Once the Data has been Exchanged

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). The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii name of this particular slave). The remaining inactive slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. It is a half duplex protocol, meaning that only one party at a time may transmit data. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. 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 GROUND line serves as a common voltage reference for the master and slave. The local and remote must share a common ground, so all serial cables include at least one ground conductor. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. Both the local and remote UARTs must be configured for the same baud rate. Modem to modem lines often use 1200, 4800, 9600, 14400, 28800, 33600, and 56000 baud. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. Newer protocols include the full duplex RS422 and the half duplex RS485 protocols, each of which drives differential 0 to 5 volt signals on the serial cable. 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. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers.

The interface can be used to support analog to digital and digital to analog converters, networks of many computers controlled by a single master, or networks of devices controlled by several coordinated masters. The Serial Peripheral Interface, SPI, is a fast synchronous serial interface. The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. There are different sets of standard baud rates in use depending on the application. The UART Wildcard supports any baud rate produced by the above formula. 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). If PT is set, all transmitted bytes with a parity bit will have an odd number of total '1' bits. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. Transmissions are always initiated by the master device, and consist of an exchange of bytes. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media.

Serial1 port the default startup serial link. 1 or 2 to specify Serial1 or Serial2, respectively, and clears the appropriate PORTJ bit to place the transceiver in receive mode. When PE is set (equal to one), the most-significant bit in each byte transmitted will be a parity bit that is either set or cleared by the serial port automatically in order to achieve even or odd parity. 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. 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 PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. The PE bit, with mask 0x02, determines whether the most-significant bit in each byte is used as a parity bit.

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