Should Fixing Rs485 Cable Take 60 Steps? > 자유게시판

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

To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. The DWOM bit determines whether Port D needs pull-up resistors; it should be set to 0. The MSTR bit determines whether the device is a master or slave. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. Use a 485 to USB serial cable, connect it to the interface of the 485 device and the computer's USB port, assign a COM port on the computer, open serial debugging assistant software, set the serial port parameters consistently, input the corresponding COM port, and monitor the data from the 485 device.

If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. This shields the inner data wires from external interference. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference.

These pins have to be connected to handshaking outputs at the PC side on the RS232 port. InitRS485() configures Port D to ensure that bit 5 is an output. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. Then reading the data that was received (by reading the SPDR) or initiating a new data transfer (by writing to the SPDR) automatically clears the SPIF flag. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. A RS232 - RS422 converter is a 4-wire, full-duplex, differential protocol with multi-drop capabilities and a recommended distance limit of 4000ft. ANSI/TIA/EIA-422-B is the American national standard for the RS422 protocol. In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message.

Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. Thus, the master has only one input, MISO, which is the slave’s only output. The arrows in the diagram point to pins configured as inputs, and originate from output pins. The central four pins remain their original RS232 functionality. The actual functionality is determined by DIP-switches on the mainboard of the PLC. You can implement the slave select lines by configuring Port A pins as outputs. Pin 1 and 6 however are reassigned to the A and B lines when the port is used in RS485 mode. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints. If your application requires use of the secondary serial port as well as other interrupt routines, rs485 cable the key is to keep the interrupt service routines short and fast. The maximum sustainable baud rate on the secondary serial port is 4800 baud. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. 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.