Information Technology Reference
In-Depth Information
User and patch cords should be twisted-pair wire, as should the station cable.
Short lengths of flat telephone-type cord may work for the lower ARCnet speeds,
though, since only one pair is used and the problem of crosstalk does not exist.
The daisy-chain twisted-pair bus is a variation of the coax bus using twisted
pair. It is useful for small networks with existing cable and in situations where the
workstations are all in one room. The one-room design is the easiest to visualize. As
with the coax bus, a twisted-pair cable is run from one workstation to the next in
line, where another cable is run onto the next workstation, forming a chain of work-
stations. Most twisted-pair ARCnet adapter cards have two common connectors, so
this is easily done. A 100-ohm terminator plug must be used in the two worksta-
tions at the ends of the chain. Of course, the wire can also be run between worksta-
tions in different rooms, in the same daisy-chain fashion.
Things are a little more complex if this is attempted with installed wiring. The
loops of the chain must be in the same physical station cable. One way to accom-
plish this is to use a special adapter cable from the wall plate that is split out to two
plugs at the NIC. The cable has two active pairs, say Pair I and Pair 2, that are split
to the two plugs, both wired for Pair 1 so they will connect to the NIC's pins 4 and
5. Then at the punch block in the wiring closet, Pair 1 on the first drop is cross-con-
nected to Pair 2 on the second drop, and so forth through the chain. The first and
last workstations in the chain will use only one of the plugs, the Pair 1 plug for the
first and the Pair 2 plug for the last, and will need a 100-ohm terminator plugged
into the other port on their NICs.
Problems in ARCnet cabling are as varied as the many types of cable topologies
that are used. The coaxial bus is subject to the same potential problems with BNC-Ts
and terminators as thinnet. If a terminator is missing, the network will at least par-
tially fail. As with the other LAN types, any break in a bus cable will also cause the
network to fail, although the twisted-pair bus may be somewhat more tolerant of this.
Of course, no signals may pass across the break! If a break occurs, look for
damaged cable, connectors, Ts, and so on. Rarely will a bad adapter card cause a
total network failure, since the cards are not active until they transmit. An adapter
failure will usually affect only that workstation.
Star-type ARCnet networks will usually have a problem only on one leg,
although that will take your network down if that leg is the server. Try plugging into
another port on the hub. Most twisted-pair hubs and some coax active hubs have
status lights that will indicate good cable. An active hub can have a total electrical
failure, or in some cases, have damage to only one port. A passive hub must have
the proper value terminator connected to all open ports. Many passive hubs have
terminators that are permanently tethered to the hub with a ball and chain, so the
terminator won't disappear.