ADM488 Analog Devices, ADM488 Datasheet - Page 8

• Manufacturer Part Number: ADM488
• Description: Full-Duplex/ Low Power/ Slew Rate Limited/ EIA RS-485 Transceivers
• Manufacturer: Analog Devices

ADM488/ADM489
GENERAL INFORMATION
The ADM488/ADM489 is a ruggedized RS-485 transceiver that
operates from a single +5 V supply.
It contains protection against radiated and conducted interference.
It is ideally suited for operation in electrically harsh environ-
ments or where cables may be plugged/unplugged. It is also
immune to high RF field strengths without special shielding
precautions. It is intended for balanced data transmission and
complies with both EIA Standards RS-485 and RS-422. It con-
tains a differential line driver and a differential line receiver, and
is suitable for full duplex data transmission.
The input impedance on the ADM488/ADM489 is 12 k ,
allowing up to 32 transceivers on the differential bus.
The ADM488/ADM489 operates from a single +5 V
power supply. Excessive power dissipation caused by bus con-
tention or by output shorting is prevented by a thermal shut-
down circuit. This feature forces the driver output into a high
impedance state if, during fault conditions, a significant tem-
perature 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 compo-
nents such as tranzorbs or surge suppressors.
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 19. This shows a full-duplex link where data may be
transferred at rates up to 250 kbps. A terminating resistor is
shown at both ends of the link. This termination is not critical
since the slew rate is controlled by the ADM488/ADM489 and
reflections are minimized.
The communications network may be extended to include
multipoint connections as shown in Figure 25. Up to 32 trans-
ceivers may be connected to the bus.
RO
RE
DI
DE
ADM488
D
Figure 19. ADM488/ADM489 Full-Duplex Data Link
GND
+5V
V
CC
R
0.1 F
A
B
Z
Y
10%
RS-485/RS-422 LINK
–8–
Tables I and II show the truth tables for transmitting and
receiving.
RE
X
X
0
1
X = Don’t Care.
RE
0
0
0
1
X = Don’t Care.
EFT TRANSIENT PROTECTION SCHEME
The ADM488/ADM489 uses protective clamping structures on
its inputs and outputs that clamp the voltage to a safe level and
dissipates 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 simu-
late 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 ap-
pearing on the line, therefore, consists of a burst of extremely
fast transient impulses. A similar effect occurs when switching
on fluorescent lights.
Inputs
DE
1
1
0
0
Table I. Transmitting Truth Table
Y
Z
B
A
Table II. Receiving Truth Table
Inputs
DE
0
1
0
0
ADM489
GND
+5V
V
R
CC
D
0.1 F
DI
1
0
X
X
RO
RE
DE
DI
A-B
Inputs O/C
X
+0.2 V
+0.2 V
Z
0
1
Hi-Z
Hi-Z
Outputs
Output
RO
1
0
1
Hi-Z
REV. 0
Y
1
0
Hi-Z
Hi-Z