ISL3170E_08 Datasheet, PDF (10/19 Pages) Intersil Corporation – RS-485/RS-422 Transceivers

ISL3170E, ISL3171E, ISL3172E, ISL3173E, ISL3174E, ISL3175E, ISL3176E, ISL3177E, ISL3178E
Test Circuits and Waveforms (Continued)
GND
RE
B
A
RO
R
15pF
SIGNAL
GENERATOR
A
0V
+1.5V
0V
-1.5V
t_PLH
t_PHL
V_CC
RO
1.5V
1.5V
0V
FIGURE 5A. TEST CIRCUIT
FIGURE 5B. MEASUREMENT POINTS
FIGURE 5. RECEIVER PROPAGATION DELAY
SIGNAL
GENERATOR
RE
GND
B
RO
AR
1kΩ
15pF
V_CC
SW ^GND
NOTE 9
RE
1.5V
3V
1.5V
0V
PARAMETER
DE
t_HZ
X
t_LZ
X
t_ZH (Note 8)
0
t_ZL (Note 8)
0
t_ZH(SHDN) (Note 11)
0
t_ZL(SHDN) (Note 11)
0
A
+1.5V
-1.5V
+1.5V
-1.5V
+1.5V
-1.5V
SW
GND
V_CC
GND
V_CC
GND
V_CC
t_ZH, t_ZH(SHDN)
NOTE 9
RO
t_ZL, t_ZL(SHDN)
NOTE 9
RO
t_HZ
OUTPUT HIGH
1.5V
V_OH - 0.25V ^VOH
0V
t_LZ
1.5V
OUTPUT LOW
V_CC
V_OL + 0.25V _VOL
FIGURE 6A. TEST CIRCUIT
FIGURE 6B. MEASUREMENT POINTS
FIGURE 6. RECEIVER ENABLE AND DISABLE TIMES (EXCEPT ISL3171E, ISL3174E, ISL3177E)
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards for use in 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 spec 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’, so the wide CMR is necessary to handle
ground potential differences, as well as voltages induced in
the cable by external fields.
Receiver 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 resistance of 96kΩ surpasses the RS-422
spec of 4kΩ and is eight times the RS-485 “Unit Load (UL)”
requirement of 12kΩ minimum. Thus, these products are
known as “one-eighth UL” transceivers and there can be up
to 256 of these devices on a network while still complying
with the RS-485 loading specification.
Receiver inputs function with common mode voltages as great
as +9V/-7V outside the power supplies (i.e., +12V and -7V),
making them ideal for long networks where induced voltages
and ground potential differences are realistic concerns.
All the receivers include a “Full Fail-Safe” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating) or shorted. Fail-safe with shorted
inputs is achieved by setting the Rx upper switching point to
-50mV, thereby ensuring that the Rx sees 0V differential as a
high input level.
10
FN6307.4
June 10, 2008