Allgemeine Beschreibung

The MAX481, MAX483, MAX485, MAX487–MAX491, and MAX1487 are low-power transceivers for RS-485 and RS-422 communication. Each part contains one driver and one receiver. The MAX483, MAX487, MAX488, and MAX489 feature reduced slew-rate drivers that minimize EMI and reduce reflections caused by improperly terminated cables, thus allowing error-free data transmission up to 250kbps. The driver slew rates of the MAX481, MAX485, MAX490, MAX491, and MAX1487 are not limited, allowing them to transmit up to 2.5Mbps.

These transceivers draw between 120µA and 500µA of supply current when unloaded or fully loaded with disabled drivers. Additionally, the MAX481, MAX483, and MAX487 have a low-current shutdown mode in which they consume only 0.1µA. All parts operate from a single 5V supply.

Drivers are short-circuit current limited and are protected against excessive power dissipation by thermal shutdown circuitry that places the driver outputs into a high-impedance state. The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit.

The MAX487 and MAX1487 feature quarter-unit-load receiver input impedance, allowing up to 128 MAX487/MAX1487 transceivers on the bus. Full-duplex communications are obtained using the MAX488–MAX491, while the MAX481, MAX483, MAX485, MAX487, and MAX1487 are designed for half-duplex applications.

 

Anwendungen

Low-Power RS-485 Transceivers

Low-Power RS-422 Transceivers

Level Übersetzer

Transceiver für EMI-empfindliche Anwendungen

Lokale Netzwerke für industrielle Steuerungen

 

Anwendungen Informationen

The MAX481/MAX483/MAX485/MAX487–MAX491 and MAX1487 are low-power transceivers for RS-485 and RS-422 communications. The MAX481, MAX485, MAX490,MAX491, and MAX1487 can transmit and receive at data rates up to 2.5Mbps, while the MAX483, MAX487, MAX488, and MAX489 are specified for data rates up to 250kbps. The MAX488–MAX491 are full-duplex transceivers while the MAX481, MAX483, MAX485, MAX487, and MAX1487 are half-duplex. In addition, Driver Enable (DE) and Receiver Enable (RE) pins are included on the MAX481, MAX483, MAX485, MAX487, MAX489, MAX491, and MAX1487. When disabled, the driver and receiver outputs are high impedance.

 

MAX487/MAX1487:128 Transceivers on the Bus

The 48kΩ, 1/ 4-unit-load receiver input impedance of the MAX487 and MAX1487 allows up to 128 transceivers on a bus, compared to the 1-unit load (12kΩ input impedance) of standard RS-485 drivers (32 transceivers maximum). Any combination of MAX487/ MAX1487 and other RS-485 transceivers with a total of 32 unit loads or less can be put on the bus.

 

MAX483/MAX487/MAX488/MAX489:Reduced EMI and Reflections

The MAX483 and MAX487–MAX489 are slew-rate limited, minimizing EMI and reducing reflections caused by improperly terminated cables. There are experiments that shows the driver output waveform and its Fourier analysis of a 150kHz signal transmitted by a MAX481, MAX485, MAX490, MAX491, or MAX1487. High-frequency harmonics with large amplitudes are evident. There are experiments that shows the same information displayed for a MAX483,MAX487, MAX488, or MAX489 transmitting under the same conditions. The results show high-frequency harmonics have much lower amplitudes, and the potential for EMI is significantly reduced.

 

Low-Power Shutdown Mode(MAX481/MAX483/MAX487)

A low-power shutdown mode is initiated by bringing both RE high and DE low. The devices will not shut down unless both the driver and receiver are disabled. In shutdown, the devices typically draw only 0.1µA of supply current.

RE and DE may be driven simultaneously; the parts are guaranteed not to enter shutdown if RE is high and DE is low for less than 50ns. If the inputs are in this state for at least 600ns, the parts are guaranteed to enter shutdown.

For the MAX481, MAX483, and MAX487, the tZH and tZL enable times assume the part was not in the lowpower shutdown state (the MAX485/MAX488–MAX491 and MAX1487 can not be shut down). The tZH(SHDN) and tZL(SHDN) enable times assume the parts were shut down (see Electrical Characteristics).

It takes the drivers and receivers longer to become enabled from the low-power shutdown state (tZH(SHDN), tZL(SHDN)) than from the operating mode (tZH, tZL). (The parts are in operating mode if the RE ,DE inputs equal a logical 0,1 or 1,1 or 0, 0.)