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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. 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. If PT is cleared, then all transmitted bytes with a parity bit will have an even number of total '1' bits. The above parity settings will also determine how incoming data is interpreted (whether the most significant bit is considered a parity bit or part of the data being transmitted, and how many bits total to expect in each byte). A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings.

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. However, verifying correct parity of bytes received with a parity bit is currently not supported. 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. The M bit, with mask 0x10, determines whether eight or nine bits total are transmitted with each byte, regardless of whether or not the most-significant bit is a parity bit. The PE bit, with mask 0x02, determines whether the most-significant bit in each byte is used as a parity bit. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled.

Even parity means that the bits sum to an even number, and odd parity means that the bits sum to an odd number. 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. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. 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 PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor.

When the keyword name is received by the Silence() routine running in the slave, the slave PDQ Board executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). There are different sets of standard baud rates in use depending on the application. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. If there are several sensors near and/or other data to be transmitted via GSM, it is more convenient to use Standard / RF versions supported on a concentrator or controller such as RXTXeasy GSM so you will be able to use one single SIM. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. This section describes the driver routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol.

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