The Ultimate Guide To Rs485 Cable > 자유게시판

The M bit, with mask 0x10, determines whether eight or nine bits total are transmitted with each byte, regardless of whether or not the most-significant bit is a parity bit. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port. If the clock idles in the high state, the leading edge of the clock is a falling edge. The serial output at the UART idles at the logic high (mark) level. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors.

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. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. 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 resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. 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.

This bit should be set only after all other SPI configuration is complete. So, for eight data bits with a parity bit, M would be set (equal to one) in order to add an extra bit to each byte transmitted, and PE would be set in order to make that extra bit be used as a parity bit. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. Data translation between different machines can be performed with ease, and applications that communicate via the one serial port can be debugged using the other serial channel. Communications capability is essential for many instrument control applications. 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. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data.

Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. The QScreen Controller, however, does not implement hardware handshaking. 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. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end. They should generally not be needed, except if you use long cables, multiple RS485 devices, and resistive termination. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel.

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