RS485 Automatic Converter - R.E.Smith Serial Communications RS485 RS232 RS422 > 자유게시판

They can easily update one PC, which will take care of all prices to the store. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). 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. The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. By setting this output LOW, the slave’s input /SS is pulled LOW. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS. Serial Monitor works by installing a filter driver on top of a serial device driver. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. In a single phase system, analog and digital instruments are selected to measure voltage, current, frequency, and power factor.

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. The CPHA bit determines whether data is valid on the leading or trailing edge of the clock. The status of a device as master or slave determines how the various pins must be configured. Alternatively, the if the SPI interrupts are enabled, the SPI interrupt handler determines what caused the interrupt by reading the SPSR register to see which of the three status bits is set. 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. If SPIF is set, reading the received data or initiating a new data transfer automatically clears the SPIF bit. Any of these conditions may generate an interrupt if the SPIE (SPI interrupt enable) bit in the SPCR control register is set. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. Transmissions are always initiated by the master device, and consist of an exchange of bytes.

Once the bytes have been exchanged, the master may write a new byte to initiate another byte exchange. 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 CPOL and CPHA bits configure the synchronous clock polarity and phase and specify when valid data is present on the MISO and MOSI data lines. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. 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. The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port. This bit should be set only after all other SPI configuration is complete.

The SPE bit turns on the SPI system. Setting SPE (SPI enable) to 1 turns on the SPI system. This configuration works for many SPI devices, including the optional battery-backed real-time clock. Consult the data sheets for any peripheral devices that you are interfacing to the SPI and, if a different configuration is needed, follow the instructions below to set up the appropriate SPI data transfer protocol. The flexibility and power of the 68HC11’s serial peripheral interface supports high speed communication between the 68HC11 and other synchronous serial devices. Serial communication also has a deterministic behaviour to avoid collisions of data packets, making it more reliable for a linkage system with many devices. This detects the presence of more than one master on the SPI bus. This means that the unit cannot initiate a communication, it can only respond when addressed by the master computer. The RS485 communication interface allows the slave unit (i.e. control module) to be interrogated and some options programmed by a remote computer. The cabling of the industrial communication systems (Modbus RS485) is different in some ways from the cabling used for power cabling and the electrician may experience some difficulties if he is not an expert in Modbus communication networks.

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