For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. Converters in general can be used to change the electrical characteristice of one communications standard into another, to take advantage of the best properties of the alternate standard selected. For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports.
Category 5 (Cat 5, Cat 5e, and Cat 6) cables are the most common Ethernet cables in use today. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. To insure that a receiver stays in an inactive state, when no data signal is present, bias is generally added to a network at one or more locations. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. Reserved Function Codes are function codes used by some companies for legacy product and are not available for public use. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.
It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS485 specifies a 2-wire, half-duplex communications bus. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. Can be work well even the temperpature change suddenly. It requires minimal wiring and can share the wiring among several seats, reducing weight. It requires a dedicated co-processor to handle fast HDLC-like token rotation. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. Because there is a chance of data collosion with this implementation, theory tells us that in this case only 37% of the bandwidth will be effectively used. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered.
If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. An RS485 transceiver is present on the QVGA Controller, and its data direction is controlled by pin 4 of port PPC of the peripheral interface adapter (PIA). The maximum sustainable baud rate on the secondary serial port is 4800 baud. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. The RS-232 driver (Maxim Part No. MAX242) can be tri-stated under software control. The QVGA Controller offers a unique addition to full duplex RS232: it can place its RS232 transmitter into a high impedance silent mode under software control. Vehicle mounted camera solution can be deployed where a rapid security response is required. RS-485, like RS-422, can be made full-duplex by using four wires. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated.
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