adm489ar-reel7 Analog Devices, Inc., adm489ar-reel7 Datasheet - Page 11

• Manufacturer Part Number: adm489ar-reel7
• Description: Full-duplex, Low Power, Slew Rate Limited, Eia Rs-485 Transceivers
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADM488/ADM489 are ruggedized RS-485 transceivers
that operate from a single 5 V supply. They contain protection
against radiated and conducted interference and are ideally
suited for operation in electrically harsh environments or where
cables can be plugged/unplugged. They are also immune to
high RF field strengths without special shielding precautions.
They are intended for balanced data transmission and comply
with both EIA RS-485 and RS-422 standards. They contain a
differential line driver and a differential line receiver, and are
suitable for full-duplex data transmission.
The input impedance on the ADM488/ADM489 is 12 kΩ,
allowing up to 32 transceivers on the differential bus. The
ADM488/ADM489 operate from a single 5 V ± 10% power
supply. A thermal shutdown circuit prevents excessive power
dissipation caused by bus contention or by output shorting.
This feature forces the driver output into a high impedance state
if, during fault conditions, a significant temperature increase is
detected in the internal driver circuitry.
The receiver contains a fail-safe feature that results in a logic
high output state if the inputs are unconnected (floating). A
high level of robustness is achieved using internal protection
circuitry, eliminating the need for external protection com-
ponents such as tranzorbs or surge suppressors. Furthermore,
low electromagnetic emissions are achieved using slew limited
drivers, minimizing interference both conducted and radiated.
The ADM488/ADM489 can transmit at data rates up to
250 kbps. A typical application for the ADM488/ADM489 is
illustrated in Figure 24 showing a full-duplex link where data is
transferred at rates of up to 250 kbps. A terminating resistor is
shown at both ends of the link. This termination is not critical
because the slew rate is controlled by the ADM488/ADM489
and reflections are minimized.
The communications network can be extended to include
multipoint connections, as shown in Figure 30. As many as
32 transceivers can be connected to the bus.
RE
RO
DI
DE
ADM488
D
V
GND
5V
CC
R
Figure 24. ADM488/ADM489 Full-Duplex Data Link
0.1µF
A
B
Z
Y
RS-485/RS-422 LINK
Y
Z
B
A
ADM489
GND
V
5V
R
CC
D
Rev. D | Page 11 of 16
0.1µF
RO
DE
RE
DI
Table 6 and Table 7 show the truth tables for transmitting and
receiving.
Table 6. Transmitting Truth Table
1
Table 7. Receiving Truth Table
1
EFT TRANSIENT PROTECTION SCHEME
The ADM488/ADM489 use protective clamping structures on
their inputs and outputs that clamp the voltage to a safe level
and dissipate the energy present in ESD (electrostatic) and EFT
(electrical fast transients) discharges.
FAST TRANSIENT BURST IMMUNITY (IEC1000-4-4)
IEC1000-4-4 (previously 801-4) covers electrical fast transient
burst (EFT) immunity. Electrical fast transients occur as a result
of arcing contacts in switches and relays. The tests simulate the
interference generated when, for example, a power relay disconnects
an inductive load. A spark is generated due to the well known
back EMF effect. In fact, the spark consists of a burst of sparks
as the relay contacts separate. The voltage appearing on the line,
therefore, consists of a burst of extremely fast transient impulses.
A similar effect occurs when switching on fluorescent lights.
The fast transient burst test, defined in IEC1000-4-4, simulates
this arcing, and its waveform is illustrated in Figure 25. It
consists of a burst of 2.5 kHz to 5 kHz transients repeating at
300 ms intervals. It is specified for both power and data lines.
Four severity levels are defined in terms of an open-circuit voltage
as a function of installation environment. The installation
environments are defined as
•
•
•
•
X = Don’t care.
X = Don’t care.
RE
0
0
0
1
RE
X
X
0
1
Well protected
Protected
Typical industrial
Severe industrial
1
1
DE
Inputs
0
0
0
0
DE
1
1
0
0
Inputs
Inputs O/C
DI
X
X
1
0
≥ +0.2 V
≤ −0.2 V
1
1
A to B
X
1
ADM488/ADM489
Hi-Z
Hi-Z
Z
0
1
Outputs
Output
Hi-Z
RO
1
0
1
Hi-Z
Hi-Z
Y
1
0