The Appeal Of What Is Rs485 Cable > 자유게시판

Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. In a multi-drop configuration, the TxD of the master is connected to the RxD of each slave, and the RxD of the master is connected to the TxD of each slave.

If the /SS input to a slave is inactive (high), the slave ignores the SCK input, does not transmit or receive data, and keeps its MISO output in a high-impedance state so that it does not interfere with the SPI bus. Thus, the master 68HC11 has only one input, MISO, which is the slave QVGA Controller’s only output. The /SS input in turn determines whether the slave responds to the SCK input, as described in a previous section. This allows the processor that is master to control the input /SS pins of other CPU’s, for example. The QVGA Controller’s on-board 12 bit A/D and 8 bit D/A converters communicate with the processor via the SPI. Initializing the 68HC11 as a slave (by clearing the MSTR bit in the SPCR control register as explained below) automatically configures the /SS pin as an input. The 68HC11 allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. Powerful APP Support: SenseCAP Mate APP allows you to set the sensor templates and quickly turn the data logger into the sensor you want. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications.

If the 68HC11 is initialized as a master by setting the MSTR bit, then bit 5 of the Port D data direction register (PORTD.DIRECTION) determines whether /SS is an input or an output. In this situation, if the /SS input is pulled low while the 68HC11 is the master, the processor detects a "mode fault" (by setting a bit in the SPI status register) meaning that there is more than one master device on the SPI bus. If the 68HC11 is initialized as a master (by setting the MSTR bit in the SPCR control register as explained below) then bit 5 of the Port D data direction register (DDRD) determines whether /SS is an input or an output. EIA-485 (formerly RS-485 or RS485) is a specification for the physical layer of a network that uses the difference in voltages between two wires (Three wire) to conveys data. The resistance of the twisted pair cable will drop the voltage difference as it travels down the wire. The voltage difference between these two wires is what conveys the information, rather than the voltage between a single wire and ground. 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.

From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. Unlike Ethernet standards like TCP that repeatedly transmit packets until acknowledged (error correction), RS-485 signals transmit without requiring confirmation. The most commonly used communication standards in industrial control systems are RS232, RS485, and Modbus. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. Figure 11 1 shows a typical multi-drop communications network. You can use the QED Board’s RS485 link to create such a multi-drop serial network. You can unsubscribe at any time. It is a half duplex protocol, meaning that only one party at a time may transmit data. 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. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register.

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