Why Does The Device Use RS485 Serial Communication

PublishTime： 2023-07-26 Article Source：usr News

1. A: Why does the device use RS485 serial communication?

Q : RS485 devices can be connected to a computer and dropped multiple times in multiple places in a network style configuration. Devices can be up to 4000 feet (1220 meters) away and up to 32 nodes (devices) can be connected before an Ethernet to RS485 Converter is required.More nodes can be connected using the Ethernet to RS485 Converter up to the addressable limit of the device used (typically 256).

2. A: How do I connect a computer to an RS485 device?

Q : You can use an Ethernet to RS485 Converter or a USB to RS485 Converter, or install an RS485 interface card in your computer. A PCI bus, PCMCIA/CardBus, or ISA bus card may be used.

You need to know how many wires are used on the RS485 port of the device you need to connect. The data (A) and data (B) lines and ground are "2-wire" 485 connections, if there are more connections and they are labeled TD (A) and TD (B) and RD (A), RD (B)) and ground, you need a "4-wire" converter.

3. A: What should I look for in Ethernet to RS485 Converter to work with Windows?

Q : It is recommended to use a converter or interface card with automatic send data control in Windows and other multitasking operating systems. This is important for half-duplex 2-wire operation. Otherwise, data may be lost. If you program with Visual Basic, sending data controls automatically can simplify your life. (Refer to B & B Electronics Technical Library for send data and RTS control illustrations)

4. A: What type of connector is used for RS485?

Q : The RS485 standard does not specify the type of connector or pinout. RS-485 connectors can be DB9, DB25, terminal block, RJ11, RJ45 or one of the circular DIN connectors.

5. A: What is the maximum connection distance for RS485 devices?

Q : RS485 without Ethernet to RS485 Converter has a maximum distance of 4000 feet (1220 meters) and a baud rate of up to 90Kbps. Add an Ethernet to RS485 Converter or Optically Isolated Repeater every 4000 feet to extend that distance.

6. A: Can I increase the RS485 signal to add more nodes or distances?

Q： Not exactly. You can repeat and refresh the signal periodically along the cable, but don't push it above spec. B & B offers different flavors of Ethernet to RS485 converters, such as 485OP or 485OPDR, that let you add another 32 nodes (or 4000 feet) to your RS-485 network.

7. A: Why do I need an Ethernet to RS485 Converter with more than 32 nodes/devices?

Q : The input impedance of a standard RS485 node is rated at 12K ohms. A standard RS485 transmitter cannot drive more than 32 devices and a pair of 120 ohm termination resistors. Another 32 nodes can be connected by simply adding an isolated repeater.

8. A: How many RS485 devices can be installed on a network?

Q : Each RS485 device must have a unique address. This typically allows up to 256 nodes.

9. A: What software protocol is specified for RS485?

Q : The EIA standard does not specify the software protocol, and the RS485 standard only defines the electrical interface. It is basically an extension of the RS422 standard to handle multidrop with many slave devices that can share the same wire pair for transmit and/or receive.Some devices do not require a specific protocol, just a software command with the correct address byte or bytes at the correct command location. Many popular protocols use RS485 as their physical layer.

10. A: Where do I use the RS485 network?

Q: Any place where you need to install multiple devices in different locations and where the device has a built-in RS485 port or can accommodate RS485 addressability. RS485 has been used in many factory environments to collect data and control addressable motor drives, cameras, PLCs, clocks, bar code scanners for process control and other factory equipment.

11. A: What does an RS485 network consist of?

Q: At least one master device with RS485 interface, such as computer, embedded controller, RTU or PLC and at least one RS485 slave device. Additional RS485 devices can be added to extend the network and functionality.

12. A: How do I make an RS485 network using an RS232 device?

Q1: A standard 4-serial Ethernet to RS485 Converter can be used on each device if the RS232 device is addressable (only responds to commands with unique bytes or byte sequences) and uses only receive, transmit, and ground connections. The master device will use a standard converter or interface.

Q2: If the RS232 device is not addressable, you can use an RS485 addressable RS485 to RS232 converter, such as a 485 DSS. Up to 256 devices can be added to the network using the Ethernet to RS485 Converter.

13. A: What are the benefits of using RS485 for data acquisition compared to RS232?

Q : Noise immunity. RS485 is less susceptible to spurious EMI/RFI "noise" than other serial protocols due to its differential signaling. In addition, one serial port can communicate with multiple devices from several feet to thousands of feet away. No additional serial port is required.Optically isolated converters can be used to protect computers from voltage and ground loops of connected equipment.

14. A: I am interested in programming software for RS485 control. Do you have an example of Visual Basic?

Q : There are VB source code demos to access our 485 SDD16 and 485 SDA12 data acquisition modules. The command code is similar to the RS232 version, 232SDD16 and 232SDA12, except for the additional RS485 functionality of the module. Note: We used a converter with automatic send data control in these RS485 demos.

15. A: What is the difference between RS422 and RS485?

Q : An RS422 device is not a tri-state transmitter, but an RS485 device. With RS422, two units cannot connect to one receiver because when one device tries to "talk", the other tries to hold the line in one state and the other tries to switch them high/low.RS422 is always full duplex (if the device is not listening only), RS485 can be full (4-wire mode) or half duplex (2-wire mode). RS422 devices can have more signals for handshaking (RTS/CTS and DTR/DSR). RS485 will never have more than 2 signals, receive and send.

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