자유게시판

RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices. Default, all the senders on the RS485 bus are in tri-state with high impedance. The extremely high volume of Category 5 cable used makes it widely available and very inexpensive, often less than half the price of specialty RS422/485 cabling. This should give you an approximate limit for that cable in feet. RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data communication equipment at a maximum speed of 20 kbps with a maximum cable length of 50 feet. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. In the picture above, the general network topology of RS485 is shown. In the picture above, noise is generated by magnetic fields from the environment. The magnetic fields are allowed to pass, but do no harm. Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable.

Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer. RS485 is the only of the interfaces capable of internetworking multiple transmitters and receivers in the same network. 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. The BitBus broadcast mechanism is used to provide a higher level mapping between NXT address (we use the Bluetooth address of the nxt), or name and the underlying BitBus address (a single byte). First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423. In most higher level protocols, one of the nodes is defined as a master which sends queries or commands over the RS485 bus.

Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions. Correspondingly, U11 takes the data found on the I/O data bus and transfers (and latches) it to the correct 8 output pins connected to the railroad. The legends and folklore, not to mention the flat out wrong information that has grown around the line length and data rate limits inherent in RS-485 are truly astounding. Articles, application notes, even data sheets from semiconductor manufacturers discuss both the data rate and line length limits in RS-485. Blocks U3 through U5 perform as the input data buffers. The receiving device on each SMINI, the U6 receiver IC, RS485 standard has a high input impedance corresponding to a ¼ unit load. 4 (DI - the driver's input pin) is high, the output voltage on pin 6 (The A output) will be positive with respect to pin 7 (the B output). The driver's impedance (when active) is not specified, but the driver needs to be capable of driving 32 unit loads and a termination resistance as low as 60 Ω.

POTS telephone cable's impedance is less controlled than CAT cable and can vary from 600 Ω at lower audio frequencies to less than 100 Ω at higher frequencies. The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100 ohms. Use 100 Ω resistors on both ends. 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. To avoid reflections on longer cables it is necessary to use appropriate termination resitors. For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections. This means that the lower the bit rate the less termination resistors are needed. For both other interfaces-RS422 and RS485-the slew rate is indefinite. The maximum slew rate also limits the maximum communication speed on the line.