If you`re sitting in the office right now, and you look to see your thermostat, there`s usually RS-485 transceivers connecting that from there to basically the control unit in the building. What it does is that`s after sensing the temperature in the room and yet when you`re making adjustment to the thermostat, it actually sends the data to the controller, which could be controlling that, the temperature, turning on and off the AC to heat or cool the room up. 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 CPOL, CPHA, SR1 and SPR0 configure the SCK pin’s clock polarity, clock phase, and clock rate. The rate of data transmission is expressed in bits per second, or baud. RS-485 is used for low-speed data communications in commercial aircraft cabins` vehicle bus. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. In a single phase system, analog and digital instruments are selected to measure voltage, current, frequency, and power factor. After having defined the type of instrument best suited to the needs, if the measurement is performed by indirect insertion, the accessories of the measuring system, what is rs485 cable such as current and voltage transformers must be chosen with care.



































The sizing of the measurement system begins with knowing the main parameters of the plant; in particular, from the characteristics of the protection switch, the type of distribution system, the rated current, the rated voltage and the type of busbar can be known. Multifunctional instruments can be used in different distribution systems. Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11`s on-chip UART and the onboard RS232 driver. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time. Transmissions are always initiated by the master device, and consist of an exchange of bytes. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. Be sure to account for these effects when designing your application. To further reduce the effects of interference, a low Baud rate of 2400 (or 1200) Baud is used. 1200 is the baud rate that you choose; you can specify any standard baud rate up to 4800 baud.



































Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. Thus, the master has only one input, MISO, which is the slave’s only output. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811`s CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). 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. The device that initiates a data transfer is the master, and all other devices on the network are slaves. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. In these distributed processing networks, a variety of machines and instruments work locally, but communicate and share data or resources with one another globally using a single serial link.



































We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively. The arrows in the diagram point to pins configured as inputs, and originate from output pins. 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. Similarly, the RS-485 and RS-422 are among the most successful standards for industrial applications, however, it is often necessary to communicate between the standards. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. The one you choose depends on the specific device, or devices you will be connecting to.