What is RS485 and how is it used in Industrial Control Systems? > 자유게시판

• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

Local capacitors on both sides of the regulator are also helpful. Both the local and remote UARTs must be configured for the same communications parameters. I would highly discourage the use of extra signal pins in the connectors for "cable connected" checking that do not meet the same specs as the RS485 signalling signals. You can use one or both of the PDQ Board’s RS485 links to create such a multi-drop serial network. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. Transmissions are always initiated by the master device, and consist of an exchange of bytes. The only way this would not work is if the secondary device only takes "write only to device" transmissions with no possibility of any acknowledgement or return result. Is it likely that the shipped cable is configured differently or do I simply have no way of knowing that (short of dissecting the cable)?

For these secondary devices you have to implement your periodic heart beat check in some device specific way. UART is an Asynchronous transmission device hence there is no clock signal to sync the data between the two devices instead it uses start and stop bits at the start and end of each data packet respectively to mark the extremities of the data being transferred. UART interface support for 7 or 8 data bits, 1 or 2 stop bits and odd / even / mark / space / no parity. An optional parity bit can be specified to enable error detection by the UART. So long as the error between the actual baud rate and that specified is less than 1.5% (or the error between transmitter and receiver is less than 3%) there should be no communication errors. Shown below is the example of how a character is a transmitter over a UART data line.

Each UART implements transmit and receive FIFO buffers to reduce the required frequency of service by the host processor. Note that this Hackster project does something similar with splicing an ethernet cable to a UART breakout module. These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. Of the processor’s three synchronous SPI (Serial Peripheral Interface) ports, two are available for inter-processor communications on multi-processor systems, and the third is brought out to the Wildcard expansion bus. Unlike what happens in many energy distribution systems, the manner in which the devices are connected in parallel is important. For devices where you have to consider the devices at the far end of your cables to be unable to be modified, either hardware or software wise, you have to get clever regarding how you determine if the device is connected. To make sure that your USB to RS-485 cable & the device RS-485 port are working, have you tried using them with a Windows PC and the manufacturer's s/w? The device ships with an Ethernet RS-485 communication interface as well as a RS-485 to USB cable, which is said to be able to communicate with a PC using proprietary Windows based software (and driver, I think).

Think of RS485 as the younger, faster brother of RS232. The USB-RS485-PCBA converter provides a USB to RS485 serial interface with a simple USB type A connector and PCB for easy integration into a design or as the basis of a converter cable. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system. A simple method is often a status check. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Resistive termination - If the PDQ Board is at the end of the RS485 cable you can terminate the cable by installing jumper caps at both jumper locations, "Term" and "RTerm". For Serial2 RS232 operation: Remove the jumper shunt from "2 485En" (J7). The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). The only difference between the master and slave devices is that the master initiates the transmission.