Introduction to Serial Communication

Drone data transmission refers to transmitting flight data collected by the aircraft - such as altitude, speed, GPS information, sensor data, etc. - to a ground control station or cloud server for analysis and processing through data transmission modules. The data transmission employs specialized modules whose working principle is similar to wireless network transmission, allowing data to be transmitted to ground equipment via radio signals.

Serial communication is one implementation method of data transmission that uses serial communication protocols for data transfer. It transmits data bit by bit sequentially, with each bit occupying a fixed time duration. Requiring only a few wires for information exchange between systems, it's particularly suitable for long-distance communication between computers, and between computers and peripherals.

In serial communication, TTL, RS-232, RS-422 and RS-485 are common electrical level standards, with their main differences lying in voltage levels and transmission methods of electrical signals.

1. TTL

TTL (transistor transistor logic) is a transistor-transistor logic level standard. The TTL level specification defines +5V as logic "1" and 0V as logic "0" (when using binary data representation). This data communication and level specification method constitutes the TTL signal system, which serves as the standard technology for communication between different components within computer processor-controlled devices.

TTL devices output low levels below 0.8V and high levels above 2.4V. For inputs, voltages below 1.2V are recognized as 0, while those above 2.0V are recognized as 1. This gives TTL levels a low-level noise margin of (0.8-0)/2=0.4V and a high-level noise margin of (5-2.4)/2=1.3V.

TTL '0' and '1' Representation

TTL Level Diagram

2. RS232

RS232 is an interface electrical characteristic standard, formally known as the "Interface Technical Standard for Serial Binary Data Exchange Between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)". Established by the Electronic Industries Association as an asynchronous transmission standard interface. "RS" stands for "Recommended Standard" with 232 being the identifier. When RS232 serial ports appear on personal computers, they are often referred to as "COM ports". Current RS232 ports use 9-wire configurations with D-SUB 9 connectors, while historical 25-wire versions are no longer in use.

The RS-232 standard specifies data rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, and 19200 baud per second. The driver allows 2500pF capacitive loading, with communication distance limited by this capacitance.

RS-232 uses unbalanced transmission (single-ended communication). With only 2-3V difference between transmission and reception levels, it has poor common-mode rejection. Combined with distributed capacitance in twisted pairs, its maximum transmission distance is approximately 15 meters at up to 20kb/s. Designed for point-to-point communication (single transmitter-receiver pair) with driver load impedance of 3-7kΩ, RS-232 is suitable for local device communication.

RS-232 '0' and '1' Representation

RS232 Wiring Diagram

3. RS422

Formally titled "Electrical Characteristics of Balanced Voltage Digital Interface Circuits", this standard defines interface circuit characteristics. A typical RS-422 interface uses four signal lines plus a ground wire (total 5 wires). With high input impedance receivers and stronger drive capability than RS232, it allows connecting up to 10 nodes on the same transmission line - one master device and multiple slave devices (slaves cannot communicate directly). RS-422 supports point-to-multipoint bidirectional communication. With receiver input impedance of 4kΩ, the maximum driver load capacity is 10×4kΩ+100Ω (termination resistor). The separate transmit/receive channels eliminate need for data direction control, with device signaling handled through software (XON/XOFF handshake) or hardware (dedicated twisted pair).

4. RS485

RS-485/422 employs balanced transmission and differential reception: the transmitter converts TTL level signals from serial ports into differential signals (A/B lines) for transmission, which are then reconverted to TTL levels at the receiver. Using twisted pair cables and differential transmission provides excellent common-mode interference rejection. The bus transceiver's high sensitivity (detecting voltages as low as 200mV) enables signal recovery over distances exceeding one kilometer.

RS-485 networks typically use terminated bus topology (daisy-chained nodes). Star or ring topologies require 485 repeaters or hubs. The international RS-485 standard doesn't specify connector types, allowing terminal blocks or DB-9/DB-25 connectors.

RS422/485 '0' and '1' Representation

RS422/485 Wiring Diagram

5. Differences Between TTL, RS232, RS422 and RS485

6. Supplementary Knowledge

1. Serial Communication Modes: Simplex, Half-Duplex and Full-Duplex

Simplex: Allows data transmission in one direction only at all times

Half-Duplex: Allows unidirectional transmission at any given time, with transmission direction selectable at different times

Full-Duplex: Allows bidirectional transmission simultaneously

2. Serial Data Frame Format

Start Bit: 1-bit logic 0 (low level) initiating data transmission

Data Bits: 5-8 bits (typically 8 bits/1 byte), transmitted LSB first

Parity Bit: Optional odd/even parity check. Even parity requires even number of 1s in data+parity bits; odd parity requires odd number

Stop Bit: 1, 1.5 or 2-bit logic 1 (high level) marking end of transmission

Idle State: High level on data line indicating no transmission

3. Differential Signaling: Transmits voltage difference between two wires. Provides strong noise immunity and long transmission distances.