(주)미라보콘크리트

• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. Systems of this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention) problems on a multi-drop network (more about solving this problem on a two-wire network in a moment). These signals are commonly called A and B in RS485 systems. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.

With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry. However, note that the functions that write to EEPROM disable interrupts for 20 msec. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). The local and remote must share a common ground, so all serial cables include at least one ground conductor. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. 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. RS485 is another protocol supported by the primary serial port on the PDQ Board.

Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. Modem to modem lines often use 1200, 4800, 9600, 14400, 28800, 33600, and 56000 baud. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. You can implement the slave select lines by configuring Port A pins as outputs. 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. This configuration works for many SPI devices, including the optional battery-backed real-time clock. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte.

It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream. The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, rs485 cable while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. The USE.SERIAL2 command means that the operating system’s terminal interface now communicates via Serial2. The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. Table 9-6 shows the connection diagram for a standard 9-pin serial cable. The serial output at the UART idles at the logic high (mark) level. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space.