ISL81485IU Intersil, ISL81485IU Datasheet - Page 7

• Manufacturer Part Number: ISL81485IU
• Description: IC TXRX RS485/422 5V HS 8-MSOP
• Manufacturer: Intersil
• Type: Transceiverr

Specifications of ISL81485IU

• Number Of Drivers/receivers: 1/1
• Protocol: RS422, RS485
• Voltage - Supply: 4.5 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Driver Features
The RS-485, RS-422 driver is a differential output device that
delivers at least 2V across a 54Ω load (RS-485), and at least
2.5V across a 100Ω load (RS-422) even with V
The drivers feature low propagation delay skew to maximize
bit width, and to minimize EMI.
The driver is three-statable via the active high DE input.
Outputs of the driver are not slew rate limited, so faster
output transition times allow data rates of at least 30Mbps.
Data Rate, Cables, and Terminations
RS-485/RS-422 are intended for network lengths up to
4000’, but the maximum system data rate decreases as the
transmission length increases. Devices operating at 30Mbps
usually are limited to lengths of a couple hundred feet, but
the larger differential output voltage of this transceiver allows
that distance to be pushed past 350’. Figure 7 illustrates the
30Mbps performance of the ISL81485 driving 350’ of CAT5
cable, terminated in 120Ω at both the driver and receiver
ends. As shown, the differential signal (A-B) delivered to the
receiver inputs at the end of the cable is still greater than
1.5V (i.e., 7.5 times the required Rx sensitivity). Thus, even
longer cables can be driven if lower noise margins are
acceptable.
Twisted pair is the cable of choice for RS-485, RS-422
networks. Twisted pair cables tend to pick up noise and other
electromagnetically induced voltages as common mode
signals, which are effectively rejected by the differential
receivers in these ICs.
Typical Performance Curves
-1.5
FIGURE 7. DRIVER AND RECEIVER WAVEFORMS DRIVING
1.5
-3
5
0
3
0
RO
A - B
DRIVER+CABLE DELAY
DI = 30Mbps
350 FEET (107 METERS) OF CAT5 CABLE
(DOUBLE TERMINATED WITH 120Ω)
TIME (20ns/DIV)
7
(~490ns)
V
CC
= 5V, T
CC
= 4.5V.
A
= 25
5
0
o
ISL81485
C; Unless Otherwise Specified
Proper termination is imperative, when using a 30Mbps
device, to minimize reflections. In point-to-point, or point-to-
multipoint (single driver on bus) networks, the main cable
should be terminated in its characteristic impedance
(typically 120Ω) at the end farthest from the driver. In multi-
receiver applications, stubs connecting receivers to the main
cable should be kept as short as possible. Multipoint (multi-
driver) systems require that the main cable be terminated in
its characteristic impedance at both ends. Stubs connecting
a transceiver to the main cable should be kept as short as
possible.
Built-In Driver Overload Protection
As stated previously, the RS-485 spec requires that drivers
survive worst case bus contentions undamaged. The
ISL81485 meets this requirement via driver output short
circuit current limits, and on-chip thermal shutdown circuitry.
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-485 spec, even at the common mode
voltage range extremes.
In the event of a major short circuit condition, the device also
includes a thermal shutdown feature that disables the drivers
whenever the die temperature becomes excessive. This
eliminates the power dissipation, allowing the die to cool.
The drivers automatically reenable after the die temperature
drops about 15 degrees. If the contention persists, the
thermal shutdown/reenable cycle repeats until the fault is
cleared. Receivers stay operational during thermal
shutdown.
-100
-120
-140
-160
160
140
120
100
FIGURE 8. DRIVER OUTPUT CURRENT vs SHORT CIRCUIT
-20
-40
-60
-80
80
60
40
20
0
-7 -6
VOLTAGE
-4
Y OR Z = LOW
-2
OUTPUT VOLTAGE (V)
0
2
4
Y OR Z = HIGH
Y OR Z = HIGH
6
8
10
12