I/O Module for Rear-I/O
RS-485 Electrically Isolated
The CU7-RS485 is a rear-I/O PHY module, destined for
CompactPCI®
systems, to be used in combination with the serial interface cards CU1-CHORUS
and CU2-QUARTET. The CU7-RS485 is provided with an electrically isolated
TIA-485 data transceiver for EMI sensitive applications.
More often known as RS-485, this standard is a popular balanced voltage signal
technology for interconnecting devices on an industrial network, up to 1200m
cable length. The CU7-1-RS485 allows for connecting various controllers in a
daisy-chain fashion across a single twisted pair cable, while the CU7-2-RS485
is suitable for a direct point-to-point interconnection between two devices
over a 4-wire line.
Designed for harsh environment, the CU7-RS485 modules are high-reliability,
rugged RS-485 interfaces, well suited for industrial and telecommunication purposes.
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The RS-485 bus network topology is also known as party-line. Using a
half-duplex transmission principle, this method requires a single twisted
pair cable only, by way of connecting together driver outputs and receiver inputs.
The CU7-1-RS485 allows for up to 32 networking nodes, sharing a common
bus. Typically, all devices receive all messages permanently, but there is only
one sender allowed to be enabled at a given time.
Point-to-point interconnections however can be established as a full-duplex 4-wire
line (sending and receiving data simultaneously). The CU7-2-RS485 is therefore
equipped with separated drivers and receivers.
While providing data transfer rates up to 2.5Mbps, both modules are protected
against ESD up to 15kV and electrically isolated up to 500V.
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CU9-4-BASE Rear I/O Module
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As data rates increase, proper termination on both ends of the RS-485 bus
becomes important. A popular approach is to use external termination, having
the resistors located within the shell of the DB9 connectors.
As an alternative, termination resistors can be stuffed directly on the
CU7-RS485 board. Due to miniaturized SMD components, a properly equipped
working place is highly recommended.
Typically, the termination resistor value is 120 Ohm or 130 Ohm, ideally
matching the twisted pair cable impedance. When calculating, the optionally
stuffed bias resistors for open-line fail-safe termination would lie in
parallel. Normally, this effect can be disregarded.
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Any party-line network must be terminated at the extreme ends of its bus.
Stubs in order to attach particular nodes are not allowed.
When establishing a point-to-point full-duplex interconnection, both data
directions, receiver and driver lines, require termination. This means, in
addition to the A/B line also the Y/Z line must be terminated.
The RS-485 receivers used in the MAX1480/MAX1490 have an internal built-in
fail-safe feature, so that the resistors R17/R18 and R22/R23 (see figure below)
normally should not be stuffed. If for some reason bias resistors for open-line
fail-safe termination are required, a nominal value of 750 Ohm or 820 Ohm is recommended.
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CU7-RS485 Termination and Fail-Safe Bias Network (Option)
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Is RS-485 a two-wire ore a three-wire system? It is most definitely a three wire system
(four plus one wire with respect to full-duplex operation).
The TIA standard (ANSI/TIA/EIA-485-A, page 15, A.4.1) requires the presence of a common return path between
all circuit grounds along the balanced line for proper operation.
The TIA standard defines a maximum common mode voltage range from -7V to +12V
on the signal lines A and B, measured against C (common ground). A TIA/EIA-485
system however with only two wires A and B (C generator and C receiver commons
not connected) can result in an unpredictable common mode voltage superimposed
on the interface lines A and B, caused either by electrostatic charging or
electromagnetic interference.
A 2-wire system often may work though due to
idle-line fail-safe resistors at the receiver inputs,
which can be considered as a loosely coupled common ground. Nevertheless this
operation mode cannot be recommended - what is working flawless in the laboratory
may not work reliable under real conditions in an industrial environment.
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Where do we get the third wire? Many times the outer cable shield is used as the
third (fifth) wire. However, EKF recommends to use a two pair cable (three pairs
for full-duplex operation), with one or both wires of the additional pair as the
dedicated common ground. Connect these additional wires directly to the
pins 3, 6 & 5, 8 of the DB9 connector for proper grounding.
The optimum cable solution would comprise an inner shield for each signal twisted pair.
The inner shield can then be used for establishing the common ground between
TIA/EIA-485 nodes (connect to pins 3, 6 & 5, 8 of the DB9 connector).
An additional outer cable shield, that may cover the inner signal and ground cable pairs,
should be connected to the pin 1 of the connector (it is equivalent and sufficient to
connect the shield with the metallic shell of the DB9 connector). This shield should
be grounded at one point only (isolate the shield at the opposite cable end in order
to avoid any contact with the connector hood).
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CU7-RS485 - Component Assembly Drawing
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Technical Feature Summary CU7-RS48 |
Form Factor
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Rear-I/O Module (60x31mm2)
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Function
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PHY Transceiver RS-485 (EIA/TIA-485-A)
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Chip
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Maxim MAX1480EA (CU7-1-RS485) oder MAX1490EA (CU7-2-RS485),electrically isolated
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RS-485 Interface
CU7-1-RS485
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2-Wire line TxD/RxD, driver and receiver connected, operation mode half-duplex,
the transmitter (driver) is enabled by signal RTS, receiver has a fail-safe
feature if the input is open circuit, external connector D-Sub 9-pos. male
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RS-485 Interface
CU7-2-RS485
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4-Wire line TxD/RxD, driver and receiver not connected, operation mode full-duplex,
the transmitter (driver) is permanently enabled, receiver has a fail-safe
feature if the input is open circuit, external connector D-Sub 9-pos. male
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Data Transfer Rate
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2.5Mbps max. (CU7-1-RS485), 2 x 2.5Mbps max. (CU7-2-RS485)
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ESD Protection
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±15kV (Human Body Model)
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Isolation Voltage
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±500V DC max.
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Failure Protection
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Short-circuit current limited, protected against excessive power by thermal shutdown
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Power Requirements
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+5V 0.3A max.
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Temperature
Humidity
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Operating temperature 0-70°C (extended temperature range on request)
Relative humidity 5-90% non-condensing
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Specifications are subject to change without further notice
Ordering Information
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Ordering Number
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Short Description
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CU7-1-RS485
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RS-485/RS-422 Rear I/O PHY interface module, 1 port, 2.5Mbps two wire
transmission line (party line), electrically isolated
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CU7-2-RS485
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RS-485/RS-422 Rear I/O PHY interface module, 1 port, 2.5Mbps 4-wire
transmission line (point-to-point), electrically isolated
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CU4-2-SOPRANO
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CompactPCI® quad serial interface, suitable for rear I/O across J2
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CU9-4-BASE
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Transition board, interconnection between CU4-SOPRANO (J2) and
rear I/O PHY interface modules, 4 ports, 3U/8HP back panel included
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