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. The SPIE bit in the SPCR (SPI control register) enables SPI interrupt handling. 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. If you restart the RS485 Master, every Brick/Bricklet of the RS485 network will send the initial enumeration message again. 100% humidity, click the "humidity 100%" button and the reference will be stored. 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, rs485 cable the /SS line may be tied low between successive transfers. Note that the data is changed by the transmitting device one half clock cycle before it is valid. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input.

In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. 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 SCK pin’s synchronous clock signal has configurable phase, polarity and baud rate so that it can interface to a variety of synchronous serial devices. RS485 interface gives some more benefits compared to RS232: simplicity and the robustness for long distance transfer. The existence of an RS485 port on the socket makes it even a little bit more difficult. Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins.

Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. 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. This function properly configures the directions of the SPI I/O pins, and configures the data transfer such that data is valid on the falling trailing edge of the clock, with the clock idling in the low state. 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. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. If normal, data will be periodically transmitted.

This automatically activates the SCK clock which synchronously transmits the data. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. 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 diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. The received data byte is accessed by reading SPDR data register. Uno shield, shift register controlled by any 3 pins. When in full RS232 mode, the six available pins are assigned to the most used RS232 signals. This ability to exchange messages means that the SPI is capable of full duplex communication. Transmissions are always initiated by the master device, and consist of an exchange of bytes. This signal synchronizes the exchange of bytes between the QScreen and its peripherals. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS.