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

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 master and slave could even exchange ascii QED-Forth commands. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. The master and slave can then exchange data. As the master transmits its data, 8 bits of data are simultaneously received. 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. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. The SPIE bit in the SPCR (SPI control register) enables SPI interrupt handling.

If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. 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. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. The distinction between master and slave is an important one. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. The GROUND line serves as a common voltage reference for the master and slave. Unlimited phone support. Schematics and data available on line. To interface devices that support synchronized serial interfaces, but are not configurable like the QScreen, determine the device’s requirements for clock phase and polarity and configure the QScreen’s CPHA and CPOL accordingly.

0), and expect valid data to be present on rising clock edges. The rate of data transmission is expressed in bits per second, or baud. Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. Although the devices would share the same network, communications would only be understandable by members of the same group. The flexibility and power of the 68HC11’s serial peripheral interface supports high speed communication between the 68HC11 and other synchronous serial devices. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate.

This detects the presence of more than one master on the SPI bus. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference. To ensure that no two devices drive the network at the same time, rs485 cable it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. The A signals of all devices should be connected with each other and the same goes for the B signals. The one you choose depends on the specific device, or devices you will be connecting to. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. In this section we will consider the most general and simple configurations. The next section describes the registers that configure and control the QScreen Controller’s SPI. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below.