Every instrument utilizes its own specific communication protocol. Common examples include Modbus, RS-232, RS-485, and HART. But how exactly do these protocols work, and what are their respective advantages and disadvantages?

      Communication Protocol: Also known as a communication specification, it is an agreement between two communicating parties regarding data transmission control. This agreement standardizes data formatting, synchronization methods, transmission speed, transmission procedures, error detection/correction mechanisms, and control character definitions. Both parties must adhere to these rules; it is also referred to as a link control procedure.

Commonly Used Instrument Communication Protocols

Modbus

RS-232

RS-485

HART

MPI (Multi-Point Interface)

Serial Communication

PROFIBUS

Industrial Ethernet

AS-i (Actuator-Sensor Interface)

PPI (Point-to-Point Interface)

Remote Wireless Communication

This article focuses primarily on the RS-232 and RS-485 communication protocols.

RS-232 Communication Protocol

      RS-232 is a serial physical interface standard established by the Electronic Industries Association (EIA). "RS" stands for "Recommended Standard," and "232" is the identifier number. RS-232 interfaces typically appear in 9-pin (DB-9) or 25-pin (DB-25) configurations. Traditional personal computers usually feature two RS-232 ports, designated as COM1 and COM2.

RS-232 Interface Configuration

      The RS-232 standard defines both 25-signal-line and 9-signal-line versions, comprising a primary channel and a secondary channel. In most applications, only the primary channel is used. For standard full-duplex communication, only a few signal lines are required: one transmit line, one receive line, and one ground line.

Transmission Rate

      The RS-232 standard specifies data transmission rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, and 19200 baud.

Remote Communication & Data Terminals

      The RS-232 standard was originally designed for connecting Data Terminal Equipment (DTE) to Data Circuit-terminating Equipment (DCE) in remote communications. Consequently, computer system requirements were not considered during its development. Today, however, it is widely adapted as a short-distance connection standard between computers (more accurately, computer interfaces) and terminals or peripherals. Clearly, some specifications of this standard are inconsistent with—or even contradictory to—modern computer systems. Understanding this historical context explains why certain RS-232 standards appear incompatible with contemporary computing.

"Transmit" and "Receive" Definitions

      In the RS-232 standard, "transmit" and "receive" are defined from the DTE perspective, not the DCE perspective. In computer systems, information is typically transferred between the CPU and I/O devices; since both are classified as DTE, both ends are capable of transmitting and receiving.

Electrical Characteristics

The EIA-RS-232 standard specifies electrical characteristics, logic levels, and signal line functions:

On TxD and RxD lines:

Logic 1 (MARK) = -3V to -15V

Logic 0 (SPACE) = +3V to +15V

On control lines (RTS, CTS, DSR, DTR, DCD):

Signal Active (ON state, positive voltage) = +3V to +15V

Signal Inactive (OFF state, negative voltage) = -3V to -15V

Disadvantages of RS-232

      High Signal Voltage: The high signal levels can easily damage interface circuit chips. Additionally, because RS-232 is incompatible with TTL logic levels, level-shifting circuits are required to interface with TTL-based systems.

      Low Transmission Speed: Maximum baud rate is limited to ≤20 Kbps in asynchronous transmission mode.

      Susceptibility to Noise: The interface uses a single signal wire and a common ground return, creating a single-ended transmission configuration. This makes it highly susceptible to common-mode noise interference.

      Limited Distance: The standard maximum transmission distance is 50 feet (practically ≤15 meters).

RS-485 Communication Protocol

      The RS-485 standard was developed as an enhancement to RS-232. It adds multi-point, bidirectional communication capability, allowing multiple transmitters to share a single bus. It also improves driver capability, adds collision protection, and extends the common-mode voltage range. It was later designated as the TIA/EIA-485-A standard.

Key Specifications

      Data Transfer Rate: Up to 10 Mbps

      Interface Design: Utilizes balanced drivers and differential receivers, significantly enhancing common-mode noise rejection and overall anti-interference performance.

      Supported Baud Rates: 1200, 2400, 4800, 9600, 19200, 38400 bps, and 125 Kbps

      Communication Mode: Asynchronous, half-duplex, serial

Data Formats:

      1 start bit, 8 data bits, 1 stop bit, no parity

      1 start bit, 8 data bits, 1 stop bit, odd parity

      1 start bit, 8 data bits, 1 stop bit, even parity

      (Note: The default data format is used when connecting to PROFIBUS fieldbus adapters.)

Transmission Distance & Multi-Station Capability

      The RS-485 standard specifies a maximum transmission distance of 4,000 feet. Theoretically, this can reach up to 3,000 meters, though in practical installations, the reliable limit is approximately 1,200 meters. Unlike RS-232, which supports only one transceiver per bus (single-station), RS-485 allows up to 128 transceivers on a single bus, enabling users to easily build device networks using a single RS-485 interface.

9-Pin Connector Pinout (RS-485)

Disadvantages & Installation Pitfalls

      When establishing RS-485 communication links, installers often simply connect the "A" and "B" terminals of each interface using a twisted pair while neglecting the signal ground connection. While this may work in many scenarios, it introduces significant hidden risks related to common-mode interference.

      Although RS-485 uses differential signaling—which theoretically does not require a reference point and only detects the potential difference between the two wires—the transceiver itself has a common-mode voltage range of -7V to +12V. The entire network can only operate reliably within this range. If the common-mode voltage on the bus exceeds these limits, communication stability will be compromised, and the interface may even be permanently damaged. Proper signal grounding is therefore essential for long-term reliability.

About Yuyao Xianglong Communication Industrial Co., Ltd.

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