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Source: ARCNET Tutorial by
Contemporary Controls
Hubs
Hubs facilitate cabling by interconnecting multiple NIMs and, in most cases,
they exercise no control over the network. The primary function of a hub is to
provide a convenient method of expanding a network. There are two types of hubs
that can perform this task — a passive hub or an active hub
Passive Hubs — Passive hubs are
inexpensive, require no power and their sole purpose is to match line
impedances, which they do with resistors. These hubs usually have four ports to
connect four coaxial star transceivers. One of the disadvantages of these hubs
is that they limit the network to 200 feet and each segment of the network to
100 feet. Also, unused ports must be terminated with a 93 ohm resistor for
proper operation. Passive hubs are used on small (four nodes or less) coaxial
star networks.
Active Hubs — Active hubs are
essentially electronic repeaters. Although they require power, active hubs
support all cabling options, support longer distances than passive hubs,
provide isolation and guard against cabling faults and reflections. These are
the hubs which are used to cable distributed star networks.
Unused ports on an active hub need not be terminated. Unlike passive hubs,
active hubs do not attenuate signals and can be cascaded. A cable failure will
affect only one port on an active hub. Active hubs are available as either
internal or external devices. Internal hubs reside inside a computer that also
has a NIM, while external hubs are stand-alone devices.
Active hubs can be configured as two port devices as well. A link is a two
port device with differing cable options on each port allowing for the
transition of one medium type to another such as coaxial to fiber conversion. A
repeater is a two-port device of the same cable option.
ARCNET Topologies
Point-To-Point — In the
point-to-point connection, only two NIMs are used. This is the simplest of
networks. Each NIM effectively terminates the other NIM; therefore, no hub is
required.
Star — The star connection
requires hubs. Each NIM connects to one point on the hub that effectively
terminates the connected NIM. Since only one NIM is connected to any one hub
port, faults in a cable or at a node can be easily isolated. Cabling a facility
is often easier with a star topology
Distributed Star (Tree) — If
several active hubs are used, a distributed star topology can be implemented.
This topology is the most flexible cabling method available in ARCNET LANs
since both node-to-hub and hub-to-hub connections are supported. Two or more
active hubs, each supporting a cluster of connected nodes, are linked together
by a “home run” cable.
The distributed star topology helps reduce cabling costs since each node
connects to a local hub, thereby eliminating the need to run each node’s
cable over to one wiring location. Like the star configuration, nodes are
isolated from one another.
Note: The "Home Run" cable from Active Hub to
Active Hub is not terminated.
Bus — In the bus configuration,
NIMs equipped with high impedance transceivers or EIA-485 drivers must be used.
Using RG-62/u coaxial cable and BNC “tees,” or twisted-pair cable, several
NIMs can be connected without the use of a hub. Termination is provided by the
installation of a resistive terminator at both ends of the cable segment. The
advantage of this configuration is that no hub is required. The disadvantage is
that one node failure could disrupt the complete network. Also, cabling
distances are less than the star or point-to-point connection.
Note: There is a
terminator at the leftmost NIM and one after the rightmost
NIM.
Multidrop
(Not Allowed!) — A multidrop topology is a
variation of the bus topology where a short “drop” cable from the tee
connection is allowed. There has not been any study on the effects and
limitation of drop cables so this topology is not allowed.
Star/Bus — To bridge a bus
topology to a star requires an active hub. In this case, the active hub acts as
both a terminator for the bus and a repeater for the network. Remove the
passive terminator from one end of the bus and connect that end to one port on
the active hub. Other ports on the active hub can now be used for other bus or
star connections.
Note: The "Bus" segment to the left of the Active
Hub has the furthest node Terminated, the cable into the Hub is not terminated.
Set the NIMs on the "Bus" segment to "Bus" mode. Set the NIMs that are directly
connected to the Hub to "Star" mode.
Daisy-Chain — Daisy chaining of
NIMs requires two connectors or a single connector with redundant connections
per NIM. Internally the two connections are bussed together and, therefore, do
not truly represent a daisy-chain connection but that of a bus. Daisy chaining
is best used with RJ-11 connectors. The unused connectors at each end of the
daisy-chain can then be used with RJ-11 style terminators.
Ring
(Not Allowed!) — ARCNET does not allow for a
ring or a loop connection. Unreliable operation of the network will be
experienced if a loop is implemented or if a distributed star topology is
violated by introducing a loop connection back to any one node.
Auto-Reconfiguration
Another feature of ARCNET is its ability to reconfigure the network
automatically if a node is either added or deleted from the network. If a node
joins the network, it does not automatically participate in the token-passing
sequence. Once a node notices that it is never granted the token, it will jam
the network with a reconfiguration burst that destroys the token-passing
sequence. Once the token is lost, all nodes will cease transmitting and begin a
timeout sequence based upon their own node address. The node with the highest
address will timeout first and begin a token pass sequence to the node with the
next highest address. If that node does not respond, it is assumed not to
exist. The destination node address is incremented and the token resent. This
sequence is repeated until a node responds. At that time, the token is released
to the responding node and the address of the responding node is noted as the
logical neighbor of the originating node. The sequence is repeated by all nodes
until each node learns its logical neighbor. At that time the token passes from
neighbor to neighbor without wasting time on absent addresses.
If a node leaves the network the reconfiguration sequence is slightly
different. When a node releases the token to its logical neighbor, it continues
to monitor network activity to ensure that the logical neighbor responded with
either a token pass or a start of a transmission sequence. If no activity was
sensed, the node that passed the token infers that its logical neighbor has
left the network and immediately begins a search for a new logical neighbor by
incrementing the node address of its logical neighbor and initiating a token
pass. Network activity is again monitored and the incrementing process and
resending of the token continues until a new logical neighbor is found. Once
found, the network returns to the normal logical ring routine of passing tokens
to logical neighbors.
"If you cut an ARCNET, you get two ARCNETs within seconds..."
This will require meditation, as termination and hubs are an important part
of the "Auto-Reconfiguration" aspect.
Transceivers
Coaxial Star — Typically, ARCNET
is cabled with RG-62/u coaxial cable (with BNC connectors) in a star topology,
each NIM connects directly to a port on an active or passive hub.
Alternatively, RG-59/u coaxial cable can be used, but at a cost of reduced
distances between a node and a hub. Overall, coaxial cable offers good
performance, good noise immunity, low propagation delay, low signal
attenuation, sufficient ruggedness and low cost. The coaxial star configuration
also provides the longest coaxial distance and simplified troubleshooting.
Coaxial Bus — RG-62/u coaxial
cable can be used in a bus configuration using BNC tee connectors with passive
terminators at each end of the cable. Although hubs are not required, cabling
options are restricted and troubleshooting is much more difficult. There is a
minimum distance between adjacent nodes. Coaxial bus is used when reliable
coaxial cable communication is required in a hubless system when shorter
distances are involved.
Twisted-Pair Star — Unshielded
twisted-pair wiring such as IBM Type 3 (#24 or #22 AWG solid copper
twisted-pair cable or telephone wiring) can be used. BALUNs are required at
both the hub and NIM to use this cable. Some twisted-pair NIMs and hubs have
internal BALUNs, so external BALUNs are not needed. Twisted-pair is convenient
to install. However, its attenuation exceeds coaxial, its noise immunity is
less, and its maximum length between a node and a hub is lower. RJ-11
connectors are often used with this cable.
Twisted-Pair Bus — The convenience
of twisted-pair wiring can be used in a bus configuration without the use of
BALUNs. Dual RJ-11 jacks are provided so modules can be wired in a “daisy
chain” fashion even though electrically they are connected as a bus.
Distances are limited as well as node count. Passive terminators are inserted
in unused jacks at the far end of the segment. For small hubless systems this
approach is attractive.
Glass Fiber Optics and EIA-485
are not covered in this page.
Cable
Coaxial Cable — RG-62u was the
original choice for cabling ARCNET systems, and is recommended over RG-59/u if
possible. RG-62/u (93 ohm) is a better impedance match to the coaxial
transceiver and has less attenuation than RG-59/u (75 ohm) yielding greater
distances. Standard BNC connectors and tees are used. Coaxial cable is
relatively inexpensive and provides the highest propagation factor compared to
other alternatives.
Coaxial Star vs. Bus
The coaxial star transceiver and the coaxial bus transceiver both receive P1
and P2 signals and generate dipulse signals. However, the star transceiver
represents a low impedance (approximately 93 ohms) at all times while the bus
transceiver represents a high impedance when idle allowing for multiple
transceivers to be attached to a common bus. Since the two transceivers have a
similar appearance, it is important to distinguish one from another. The
following practice is recommended for identification purposes. For star
transceivers, use black bodied BNC connectors on the printed-circuit board. For
bus transceivers, use white.
The capabilities of the two transceivers differ significantly. The star
transceiver can drive 2000 feet (610 m) of RG-62/u cable while the bus can only
drive 1000 feet (305 m). However, the bus transceiver can support eight nodes
on a single segment. Connections between nodes are made with BNC tee connectors
and coaxial cables of at least six feet (2 m) in length. Passive termination is
required at the ends of bus segments. The isolation of the two transceivers is
typically 1000 volts DC.
Twisted-Pair — Unshielded
twisted-pair cabling can be used with several transceivers including those for
EIA-485. We recommend IBM type 3 (although other unshielded twisted-pair cable
with similar characteristics will also work). Twisted-pair cable is inexpensive
and convenient to use and easy to terminate. However, twisted-pair cable has
much greater attenuation than coaxial cable and, therefore, has limited
distance capability
Twisted-pair is also a popular cabling technology. It is inexpensive and
easy to terminate. However, it has much higher attenuation than coaxial cable
limiting its use to shorter distances. Frequently, modular jacks and plugs are
used to interconnect segments. Twisted-pair cable can be used with conventional
coaxial star transceivers if a BALUN is used between the cable and the
transceiver. A MUX LAB 10070 is recommended for use as an external BALUN. It
has a male BNC connector at one end and a RJ-11 jack at the other, and it must
be used only with coaxial star transceivers. For convenience, some vendors
provide a product that eliminates the need for external BALUNs. The
twisted-pair star transceiver incorporates an internal BALUN along with a
coaxial star transceiver together as one unit. Simply connect to the provided
RJ-11 jack. When using BALUNs, only star and distributed star topologies are
supported. No phase reversal of the wiring is allowed. Many modular plug patch
cables invert the wiring. To test for this, hold both ends of the cable side by
side with the retaining clips facing the same direction. The color of the wire
in the rightmost position of each plug must be the same if there is no
inversion of the cable. If this is not the case, the cable is inverted
Twisted-Pair Bus
For hubless systems, twisted-pair bus transceivers can be used. Since
modular jacks are used and a bus connection is required, two jacks, internally
wired together, are provided on each NIM. Field connections are then made in a
daisy-chain fashion to each successive NIM. The remaining end jacks are then
plugged with passive terminators. A modular plug terminator is available for
this use. Each daisy-chain cable must not invert the signals and must be at
least six feet long for reliable operation.
Hubs can be used to extend twisted-pair bus segments. Use a twisted-pair
star hub port in place of the passive terminator at one end of the segment.
Connect this last port on the NIM to the twisted-pair star port on the hub
using an “inverted” modular plug cable. This is necessary since the BALUN
in the twisted-pair star port creates a signal inversion that is not compatible
with the twisted-pair bus port. The interconnecting inverted cable “rights”
the signal. Connect the second twisted-pair bus segment in a similar fashion
using an additional twisted-pair star port.
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