Datasheet

___________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
condition, V
ID
is approximately 0 and a fault occurs.
MAX3097E/MAX3098E
±15kV ESD-Protected, 32Mbps, 3V/5V,
Triple RS-422/RS-485 Receivers with Fault Detection
10 ______________________________________________________________________________________
CHARGE-CURRENT
LIMIT RESISTOR
DISCHARGE
RESISTANCE
STORAGE
CAPACITOR
C
s
150pF
R
C
50MΩ to 100MΩ
R
D
330Ω
HIGH-
VOLTAGE
DC
SOURCE
DEVICE
UNDER
TEST
t
r
= 0.7ns to 1ns
30ns
60ns
t
100%
90%
10%
I
PEAK
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