ISL81487L Intersil Corporation, ISL81487L Datasheet - Page 8

• Manufacturer Part Number: ISL81487L
• Description: 15kV ESD Protected/ 1/8 Unit Load/ 5V/ Low Power/ High Speed or Slew Rate Limited/ RS-485/RS-422 Transceivers
• Manufacturer: Intersil Corporation

Test Circuits and Waveforms
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
(SHDN) for ISL8487E and ISL81487L only.
SIGNAL
GENERATOR
t
t
ZH(SHDN)
ZL(SHDN)
PARAMETER
t
t
ZH
ZL
(Note 6)
(Note 6)
t
t
HZ
LZ
SIGNAL
GENERATOR
(Note 7)
(Note 7)
+1.5V
FIGURE 4A. TEST CIRCUIT
FIGURE 5A. TEST CIRCUIT
RE
A
B
B
A
RE
DE
R
0
0
0
0
0
0
8
R
RO
RO
+1.5V
+1.5V
+1.5V
-1.5V
-1.5V
-1.5V
FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES
A
ISL8487E, ISL81487L, ISL81487E
15pF
(Continued)
FIGURE 4. RECEIVER PROPAGATION DELAY
1kΩ
15pF
SW
GND
GND
GND
V
V
V
SW
CC
CC
CC
GND
V
CC
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 ±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 spec.
Receiver inputs function with common mode voltages as
great as ±7V outside the power supplies (i.e., +12V and
-7V), making them ideal for long networks where induced
voltages are a realistic concern.
t
t
ZH
ZL
RO
A
RE
RO
RO
, t
NOTE 7
, t
NOTE 7
ZH(SHDN)
ZL(SHDN)
NOTE 7
FIGURE 4B. MEASUREMENT POINTS
FIGURE 5B. MEASUREMENT POINTS
t
1.5V
PLH
1.5V
OUTPUT HIGH
50%
1.5V
1.5V
OUTPUT LOW
t
PHL
1.5V
t
t
HZ
1.5V
LZ
50%
V
V
OH
OL
- 0.5V
+ 0.5V
3V
0V
3V
0V
V
0V
CC
V
V
V
0V
OH
CC
OL