ISL43485_05 Datasheet, PDF (7/11 Pages) Intersil Corporation – 3.3V, Low Power, 30Mbps, RS-485/RS-422 Transceiver

ISL43485
Test Circuits and Waveforms (Continued)
GND ^RE
+1.5V
B
RO
A
R
15pF
3V
A
1.5V
1.5V
0V
SIGNAL
GENERATOR
t_PLH
t_PHL
V_CC
RO
50%
50%
0V
FIGURE 4A. TEST CIRCUIT
FIGURE 4B. MEASUREMENT POINTS
FIGURE 4. RECEIVER DATA RATE AND PROPAGATION DELAY
SIGNAL
GENERATOR
RE
B
GND
A
RO
R
1kΩ
15pF
V_CC
SW ^GND
NOTE 7
RE
1.5V
3V
1.5V
0V
PARAMETER
t_HZ
t_LZ
t_ZH (Note 6)
t_ZL (Note 6)
t_ZH(SHDN) (Note 9)
t_ZL(SHDN) (Note 9)
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 7
RO
t_ZL, t_ZL(SHDN)
NOTE 7
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 5A. TEST CIRCUIT
FIGURE 5B. MEASUREMENT POINTS
FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES
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
This device utilizes a differential input receiver for maximum
noise immunity and common mode rejection. Input
sensitivity is ±200mV, as required by the RS422 and RS-485
specifications.
Receiver input impedance surpasses the RS-422 spec of
4kΩ, and meets the RS-485 “Unit Load” requirement of 12kΩ
minimum.
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 are a realistic concern.
All the receivers include a “fail-safe if open” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating).
The receiver easily meets the data rate supported by the
driver, and the receiver output is tri-statable via the active
low RE input.
7
FN6071.1
March 15, 2005