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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. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. 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 standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. It seems if there is a lot of RFI in the installation, shielded twisted cable can be justified. It requires minimal wiring and can share the wiring among several seats, reducing weight. 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.

The CPOL, CPHA, SR1 and SPR0 configure the SCK pin’s clock polarity, clock phase, and clock rate. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. And the results before was a degraded signal that the devices near the end would interpret the signal incorrectly. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices.

If a slave device has already stored a byte into its SPDR register, rs485 cable that byte will be exchanged with the master’s byte. A data transfer is initiated by a master device when it stores a message byte into its SPDR 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. The MODF bit is cleared by a read of the SPSR followed by a write to the SPCR. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. 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. Any of these conditions may generate an interrupt if the SPIE (SPI interrupt enable) bit in the SPCR control register is set.

It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. These characteristics make RS-485 useful in industrial control systems and similar applications. Electrical signaling is balanced, and multipoint systems are supported. These distances I'm talking about are greater than 100 meters. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. The flexibility and power of the 68HC11’s serial peripheral interface supports high speed communication between the 68HC11 and other synchronous serial devices. 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. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.