

1. A PC connects to the modem via RS-232 (DB-9).
2. The PC sends AT (attention) commands (e.g., AT+CGATT=1 to attach to the network).
3. The modem responds with OK/ERROR over RXD.
4. Handshake: RTS/CTS ensures the modem isn’t overwhelmed by rapid commands.

5. Synchronous vs. Asynchronous Communication

When to Use Which?

• Asynchronous: Low-speed, simple devices (sensors, GPS, modems).
• Synchronous: High-speed, multi-device systems (SPI for sensors, I²C for EEPROM).

6. Advantages of Serial I/O Over Parallel

1. Longer cable distances: Less signal degradation (e.g., RS-232 supports up to 15m).
2. Fewer wires: Reduces cost and complexity (e.g., USB uses 4 wires vs. 8 for parallel).
3. Easier interfacing: Standard protocols (UART, RS-232) work across vendors.
4. Scalability: Supports daisy-chaining (e.g., I²C buses).

Disadvantage:

• Slower for short distances: Parallel is faster for CPU-memory communication.

7. Worked Example: 8086 Assembly Program for Serial I/O (USART)

Task: Send the string "HELLO" via USART (8251) at 9600 baud, 8N1 (8 data bits, no parity, 1 stop bit).

Steps:

1. Initialize 8251:
   • Set mode (async, 8N1, baud rate divisor).
   • Enable transmitter/receiver.
2. Send each character:
   • Load data into transmit buffer.
   • Wait for transmit buffer empty (TBE) flag.

Assembly Code (8086):

; Assume 8251 base address at 0x80 (simplified)
MOV AL, 0x40    ; Mode command: async, 8N1, divisor=1 (9600 baud)
OUT 0x80, AL    ; Send mode command to 8251
MOV AL, 0x0E    ; Enable transmitter/receiver
OUT 0x80, AL

; Send "HELLO"
MOV SI, OFFSET STRING
MOV CX, 5       ; 5 characters
NEXT_CHAR:
    MOV AL, [SI]
    OUT 0x81, AL ; Send to TX buffer
    INC SI
    LOOP NEXT_CHAR
    HLT

STRING DB 'H', 'E', 'L', 'L', 'O'

Real-World Tie-In: eSewa Terminal Serial Communication

eSewa’s payment terminals use USART to communicate with:

• Card readers (Magnetic stripe/EMV chips) via UART.
• Printers (for receipts) via RS-232.
• Central server (for transaction validation) via GSM modem (AT commands over UART).

8. Short Notes (Exam-Friendly Summaries)

a) Asynchronous Serial Data Communication

• No clock signal: Uses start/stop bits for synchronization.
• Baud rate: Must match sender/receiver (e.g., 9600 baud).
• Frame format: Start bit + data + parity + stop bit.
• Example: UART in microcontrollers (Arduino, Raspberry Pi).

b) RS-232 Standard

• Electrical levels: ±3V to ±15V (inverted logic).
• Connectors: DB-25 (full) or DB-9 (simplified).
• Handshake: RTS/CTS for flow control.
• Limitations: Max 15m distance, no built-in error correction.

c) Addressing Modes (Bonus for ALP Questions)

• Direct: MOV AX, [SI] (SI holds address).
• Indirect: MOV AX, [BX] (BX holds address).
• Register Indirect: MOV AX, [DI] (DI holds address).
• Base+Index: MOV AX, [BX+SI] (combines two registers).

In the Real World

1. eSewa Payment Terminals

   • Idea Used: USART (asynchronous serial) for communication between the terminal’s MCU and:
     • Card reader (UART at 115200 baud, 8N1).
     • Printer (RS-232 for receipt printing).
   • Why? Low-cost, reliable serial links for secure transactions.

2. Ncell 4G Modems (AT Commands)

   • Idea Used: RS-232 serial for AT command-based configuration.
   • Example Command:

     AT+CGATT=1  ; Attach to GPRS network
     AT+CMGF=1   ; Set SMS to text mode
     AT+CMGS="+97798XXXXXXXX"  ; Send SMS
     

   • Handshake: RTS/CTS ensures the modem processes commands without overflow.

3. Industrial PLCs (Programmable Logic Controllers)

   • Idea Used: Synchronous serial (Modbus RTU) over RS-485 for:
     • Remote sensor data collection.
     • Motor control signals.
   • Why? Long-distance (up to 1.2 km), noise-resistant twisted-pair wiring.

Exam Tip

1. Diagrams Are Mandatory:

   • For 8251 USART, draw the block diagram (CPU, control/status registers, buffer, baud generator).
   • For RS-232, sketch the DB-9 pinout and label TXD/RXD/RTS/CTS.
   • For asynchronous frames, show the start/stop bits + data waveform.

2. Short Notes = Bullet Points:

   • Questions like "Explain RS-232" expect electrical levels + connectors + handshake lines.
   • "Advantages of serial I/O" → longer distance, fewer wires, cost.

3. Assembly Programming:

   • If asked to send a string via USART, show:
     • Initialization (mode command).
     • Loop to send each character.
     • Wait for TBE flag (transmit buffer empty).

4. Real-World Examples:

   • Link USART to eSewa terminals or RS-232 to Ncell modems.
   • For baud rate calculations, use: $$\text{Baud Rate}=\frac{\text{Clock Frequency}}{\text{Divisor}}$$ (e.g., 1.8432 MHz / 192 = 9600 baud).

5. Common Pitfalls:

   • Forgetting stop bits in async frames (always include 1 or 2).
   • Mixing RS-232 levels (remember: 0 = +3V to +15V, 1 = -3V to -15V).
   • Not waiting for TBE in assembly programs (leads to data loss).