Again the amount of loss depends of the values of the series and parallel resistance and the loss will increase with frequency. 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. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. The CPOL, CPHA, SR1 and SPR0 configure the SCK pin’s clock polarity, clock phase, and clock rate. The clock’s polarity is controlled by a bit named CPOL (clock polarity) and its phase is controlled by CPHA (clock phase). 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. 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. 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.
It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS. Ultimately, it can be thought of in a way that serial communication is more made for this usage as compared to the common USB and ethernet. A 120 Ω resistor on a 100 Ω cable will dramatically reduce the ringing compared to no termination. Low data rates are primarily limited by the DC resistance of the cable (the effects of the DC resistance of the cable are made worse if a termination resistor is used) and high data rates are limited by the AC effects of the cable on signal quality. Both types of cables typically have a characteristic impedance of 120 ohms, which helps to ensure signal integrity and reduce reflections.
The range of RS232, RS422, and RS485 can be extended using repeaters, which regenerate the signal to overcome signal attenuation over long distances. It is known for being able to be used effectively over long distances and in electrically noisy environments. The maximum speed of RS422 is 10 Mbps at distances up to 12 meters. The transmission speed can range from 300 baud to 10 Mbps and can support up to 32 devices on a single bus. RS485 is made for serial communication with high speeds, support for long distances, and support for multiple devices (slaves). For extended distances, the need for proper grounding, shielding, and consideration of cable impedance can become more critical, adding to the overall system complexity. Many cable manufacturers can recommend a 120 Ω cable intended to work with RS-422 or RS-485. The maximum distance can vary depending on factors like cable quality, noise environment, and speed. First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423.
RS422 and RS485 are often used in industrial control systems, data acquisition systems, and other situations where long-distance, high-speed, noise-immune communication is needed. In some circumstances a one-way data flow may suffice. 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. Thus, the master has only one input, MISO, which is the slave’s only output. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. 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. 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. This setting is only relevant for the master device, as it is the master’s clock which drives the transfer. The CPHA bit determines whether data is valid on the leading or trailing edge of the clock.
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