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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 2116
Maxim > Design Support > Technical Documents > Application Notes > Protection and Isolation > APP 2116
Keywords: RS-485, rs485, full duplex, isolated, isolation, isolated circuits, isolated lines, 250kbaud,
2.5Mbaud, 250 kilobaud, 2.5 megabaud, bi-directional, 1600V isolation, 2000V isolation, transformer
coupled, opto coupled, optocouplers, internal isolated
APPLICATION NOTE 2116
RS-485 Data Interface Gives Isolated, Full-Duplex
Operation
Jul 09, 1998
Abstract: A circuit is described that provides fully isolated RS-485 full-duplex data communication with
only two integrated circuits (ICs). An internal transformer-coupled isolated supply and internal
optocouplers in the data path provide typical isolation to 1600Vrms for 1 minute or 2000Vram for 1
second. Data rates of 250kbaud are achieved with internally slew-rate-limited circuits or to 2.5Mbaud
with non-slew-rate limited circuits, depending upon choice of available ICs.
The simple RS-485 circuit of Figure 1 provides full-duplex communications (simultaneous transmission
and reception) with only two essential packages (IC2 and IC3). Its balanced and differential data lines are
necessary for high-noise environments or for long-distance transmission between a computer and its
peripherals. Such transmissions are difficult, if not impossible, with the single-ended circuitry of an RS-
232 transceiver.
The RS-485 standard allows for bidirectional, multi-point, party-line communications, with data rates to
10M bits/second (10Mbps) and line lengths to 1200 meters. Differential transmission provides noise
immunity. The circuit shown features controlled-slew-rate drivers that minimize EMI and the reflections
caused by improperly terminated cables. It also enables error-free transmissions to 250kbps. To achieve
data rates to 2.5Mbps, substitute a full-slew-rate MAX1480A for IC2, a MAX485 for IC3, and R2—R5
values per Table 1.
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