One Article to Understand: The difference between serial communication RS232 vs RS485

If we compare serial communication to traffic, then UART can be likened to a bus station, and a frame of data to a vehicle. Just as a vehicle on the road must adhere to traffic rules, the speed limits vary depending on the road—30 or 40 km/h in urban areas, and up to 120 km/h on highways. The specific route and speed limits depend on the defined protocol. Among the common serial protocols in CPDEVICE's rugged vehicle-mounted smart tablets are RS232 and RS485, each with their subtle differences. Let's delve into these protocols together.

RS232 and RS485: Paradigms of Serial Communication

Before we explore the differences between RS232 and RS485, it's crucial to understand serial and parallel communications, which are two primary methods for data transfer between computers and other electronic devices.

What is Serial Communication?

Serial communication involves encoding data or information in a specific format and then transmitting it bit by bit in sequence over a single wire. After sending one character, the next is sent. During data reception, each bit is received sequentially from a single wire and compiled into a character for further processing by the CPU. This method is suitable for long-distance communications, requiring fewer communication lines and capable of transmitting over long distances. RS232 and RS485 are typical representatives of serial communication.

What is Parallel Communication?

In parallel communication, all bits of data are transmitted simultaneously. This method is faster but requires multiple communication lines. For example, transmitting 8-bit data requires eight data lines, along with several control signal lines. As the transmission distance increases, the cost of communication lines becomes significant, and reliability decreases with distance. Thus, parallel communication is suitable for short-distance data transfer.

RS232 and RS485: Serial Communication Methods and Interface Definitions

Having understood serial communication, the next step is to grasp the communication methods and interface definitions of RS232 and RS485.

Different Communication Modes of RS232 and RS485

The common serial communication methods include simplex, half-duplex, and full-duplex. Although RS232 and RS485 share serial communication traits, their operational modes differ:

• Simplex: Data transfer in simplex mode only supports one-way transmission. That is, 'A' can only send signals, and 'B' can only receive. Communication is one-directional, similar to a lighthouse emitting light signals while a positioning system only receives.

• Half-Duplex: Half-duplex allows data transfer in both directions but only one direction at a time. This mode is effectively a direction-switching simplex communication. Imagine a walkie-talkie, where you don't always receive an immediate reply, and communication isn't possible if both parties are either sending or receiving at the same time.

• Full-Duplex: Full-duplex supports simultaneous data transmission in both directions, combining two simplex communications. This mode allows 'A' to send signals to 'B' while 'B' can simultaneously send signals to 'A', akin to a phone conversation where both parties can speak and listen.

Different Interface Definitions of RS232 and RS485

RS232 is the most widely used serial interface in computing and communication industries, operating in full-duplex mode and requiring a ground, transmit, and receive line. It only supports point-to-point communication.

RS485 uses balanced transmission and differential reception, offering excellent interference resistance, with signal transmission over kilometers. It supports two-wire and four-wire configurations. The two-wire mode operates in half-duplex, preventing simultaneous sending and receiving.

Why Does RS485 Have Superior Interference Resistance?

When using common-mode transmission, common-mode noise can superimpose on the output signal, corrupting the original signal, as in RS232.

However, RS485 uses differential (or balanced) transmission, where the transmitted signals + and - are in opposite phases.

For common-mode noise, ideally, it is equal and in-phase on both lines, while the receiving end, acting like a subtractor, retains the useful signal due to the opposite phases, and noise is canceled out.

TTL, RS232, RS485 Logic Levels

• TTL Standards:
  • Output Low (L): <0.8V; High (H): >2.4V
  • Input Low (L): <1.2V; High (H): >2.0V
• RS232 Levels:
  • Logical 1: -3 to -15V
  • Logical 0: +3 to +15V
• RS485 Levels:
  • Voltage difference between Output A and B: High (H): +2 to +6V, Low (L): -2 to -6V
  • Voltage difference between Input A and B: High (H): >+200mV, Low (L): <+200mV

Other Differences Between RS232 and RS485

CPDEVICE's latest generation of vehicle-mounted smart tablets, including APOLLO 12 PRO, SPRING 10 PRO, and SPRING 10 PLUS, support a maximum data transfer rate of 1Mbps for RS232, simply by choosing the appropriate transceiver chip for high-speed applications.