NVE offers a broad line of single-chip isolated RS-422, RS-485, and PROFIBUS transceivers.
IsoLoop logic isolators can also be used as part of multi-chip designs to isolate
signals using particular non-isolated transceivers.
AB-19: PROFIBUS Compliance:
A Hardware Design Guide (.pdf) Hardware design best practices for Profibus DP nodes. Covers three important
but sometimes tricky design considerations: isolation, Profibus compatibility,
and bus node polarity.
NVE’s single-chip
isolated RS-485 transceivers include digital- and passive-input versions,
fractional-load versions, and PROFIBUS-compliant versions. Termination resistors
can be added to maximize speed and transmission length. Fail-safe resistors guarantee
a known state on a terminated bus with no active transceivers. NVE transceivers
are available with data rates are up to 40 Mbps. Parts are available in 0.3"
and 0.15" 16-pin SOIC packages.
Faster and longer networks should be be terminated. Reflections are minimized
by terminating both ends of the bus but not every node. Two 120 ohm termination
resistors (on each end of the bus) are used to match a 60 ohm cable impedance.
With terminated networks the differential voltage across the conductor pair will
be close to zero with no active drivers. In this case, the idle bus is indeterminate
and susceptible to noise. External fail-safe biasing resistors (RFS-EXT) at one
end of the bus ensure fail-safe operation with a terminated bus. Biasing should
provide at least 200 mV across the conductor pair to meet the RS-485 input
sensitivity specification.
The more loads on the bus, the lower the required values of the biasing resistors.
The reference design has a 120 ohm termination resistor and 560 ohm
biasing resistors for fail-safe operation for up to four Unit Loads.
An IL610
can be used as a simple isolated RS-485/RS-422 receiver. No external resistors
are required, and cabling is greatly simplified by eliminating the need to power
the input side of the receiving board. The circuit is failsafe because the IL610
is guaranteed to switch to the high state when the coil input current is less
than 500 µA. No current-limiting resistor is needed for a single receiver
because it will draw less current than the driver maximum. There is also no need
for line termination resistors in most IL610 line receiver applications below
a data rate of about 10 Mbps because the IL610 coil resistance of approximately
70 Ohms is close to the characteristic impedance of most cables.
The IL610
provides 2.5 kVrms isolation (one minute), and 20 kV/µs transient immunity.
The IL610-1 is a unique MSOP isolator for space-critical boards.
NVE’s single-chip
isolated RS-422 transceivers include digital- and passive-input versions,
and fractional-load versions. Termination resistors can be added to maximize speed
and transmission length. Fail-safe resistors guarantee a known state on a terminated
bus with no active transceivers. NVE transceivers are available with data rates
are up to 40 Mbps. Parts are available in 0.3" and 0.15" 16-pin SOIC packages.
This circuit shows a two-receiver RS-422 network using IL610s.
Two 17 Ohm current-limiting resistors prevent two IL610 coils from exceeding
the maximum load of the transceiver chip. Placement of the current-limiting resistors
on both lines provides better dynamic signal balance.
Simple, low cost, isolated repeater systems can be made with an RS-422 transmitter/receiver
pair (or an RS-485 transceiver in drive mode) and an IL610.
Repeaters can extend the useful cable length and node count indefinitely, provided
the network controller can tolerate the additional propagation delays.
NVE offers a broad line of single-chip isolated RS-485 transceivers, but the
unique IL614
three-channel isolator can also be used as part of a multi-chip design with non-isolated
transceivers. The IL614 provides 2.5 kVrms isolation (1 minute) and
20 kV/µs transient immunity. The IL614-3 is in a narrow-body (0.15"-wide)
package when board space is critical.
Isolated
PROFIBUS Using IL3685 Single-Chip Transceiver
The 220 ohm termination resistor, along with another 220 ohm resistor and the
other end of the bus, matches the bus impedance to minimize reflections. The 390
ohm "fail-safe" resistors guarantee a known state with no active transceivers.
Bypass capacitors should be placed as close as possible to the appropriate IC.
NVE offers the unique IL3685 single-chip isolated PROFIBUS transceiver,
but IL700 high-speed digital signal isolators can also be used as part of multi-chip
designs using non-isolated PROFIBUS transceivers. Circuits can be configured with
three IL710s
(circuit above) or one IL710 and one IL711
(circuit below). The IL710 and IL711 provide 2.5 kVrms isolation (1 minute)
and 30 kV/µs typical common-mode transient immunity. The IL710-1
and IL711-1 are unique MSOP one- and two-channel isolators to minimize
board space.
RS-485
Tips (04:03)
