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ADM488 Datasheet PDF : 16 Pages
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ADM488/ADM489
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.
5V
5V
0.1µF
0.1µF
VCC
RE
A
RO
R
B
ADM488
Z
DI
D
Y
DE
GND
RS-485/RS-422 LINK
VCC
DE
Y
DI
Z
D
ADM489
B
A
R
RO
RE
GND
Figure 24. ADM488/ADM489 Full-Duplex Data Link
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.
Table 6 and Table 7 show the truth tables for transmitting and
receiving.
Table 6. Transmitting Truth Table
Inputs
RE
DE
DI
X1
1
1
X1
1
0
0
0
X1
1
0
X1
Outputs
Z
Y
0
1
1
0
Hi-Z
Hi-Z
Hi-Z
Hi-Z
1 X = Don’t care.
Table 7. Receiving Truth Table
Inputs
RE
DE
A to B
0
0
≥ +0.2 V
0
0
≤ −0.2 V
0
0
1
0
Inputs O/C
X1
Output
RO
1
0
1
Hi-Z
1 X = Don’t care.
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
• Well protected
• Protected
• Typical industrial
• Severe industrial
Rev. D | Page 11 of 16
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