• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

It provides a convenient means of connecting the QVGA Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. This configuration works for many SPI devices, including the optional battery-backed real-time clock. Different Unit Loads in transceivers allow more devices on a network, including full, half, quarter, or eighth-unit load circuits. The receiver load impedence is 12K ohms (or higher) and transmitter "leakage" current is ±100µA (or less) in either the powered or unpowered state. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line. The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire. You can use the QED Board’s RS485 link to create such a multi-drop serial network.

Network topology refers to the arrangement of various nodes (such as computers, servers, switches) in a computer network. The QVGA Controller makes it easy to configure a multi-drop network of QVGA Controllers controlled by a single master desktop computer. When there is no data activity on an RS485 network (or in many instances RS422 networks, other than point-to-point), the communications lines are "floating" and, thus susceptible to external noise or interference. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QVGA Controller) is ready to accept data. However, note that the ReadWatch() and SetWatch() functions disable interrupts for about one millisecond (msec.), and the functions that write to EEPROM disable interrupts for 20 msec. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. By converting from single-ended RS232 to differential RS422 and then, converting back from RS422 to RS232 at the other end of the line, distance and noise immunity can be greatly improved.

Maintain as much distance as possible and cross any power cable at a right angle. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. While it goes against conventional wisdom, and can potentially cause a problem with circulating currents by grounding a shielded cable at both ends, this method is very effective at keeping induced lightening noise away from the communications lines. Master Receives: The master device, like the slaves, is constantly monitoring the voltage difference between lines A and B, so it will receive the response from the slave. To interface devices that support synchronized serial interfaces, but are not configurable like the QScreen, determine the device’s requirements for clock phase and polarity and configure the QScreen’s CPHA and CPOL accordingly. The QVGA 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. To enable the secondary serial port, turn dip switch 5 (labeled "2COM") on the QED Board ON.

An "Automatic Converter" (RS232 to RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. DIP switch number 7, or turn the system off and on again. Otherwise, dip switch 5 should be OFF so that it frees bit 3 of PORTA for use as general-purpose I/O. When the 68HC11 controls the network, it is referred to as a "master"; otherwise, it is a "slave". Thus, the master has only one input, MISO, which is the slave’s only output. By "isolating" sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network. The QED-Forth kernel includes pre-coded drivers that configure the SPI for maximum speed data transfer using a format that is compatible with the on-board D/A and 12 bit A/D. We have built sophisticated instruments using the QVGA Controller that operate very reliably using multiple interrupts in addition to the software UART.

If you cherished this posting and you would like to receive a lot more info about what is rs485 cable kindly pay a visit to the website.