___________Applications Information

Using the MAX3097E/MAX3098E as Shaft

Encoder Receivers

The MAX3097E/MAX3098E are triple RS-485 receivers

designed for shaft encoder receiver applications. A

shaft encoder is an electromechanical transducer that

converts mechanical rotary motion into three RS-485

differential signals. Two signals, A (A and A) and B (B

and B) provide incremental pulses as the shaft turns,

while the index signal, Z (Z and Z) occurs only once

per revolution to allow synchronization of the shaft to a

known position. Digital signal processing (DSP) tech-

niques are used to count the pulses and provide feed-

back of both shaft position and shaft velocity for a

stable positioning system.

Shaft encoders typically transmit RS-485 signals over

twisted-pair cables since the signal often has to travel

across a noisy electrical environment (Figure 7).

Detecting Faults

Signal integrity from the shaft encoder to the DSP is

essential for reliable system operation. Degraded sig-

nals could cause problems ranging from simple mis-

counts to loss of position. In an industrial environment,

many problems can occur within the three twisted

pairs. The MAX3097E/MAX3098E can detect various

types of common faults, including a low-input-level sig-

nal, open-circuit wires, short-circuit wires, and an input

signal outside the common-mode input voltage range

of the receiver.

Detecting Short Circuits

In Figure 8, if wires A and A are shorted together, then A

and A will be at the same potential, so the difference in

the voltage between the two will be approximately 0. This

causes fault A to trigger since the difference between A -

A is less than the differential fault threshold.

Detecting Open-Circuit Conditions

Detecting an open-circuit condition is similar to detect-

ing a short-circuit condition and relies on the terminat-

ing resistor being across A and A. For example, if the

wire drops out of the A terminal, A pulls A through the

terminating resistor to look like the same signal. In this

±15kV ESD-Protected, 32Mbps, 3V/5V,

Triple RS-422/RS-485 Receivers with Fault Detection

10

______________________________________________________________________________________

CHARGE-CURRENT

LIMIT RESISTOR

DISCHARGE

RESISTANCE

STORAGE

CAPACITOR

Cs

150pF

50M

Ω to 100MΩ

330

Ω

HIGH-

VOLTAGE

DC

SOURCE

DEVICE

UNDER

TEST

30ns

60ns

t

100%

90%

10%

I

A

A

B

B

Z

Z

Figure 7. Typical Shaft Encoder Output

Figure 6a. IEC 1000-4-2 ESD Test Model

Figure 6b. IEC 1000-4-2 ESD Generator Current Waveform