이름 : Melissa
이메일 : melissa.minogue@bol.com.br
연락처 :
예식일 : Consult their Glossary Entries for Details
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It is supported by virtually all personal computers, rs485 cable and is the default protocol for both of the QScreen Controller’s serial ports. 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. 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. 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. Thus RS485 is the standard protocol of choice when multi-drop communications are required. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. 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. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). Modem to phone line communications use rates of 110, 150, 300, 600, 1200, 2400, 3000, and 3200 baud.

Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. A serial communications cable is also supplied with QScreen Starter Kits. 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. 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 serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data.

To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. 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. Each of the two UARTs on the wildcard is capable of full-duplex communications, meaning that both transmission and reception can occur simultaneously (although the RS485 protocol is half duplex as explained below). The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii name of this particular slave). When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. 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. This section describes the driver routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol.

The master and slave could even exchange ascii QED-Forth operating system commands. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. The UART Wildcard provides two simultaneous communications links, each configurable as RS232, RS485 or RS422. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. 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 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 communications is asynchronous because no synchronizing clock signal is transmitted along with the data. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost.