• 參數(shù)資料
    型號: MAX1480ACPI
    廠商: MAXIM INTEGRATED PRODUCTS INC
    元件分類: 通用總線功能
    英文描述: Complete, Isolated RS-485/RS-422 Data Interface
    中文描述: LINE TRANSCEIVER, PDIP28
    封裝: 0.600 INCH, PLASTIC, DIP-28
    文件頁數(shù): 20/21頁
    文件大小: 487K
    代理商: MAX1480ACPI
    M
    Complete, Isolated RS-485/RS-422
    Data Interface
    20
    ______________________________________________________________________________________
    Doubly Isolated RS-485 Repeater
    The RS-422/RS-485 standard is specified for cable
    lengths up to 4000 feet. When approaching or exceed-
    ing the specified maximum cable length, a ground-
    potential difference of several tens of volts can easily
    develop. This difference can be either DC, AC, at
    power-line frequency, or any imaginable noise or
    impulse waveform. It is typically very low impedance so
    that if a connection between the two grounds is
    attempted, very large currents may flow. These cur-
    rents are by their nature unstable and unpredictable. In
    addition, they may cause noise to be injected into sen-
    sitive instrumentation and, in severe cases, might actu-
    ally cause physical damage to such equipment.
    Figure 15 shows a half-duplex (two-wire), bidirectional,
    party-line repeater system that prevents interference
    and/or damage from ground-potential differences. Two
    MAX1480A/MAX1480B/MAX1480C isolated RS-485
    transceivers are used to isolate each of the network
    segments from the electrical environment of the
    repeater. The MAX1480A/MAX1480B/MAX1480C also
    regenerate bus signals that may have been degraded
    by line attenuation or dispersion.
    In the idle state, both transmitters are disabled, while all
    receivers in the system are enabled. If any device on
    the system has information for any other device, it starts
    sending its data onto the bus. Each data transmission
    on the bus retriggers the one-shot, keeping the sending
    transmitter enabled until there are no more transmis-
    sions. All receivers receive all data; if this is undesir-
    able, the protocol must allow for an address field so
    receivers can ignore data not directed to them.
    Each node must refrain from transmitting when data
    already exists on the bus, and must resend data that is
    corrupted by the collisions that inevitably occur with a
    party-line system. With the repeater of Figure 15, there
    might be transmitters up to 8000 feet apart. That repre-
    sents more than 8μs (assuming 1ns/foot of delay) in
    which two nodes could be transmitting simultaneously.
    The circuit in Figure 15 can be used either directly as
    shown, with the slew-rate-limited MAX1480B/
    MAX1480C, for data transfer rates up to 250kbps, or
    with the MAX1480A for data rates up to 2.5Mbps (see
    Table 1 for pullup and LED resistor values when using
    the MAX1480A or MAX1480B). If dual-port isolation is
    not needed, one of the MAX1480C devices can be
    replaced by a MAX487 for 250kbps applications.
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