The use of shielded twisted-pair cables helps to reduce and eliminate the distributed capacitance between the two 485 communication lines and the common mode interference generated around the communication lines. The GROUND line serves as a common voltage reference for the master and slave. Not too long ago I was running 240 VAC out to a well pump and got a shock off the protective ground lead. Can be work well even the temperpature change suddenly. 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. The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. SenseCAP is compatible with a variety of communication protocols, including LoRa, 2G, 4G, NB-IoT, and more. This configuration works for many SPI devices, including the optional battery-backed real-time clock. It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. 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.

There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. The CPOL, CPHA, SR1 and SPR0 configure the SCK pin’s clock polarity, clock phase, and clock rate. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. As the master transmits its data, 8 bits of data are simultaneously received. Transmissions are always initiated by the master device, and consist of an exchange of bytes. This ability to exchange messages means that the SPI is capable of full duplex communication. 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. This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. The communication line should be as far away as possible from interference sources such as high-voltage wires, fluorescent lamps, etc. When the communication line cannot be avoided from interference sources such as power lines, the communication line should be perpendicular to the power line, not parallel, and cannot be bundled together, and use high-quality twisted-pair cables.

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. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition. The SPE bit turns on the SPI system. Setting SPE (SPI enable) to 1 turns on the SPI system. This setting is only relevant for the master device, as it is the master’s clock which drives the transfer. Also, in the diagram, the master’s /SS (slave select) is configured as an output. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers.

The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. 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. You can implement the slave select lines by configuring Port A pins as outputs. 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. By setting this output LOW, the slave’s input /SS is pulled LOW. 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.

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