components with large amplitudes are evident. Figure

15 shows the same signal displayed for a MAX3083/

transmitting under the same conditions. Figure 15’s

high-frequency harmonic components are much lower

in amplitude, compared with Figure 14’s, and the poten-

tial for EMI is significantly reduced. Figure 16

shows the same signal displayed for a MAX3080/

MAX3081/MAX3082, and MAX3089 with SRL = uncon-

nected, transmitting under the same conditions. Figure

16’s high-frequency harmonic components are even

lower.

In general, a transmitter’s rise time relates directly to the

length of an unterminated stub, which can be driven with

only minor waveform reflections. The following equation

expresses this relationship conservatively:

For example, the MAX3080’s rise time is typically

1320ns, which results in excellent waveforms with a stub

length up to 90 feet. A system can work well with longer

unterminated stubs, even with severe reflections, if the

waveform settles out before the UART samples them.

Fail-Safe, High-Speed (10Mbps),

Slew-Rate-Limited RS-485/RS-422 Transceivers

______________________________________________________________________________________

17

R

B

RECEIVER

OUTPUT

ATE

A

R

Figure 13. Receiver Propagation Delay Test Circuit

100kHz/div

0Hz

1MHz

20dB/div

Figure 14. Driver Output Waveform and FFT Plot of

MAX3086/MAX3087/MAX3088, and MAX3089 with

SRL = GND, Transmitting a 20kHz Signal

100kHz/div

0Hz

1MHz

B

A

20dB/div

Figure 15. Driver Output Waveform and FFT Plot of

MAX3083/MAX3084/MAX3085, and MAX3089

100kHz/div

0Hz

1MHz

B

A

20dB/div

Figure 16. Driver Output Waveform and FFT Plot of

MAX3080/MAX3081/MAX3082, and MAX3089

with SRL = Unconnected, Transmitting a 20kHz Signal