±15kV ESD-Protected, Slew-Rate-Limited,

Low-Power, RS-485/RS-422 Transceivers

12

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The major difference between tests done using the

Human Body Model and IEC1000-4-2 is higher peak

current in IEC1000-4-2, because series resistance is

lower in the IEC1000-4-2 model. Hence, the ESD with-

stand voltage measured to IEC1000-4-2 is generally

lower than that measured using the Human Body

Model. Figure 7 shows the current waveform for the 8kV

IEC1000-4-2 ESD contact-discharge test.

The air-gap test involves approaching the device with a

charged probe. The contact-discharge method connects

the probe to the device before the probe is energized.

Machine Model

The Machine Model for ESD tests all pins using a

200pF storage capacitor and zero discharge resis-

tance. Its objective is to emulate the stress caused by

contact that occurs with handling and assembly during

manufacturing. Of course, all pins require this protec-

tion during manufacturing—not just inputs and outputs.

Therefore, after PC board assembly, the Machine Model

is less relevant to I/O ports.

MAX487E/MAX1487E:

128 Transceivers on the Bus

The 48k

Ω, 1/4-unit-load receiver input impedance of the

MAX487E and MAX1487E allows up to 128 transceivers

on a bus, compared to the 1-unit load (12k

Ω input

impedance) of standard RS-485 drivers (32 transceivers

maximum). Any combination of MAX487E/MAX1487E

and other RS-485 transceivers with a total of 32 unit

loads or less can be put on the bus. The MAX481E,

MAX483E, MAX485E, and MAX488E–MAX491E have

standard 12k

Ω receiver input impedance.

MAX483E/MAX487E/MAX488E/MAX489E:

Reduced EMI and Reflections

The MAX483E and MAX487E–MAX489E are slew-rate

limited, minimizing EMI and reducing reflections

caused by improperly terminated cables. Figure 16

shows the driver output waveform and its Fourier analy-

sis of a 150kHz signal transmitted by a MAX481E,

MAX485E, MAX490E, MAX491E, or MAX1487E. High-

frequency harmonics with large amplitudes are evident.

Figure 17 shows the same information displayed for a

MAX483E, MAX487E, MAX488E, or MAX489E transmit-

ting under the same conditions. Figure 17’s high-fre-

quency harmonics have much lower amplitudes, and

the potential for EMI is significantly reduced.

Low-Power Shutdown Mode

(MAX481E/MAX483E/MAX487E)

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 0.5µA 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.

low-power shutdown state (the MAX485E, MAX488E–

MAX491E, and MAX1487E can not be shut down). The

parts were shut down (see

Electrical Characteristics).

500kHz/div

0Hz

5MHz

10dB/div

Figure 16. Driver Output Waveform and FFT Plot of

MAX485E/MAX490E/MAX491E/MAX1487E Transmitting a

150kHz Signal

500kHz/div

0Hz

5MHz

10dB/div

Figure 17. Driver Output Waveform and FFT Plot of

MAX483E/MAX487E–MAX489E Transmitting a 150kHz Signal