Datasheet

MAX3097E/MAX3098E
±15kV ESD-Protected, 32Mbps, 3V/5V,
Triple RS-422/RS-485 Receivers with Fault Detection
______________________________________________________________________________________ 11
Common-Mode Range
The MAX3097E/MAX3098E contain circuitry that de-
tects if the input stage is going outside its useful com-
mon-mode range. If the received data could be
unreliable, a fault signal is triggered.
Detecting Low Input Differential
Due to cable attenuation on long wire runs, it is possi-
ble that V
ID
< 200mV, and incorrect data will be
received. In this condition, a fault will be indicated.
Delayed Fault Output
The delayed fault output provides a programmable
blanking delay to allow transient faults to occur without
triggering an alarm. Such faults may occur with slow
signals triggering the receiver alarm through the zero
crossover region.
Figure 9 shows the delayed alarm output.
ALARMD performs a logic OR of ALARMA, ALARMB,
and ALARMZ (Figure 10). A NOR gate drives an N-
channel MOSFET so that in normal operation with no
faults, the current source (10µA typ) is shunted to
ground. Upon activation of any alarm from receiver A,
B, or Z, the MOSFET is turned off, allowing the current
source to charge C
DELAY
. When V
DELAY
exceeds the
DELAY threshold, the comparator output, ALARMD,
goes high. ALARMD is reset when all receiver alarms
go low, quickly discharging C
DELAY
to ground.
Setting Delay Time
ALARMD’s delay time is set with a single capacitor
connected from DELAY to GND. The delay comparator
threshold varies with supply voltage, and the C
DELAY
value can be determined for a given time delay period
from the Capacitance vs. ALARMD Output Delay graph
in the Typical Operating Characteristics or using the
following equations:
t
D
= 15 + 0.33 x C
DELAY
(for V
CC
= 5V)
and
t
D
= 10 + 0.187 x C
DELAY
(for V
CC
= 3V)
where t
D
is in µs and C
DELAY
is in pF.
ALARMA
ALARMB
ALARMD
t
D
t
D
ALARMA
ALARMB
ALARMZ
ALARMD
DELAY
COMPARATOR
NMOS
DELAY
CURRENT
SOURCE
C
DELAY
*
(EXTERNAL)
G1
DELAY THRESHOLD
t
DLY
ALARM_
DELAY
ALARMD
*The capacitor (C
DELAY
) charges up slowly, but discharges rapidly.
If the duration of an ALARM_ pulse is less than t
DLY
, no alarm
output will be present at ALARMD.
Figure 9. Delayed Alarm Output
Figure 10. ALARMD Simplified Schematic
Figure 8. Short-Circuit Detection
A
NORMAL OPERATION SHORT CIRCUIT A TO A
A