In either of these cases, a source of noise that caused one bit to be received incorrectly would invalidate the received byte, since the total number of '1' bits would be odd rather than even. This is an extra single bit appended to the end of each byte or character transmitted, which is set or cleared as necessary to ensure that the total number of '1' bits in the byte is always odd or even. So, for eight data bits with a parity bit, M would be set (equal to one) in order to add an extra bit to each byte transmitted, and PE would be set in order to make that extra bit be used as a parity bit. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2.

RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. The DECIMAL command makes sure that numbers are interpreted in the decimal base. The USE.SERIAL2 command means that the operating system’s terminal interface now communicates via Serial2. No parity means that there is no parity bit. 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. 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 you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). If two bits are received incorrectly, the error will go unnoticed by parity checking. Parity checking is not often used, because it is not a robust method of error detection.

The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. We’ll use code from the GETSTART.c program which was introduced in the chapter titled Your First Program. Serial data is shifted out least-significant-bit first. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. 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.

This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. 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 PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. RS485Init() configures PORTJ to ensure that bits 0 and 1 are outputs, and disables both RS485 transmitters, leaving the Serial1 and Serial2 RS485 channels in receive mode. Because all of the serial I/O routines on the PDQ Board are revectorable, it is very easy to change the serial port in use without modifying any high level code. Once the project is open, click Build→ Build, and after the compilation is done, enter the Mosaic Terminal by clicking Tools→ Mosaic Terminal and use the Send File menu item to send GETSTART.DLF to the PDQ Board.

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