RS485 vs Modbus - What is the difference

Is the RS485 communication tutorial the same as the communication protocol

RS485 is a serial data transmission standard widely used in industrial implementations. When implementing RS485 cables communication, the communication protocol protocol is usually used. This distinguishes max485 from the industry standard protocol for serial communication connections, the RS232 protocol communicates via ANSI code character transmission. If you will be using equipment that uses the RS485 arduino protocol, it is important to understand the communication protocol.

If you’re a technician responsible for supporting RS485 devices, there are a few tips to make your job easier. We provide this short tutorial that demonstrates the difference between the communication protocol and the RS485 protocol.

What is RS485 and RS232

RS485 and RS232 are only the physical protocol of communication (ie interface standard), RS485 is the differential transmission mode, RS232 is the single-ended transmission mode, but the communication program does not have much difference.

RS485is is an industry standard protocol similar to serial communication connection, used to realize serial data communication. The two protocols use different electrical signals to achieve data transmission.

One of the reasons for using the RS485 interface in industrial environments is its ability to serve multiple devices on the same bus. This eliminates the need for multiple available interfaces when querying multiple devices. This can be done by using a bus terminator, moving a switch, or screwing a small resistor on the terminal. Care must be taken to ensure the correct interface is used, as the RS485 and RS232 protocols are not fully compatible. You can build a gateway between an industry standard serial communication connection and an RS485 interface, but typically you will see the RS485 protocol connected directly to a RS485 to USB, GSM or Ethernet port. This lower cost method does not require the use of any additional components.

Devices that use the RS485 port usually use the communication protocol protocol. When the American Standard Code for Information Interchange protocol for industry standard equipment connected by serial communication is no longer used, it is recommended to study the differences between the communication protocol protocols. That’s the purpose of this short communication protocol tutorial.

The following table summarizes the characteristics of the RS485 protocol. RS485 provides half-duplex transmission on balanced lines, and the transmission distance can reach 1.2 kilometers.

If you need more information about RS485, you can consult EIA (Electronic Industries Association) or a company that manufactures equipment compatible with this protocol, such as asTexas Instruments or Maxim Semiconductor. 

What is RS485 connector

RS-485 is a serial communications standard that provides asynchronous communication capabilities, such as hardware flow control, software flow control, and parity check. It has been widely used for decades.

What is a half-duplex system

A half-duplex system consists of one or more transmitters and receivers, and only one transmitter can work at a time. Communication is achieved by the sender making a request to a specific receiver. The transmitter then waits for a predetermined length of time for an answer, or decides that no answer will be received from the receiver.

The RS485 network is built on the master/slave framework. The master acts as a transmitter, making requests to the designated slave as a receiver. The master device listens for the response, and if it does not get a response within the appropriate time frame, it will terminate the communication.

What is Modbus

Communication protocol message

The principle of communication protocol message. In a Modbus RS485 network, communication begins when a master device sends a query to a connected slave device. The slave spends time monitoring the network for queries specific to it. When a query is received, it performs an action or replies with a response to the main server. The query is only initiated by the master device. Communication Protocol The protocol provides the host with the choice of sending a message to a specific slave device or communicating with all slave devices at the same time. This is achieved through a special “broadcast” address 100. Products such as Integra and the Society for Psychical Research do not support the use of this broadcast address.

Use coils to send read and write operations via communication protocol messages. Coils consist of 16-bit words and binary registers. The slave can only respond to received messages, but cannot initiate communication with the master.

Each slave device connected to the RS485 bus in parallel is assigned a unique communication protocol slave ID. All communication protocol communication starts by sending the slave ID, either to remind the slave to accept the query, or to inform the master which device provides Reply.

In the same way that an industry standard connection for a serial communication connection works, the slave and master devices also need to be properly configured. Parameters such as speed and parity need to be synchronized across the network. 

Communication Protocol Message Format

The message format used in the communication protocol communication between the master device and the slave device is defined in the protocol.

An Amodbus query consists of a device (or broadcast) address, a function code that defines any requested action, the data returned by the request, and error checking fields.

An Amodbus response consists of fields to verify that the requested operation has been performed, data sent with the response, and error checking fields. If the slave device cannot satisfy the request, or if an error affects the receipt of the message, the slave device will create an error message in its response.

Serial transfer mode

Communication Protocol Communication adopts two different serial transmission modes: ASCII and RTU.

Modbus ASCII (American Standard Code for Information Interchange) mode sends each 8-bit message in ASCII as two ASCII characters. The advantages of mode include the ability to easily monitor messages on a text console. This mode also allows a one-second interval to be acceptable without initiating a timeout.

Modbus RTU (Remote Terminal Unit) mode transmits information in different formats. Here, an 8-bit message contains two four-digit hexadecimal characters. Data using this transmission mode needs to be sent in a continuous stream, which provides higher throughput at the same baud rate than the ASCII mode.

Conclusion

RS485 is a serial data transmission standard widely used in industrial implementations. The Modbus protocol is commonly used when implementing RS485 communication. This differentiates RS485 from the RS232 protocol which communicates by transmitting with ASCII characters.

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