Do away with Rs485 Cable Once and For All > 자유게시판

The older RS232 standard uses single-ended signals, is more sensitive to noise, and is only rated for distances up to 50 ft (15 meters). Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. In fact, rs485 cable a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. So in order to transfer data from a RS232 serial interface engineers need a converter to be able to connect to a RS485 or RS422 interface. 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. 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. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above.

Both the local and remote UARTs must be configured for the same baud rate. Both the local and remote UARTs must be configured for the same communications parameters. 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. 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. RS485 is a half duplex version of RS422 that is capable of multi-drop operation. 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. Characters may be lost if the transmit driver is turned off while pending characters are still being transmitted.

Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. 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. In this implementation, software running on a PC monitors and controls a variety of subsystems. It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications. 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. The primary channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. 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.

An RS485 system should ideally consist of a single linear cable (with no branching) with 120-ohm resistors linked across the two wires at each end. Consequently, this arrangement restricts the system to half-duplex data transmission, where data can only move in one direction at a time between devices. 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. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector). The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). The power of the cable is from the TER port of the PLC. Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. Now select the "Comm" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above).