이름 : Connor Roxon
이메일 : connor_roxon@hotmail.co.uk
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예식일 : The pros And Cons Of Rs485 Cable
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Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. We’ll use code from the GETSTART.C program. Should I use half-duplex or full-duplex RS485? The RS485 converter interface uses two wires for communication, one for transmitting and one for receiving data, which is referred to as a half-duplex communication converter. 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. A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. A FIFO is a First In/First Out buffer that can queue a burst of outgoing characters for transmission, or save a set of incoming characters until the host can read them. Each UART implements transmit and receive FIFO buffers to reduce the required frequency of service by the host processor. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications.

When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. In the simplest scheme, all RS485 transceivers come up in receive mode when the interface is initialized, and each transceiver node has a unique address known to it and the master. After the slave transmission is complete, the slave puts itself back into receive mode so that the master can transmit additional commands. Care must be taken when changing an RS485 node from transmit mode to receive mode, or visa versa. I am beginning to suspect that the board is not writing to the RS485 adapter correctly, or that pins are not mapped correctly. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. 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.

There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. 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. 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 communications is asynchronous because no synchronizing clock signal is transmitted along with the data. If the clock idles in the high state, the leading edge of the clock is a falling edge. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. 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.

RS232 is by far the most common protocol. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. 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. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. The protocols are described in turn. What communication protocols are supported by RS485? 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.

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