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It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. 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. The RS485 connections are not brought out to the Serial 1 Connector. If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. 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 secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent.

These are used in programmable logic controllers and on factory floors. The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. The terminal’s serial receiver chip re-inverts the signal to its positive sense. 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. 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. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. RS485 is a half duplex version of RS422 that is capable of multi-drop operation. It is a half duplex protocol, meaning that only one party at a time may transmit data.

The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. 0), and expect valid data to be present on rising clock edges. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. RS485 is another protocol supported by the primary serial port on the QScreen Controller. RS485Init() configures PORTJ to ensure that bits 0 and 1 are outputs, and disables both RS485 transmitters, leaving the Serial1 and Serial2 RS485 channels in receive mode.

The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. And the results before was a degraded signal that the devices near the end would interpret the signal incorrectly. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. The QScreen Controller does not differentiate between these. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself.

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