MAX1480E Datasheet, PDF (18/19 Pages) Maxim Integrated Products – 15kV ESD-Protected, Isolated RS-485/RS-422

±15kV ESD-Protected, Isolated RS-485/RS-422
Data Interfaces
rents may flow. These currents are by their nature unsta-
ble and unpredictable. In addition, they may cause noise
to be injected into sensitive instrumentation and, in
severe cases, might actually cause physical damage to
such equipment.
Figure 17 shows a half-duplex (2-wire), bidirectional,
party-line repeater system that prevents interference
and/or damage from ground-potential differences. Two
MAX1480EA/MAX1480EC isolated RS-485 transceivers
are used to isolate each of the network segments from
the electrical environment of the repeater. The
MAX1480EA/MAX1480EC also regenerate bus signals
that may have been degraded by line attenuation or dis-
persion.
In the idle state, both transmitters are disabled, while all
receivers in the system are enabled. If any device on the
system has information for any other device, it starts
sending its data onto the bus. Each data transmission on
the bus retriggers the one-shot, keeping the sending
transmitter enabled until there are no more transmis-
sions. All receivers receive all data; if this is undesirable,
the protocol must allow for an address field so receivers
can ignore data not directed to them.
Each node must refrain from transmitting when data
already exists on the bus, and must resend data that is
corrupted by the collisions that inevitably occur with a
party-line system. With the repeater of Figure 17, there
might be transmitters up to 8000 feet apart. That repre-
sents more than 8µs (assuming 1ns/foot of delay) in
which two nodes could be transmitting simultaneously.
The circuit in Figure 17 can be used either directly as
shown, with the slew-rate-limited MAX1480EC, for data
transfer rates up to 160kbps, or with the MAX1480EA for
data rates up to 2.5Mbps (see Table 1 for pullup and
LED resistor values when using the MAX1480EA). If dual-
port isolation is not needed, one of the MAX1480EC
devices can be replaced by a MAX487E for 250kbps
applications.
Reliability
These products contain transformers, optocouplers, and
capacitors, in addition to several monolithic ICs and
diodes. As such, the reliability expectations more closely
represent those of discrete optocouplers rather than the
more robust characteristics of monolithic silicon ICs. The
reliability testing programs for these multicomponent
devices may be viewed on the Maxim website
(www.maxim-ic.com) under Technical Support,
Technical Reference, Multichip Products.
NETWORK SEGMENT A
A 23
B ²⁵
26
3kΩ
3kΩ
19
22
17
21
24
200Ω
15
+5V
2 8 10 14
3kΩ
13
1
2
+5V
200Ω
2 8 10 14
9
MAX1480EC
200Ω
9
74HC04
4
3
200Ω
11
DRIVER
ENABLE
B>A
3kΩ
13
DRIVER
ENABLE
A>B
200Ω
11
MAX1480EC
5 7 12
+5V _{51kΩ 1000pF}
16
2B
15 14
Q 13
74HC123
1A
Q4
CLR
3
5 7 12
+5V _{51kΩ 1000pF}
10 B
76
Q5
74HC123
9A
Q 12
CLR
11
NETWORK SEGMENT B
23 A
25
B
26
3kΩ
19
3kΩ
22
17
21
24
200Ω
15
Figure 17. Double-Isolated RS-485 Repeater
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