Each instrument utilizes its unique communication protocol, with common ones including Modbus, RS-232, RS-485, and HART protocols, among others. How do these communication protocols actually operate, and what are their pros and cons?

Communication Protocol: Also known as communication procedures, it refers to an agreement between the two parties involved in communication regarding the control of data transmission. The agreement includes unified provisions for data formats, synchronization methods, transmission speed, transmission steps, error detection and correction methods, as well as control character definitions. Both parties must adhere to these rules, and it is also referred to as Link Control Procedures.

Common Instrument Communication Protocols:

Modbus Communication Protocol

RS-232 Communication Protocol

RS-485 Communication Protocol

HART Communication Protocol

MPI Communication

Serial Communication

PROFIBUS Communication

Industrial Ethernet

ASI Communications

PPI Communication

Remote wireless communication

Today, we'll mainly introduce the communication protocols RS-232 and RS-485.

RS-232 Communication Protocol

RS-232 is the Electronic Industries Association EIA (Electro) standard.The RS-232 standard is a serial physical interface specification established by the Electronic Industries Association (EIA). RS stands for "Recommended Standard," and the 232 refers to the number of bits. The RS-232 interface typically appears in two forms: with 9 pins (DB-9) or 25 pins (DB-25). Most personal computers have two RS-232 interfaces, known as COM1 and COM2.

RS-232 Interface

The RS-232 standard does not come in two versions with 25 signal lines and another with 9 signal lines. It includes one main channel and one auxiliary channel. In most cases, the main channel is used predominantly. For general full-duplex communication, only a few signal lines are needed, such as one transmit line, one receive line, and one ground line.

Transmission Speed

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

Remote Communication Connection Data Terminal

The RS-232 standard was originally developed for remote communication connections between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE), and did not take into account the application requirements of computer systems. However, it is now widely borrowed for local connections between computers (more accurately, computer interfaces) and terminals or peripherals. Clearly, some provisions of this standard are inconsistent and even contradictory with computer systems. With an understanding of this background, it becomes easier to comprehend the incompatibilities between the RS-232 standard and computers.

"Send" and "Receive"

In the RS-232 standard, "transmit" and "receive" are defined from the perspective of DTE, not DCE. Since in computer systems, information is often transmitted between the CPU and I/O devices, both of which are DTEs, both parties are capable of transmitting and receiving.

Electrical Characteristics

The EIA-RS-232 specifies electrical characteristics, logical levels, and various signal line functions.

On TxD and RxD:

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

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

On control lines such as RTS, CTS, DSR, DTR, and DCD:

Signal Valid (Connected, ON State, Positive Voltage) = +3V ~ +15V

Invalid signal (disconnected, OFF state, negative voltage) = -3V ~ -15V

RS-232 Drawbacks:

The interface signal level is high, which is prone to damaging the chip in the interface circuit. Additionally, due to incompatibility with TTL levels, a level conversion circuit is required to connect with TTL circuits.

(2) The transmission rate is relatively low, with a baud rate of ≤20Kbps during asynchronous transmission.

(3) The interface is formed by a signal line and a signal return line, constituting a common ground transmission. This common ground transmission is prone to common-mode interference, thus exhibiting weak noise immunity.

(4) Limited transmission distance; standard distance is 50 feet (actual ≤ 15 meters).

RS-485 Communication Protocol

The RS-485 standard was developed from the RS-232, enhancing multi-point, bidirectional communication capabilities—allowing multiple transmitters to connect to the same bus while also boosting the transmitter's driving power and collision protection features. It expanded the common-mode range of the bus, later named the TIA/EIA-485-A standard.

Propagation Rate:

The data transmission rate of RS-485 is 10Mbps.

RS-485 Interface:

The combination features a differential receiver with balanced drivers, which enhances the common-mode rejection ability, i.e., excellent noise interference resistance.

Porter Ratio:

1200bps、2400bps、4800bps、9600bps、19200bps、38400bps、125K

Communication Interface Method:

RS485 Interface: Asynchronous, Half-Duplex, Serial

Data Format:

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

When connected to the PROFIBUS fieldbus adapter, use the default data format.

The RS-485 interface has a standard transmission distance of 4,000 feet, but can actually reach up to 3,000 meters (theoretical data; in practical operation, the maximum distance is around 1,200 meters). Additionally, the RS-232-C interface allows only one transmitter to be connected to the bus, meaning single-station capability. In contrast, the RS-485 interface permits up to 128 transmitters to be connected to the bus, offering multi-station capability. This allows users to conveniently establish a device network using a single RS-485 interface.

9-Pin Interface

RS485 Interface Signal Meaning

RXD - Receive Data

4 RXD+ Data Received

TXD+ Transmits Data

TXD - Data Transmission

Flaw:

In many cases, when connecting RS-485 communication links, it is merely a simple matter to connect the "A" and "B" terminals of each interface with a pair of twisted wires, overlooking the connection of the signal ground. While this method often functions properly in various scenarios, it harbors a significant隐患: common-mode interference issues. The RS-485 interface uses differential signaling, which does not require detection of the signal relative to a reference point; the system only needs to detect the potential difference between the two wires. However, people often overlook the common-mode voltage range of the transceivers, which is -7 to +12V for RS-485 transceivers. The entire network can only operate normally when these conditions are met. When the common-mode voltage in the network lines exceeds this range, it can affect the stability and reliability of communication, and even damage the interfaces.

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