ISL32470E Datasheet, PDF (11/19 Pages) Intersil Corporation – Fault Protected, Extended Common Mode Range, RS-485/RS-422 Transceivers

ISL32470E, ISL32472E, ISL32475E, ISL32478E
Test Circuits and Waveforms (Continued)
RE
B
B
15pF
RO
A
R
A
SIGNAL
GENERATOR
SIGNAL
GENERATOR
RO
V_CM
0V
t_PLH
50%
750mV
0V
-750mV
t_PHL
50%
V_CC
0V
FIGURE 7A. TEST CIRCUIT
FIGURE 7B. MEASUREMENT POINTS
FIGURE 7. RECEIVER PROPAGATION DELAY AND DATA RATE
SIGNAL
GENERATOR
RE
B
A
RO
R
1kΩ
15pF
V_CC
SW ^GND
RE
(Note 11)
1.5V
3V
1.5V
0V
PARAMETER
t_HZ
t_LZ
t_ZH (Note 10)
t_ZL (Note 10)
t_ZH(SHDN) (Note 13)
t_ZL(SHDN) (Note 13)
DE
A
SW
0
+1.5V
GND
0
-1.5V
V_CC
0
+1.5V
GND
0
-1.5V
V_CC
0
+1.5V
GND
0
-1.5V
V_CC
t_ZH, t_ZH(SHDN)
(Note 11)
RO
t_ZL, t_ZL(SHDN)
(Note 11)
RO
t_HZ
OUTPUT HIGH
1.5V
V_OH - 0.5V
V_OH
0V
t_LZ
1.5V
OUTPUT LOW
V_OL + 0.5V
V_CC
V_OL
FIGURE 8A. TEST CIRCUIT
FIGURE 8B. MEASUREMENT POINTS
FIGURE 8. RECEIVER ENABLE AND DISABLE TIMES
Application Information
RS-485 and RS-422 are differential (balanced) data transmission
standards used for long haul or noisy environments. RS-422 is a
subset of RS-485, so RS-485 transceivers are also RS-422
compliant. RS-422 is a point-to-multipoint (multidrop) standard,
which allows only one driver and up to 10 (assuming one-unit load
devices) receivers on each bus. RS-485 is a true multipoint standard,
which allows up to 32 one-unit load devices (any combination of
drivers and receivers) on each bus. To allow for multipoint operation,
the RS-485 specification requires that drivers must handle bus
contention without sustaining any damage.
Another important advantage of RS-485 is the extended common
mode range (CMR), which specifies that the driver outputs and
receiver inputs withstand signals that range from +12V to -7V.
RS-422 and RS-485 are intended for runs as long as 4000 feet; thus,
the wide CMR is necessary to handle ground potential differences, as
well as voltages induced in the cable by external fields.
The ISL32470E, ISL32472E, ISL32475E, ISL32478E is a family of
ruggedized RS-485 transceivers that improves on the RS-485 basic
requirements and therefore increases system reliability. The CMR
increases to ±15V, while the RS-485 bus pins (receiver inputs and
driver outputs) include fault protection against voltages and
transients up to ±60V. Additionally, larger-than-required differential
output voltages (V_OD) increase noise immunity, while the ±16.5kV
built-in ESD protection complements the fault protection.
Receiver (Rx) Features
These devices utilize a differential input receiver for maximum noise
immunity and common mode rejection. Input sensitivity is better
than ±200mV, as required by the RS-422 and RS-485 specifications.
Receiver input (load) current surpasses the RS-422 specification of
3mA and is four times lower than the RS-485 “Unit Load (UL)”
requirement of 1mA maximum. Thus, these products are known as
“one-quarter UL” transceivers, and there can be up to 128 of these
devices on a network while still complying with the RS-485 loading
specification.
The Rx functions with common mode voltages as great as ±15V,
making them ideal for industrial or long networks where induced
voltages are a realistic concern.
All the receivers include a “full fail-safe” function that guarantees a
high-level receiver output if the receiver inputs are unconnected
(floating), shorted together, or connected to a terminated bus with all
the transmitters disabled (i.e., an idle bus).
Rx outputs feature high drive levels (typically 22mA @ VOL = 1V) to
ease the design of optically coupled isolated interfaces.
11
FN7784.0
January 21, 2011