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예식일 : Where Will Rs485 Cable Be 6 Months From Now?
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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). If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. 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. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. The GROUND line serves as a common voltage reference for the master and slave. The data exchange format may be a line of ascii text. A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. Termination resistors also reduce electrical noise sensitivity due to the lower impedance.

Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register. 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. 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. As the master transmits its data, 8 bits of data are simultaneously received. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte.

In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code! A hardware reset (pressing down on the reset switch) has the same effect. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. By setting this output LOW, the slave’s input /SS is pulled LOW. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. 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.

Setting the MSTR bit initializes the QScreen as a master, and clearing the MSTR bit initializes it as a slave. 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. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. This ability to exchange messages means that the SPI is capable of full duplex communication. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. At the start of a transmitted character, the service routine takes about 65 µs.

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