클잼 로고

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

Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground. Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below. Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485.

The UART Wildcard supports any baud rate produced by the above formula. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. Thus RS485 is the standard protocol of choice when multi-drop communications are required. There are different sets of standard baud rates in use depending on the application. You may use nonstandard baud rates if both devices support them. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector.

The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. Chassis and signal grounds are connected together to the digital ground (GND) signal. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. A jumper, J3, rs485 cable configures the primary serial port for either RS232 or RS485 operation. RS232 is by far the most common protocol. RS485 is another protocol supported by the primary serial port on the QScreen Controller.

The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. Using the primary serial port is easy. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as 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. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data.