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Thin Ethernet (10base2)

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I have seen now so many
messages on the News-Groups, and also received so many
messages on this topic, that I am putting it up now like
this:
on a 10base2 coax network-cable,
there MUST be a 50 Ohm terminator
on each end : One
terminator MUST be grounded, the other NOT !
(And it does NOT work to plug the
BNC-connector of the coax-cable directly onto the
BNC-connector of the NIC without T-connector !)
The dangerous issue: if you put on TWO not-grounded
terminator or
if you ground BOTH terminators, the network does NOT fail
completely, but I can guarantee you problems: slow,
unreliable, errors during data-transfer ! |
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Thin Ethernet (10base2), sometime also called "Cheapernet", is based on using a coax-cable, which is specified as RG58
(please, do NOT use other type of coax-cable, it has other
electrical properties and will not work properly) and which runs
from system to system.

The cable is connected via BNC-T-connectors
to the network card installed in the PC.

The T-connector must be put on the
BNC-connector of the network card !
(if you have a Combo / Multi-Connector
Board, check, if you need to
select the BNC-port !)
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It is NOT allowed to put
in any extension/cable between the
T-connector and the BNC-connector on the network card !
If you do that, your network will either NOT work or
becomes unreliable,
working slow or sometimes failing ! |
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The T-connector is
put directly on the
BNC-connector of the
Network board |
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If one of the systems
is
located away from the
cable, you CANNOT use
a drop-cable from the
t-connector to the
BNC of the Network
Card (NIC) !
It either does NOT work
or works unreliable /
slow ! |
Solution 1)
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Run the cable to the
system and then
run it back
(watch out NOT
to exceed the
Maximum allowed
cable-length ) ! |
Solution 2)
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Put in a separate
"Repeater",
which allows to connect the
remote located system on its
own segment, which need
to have its own terminators. |
So, a more realistic view on an installation is:

The cable swings from system to system (it is NOT allowed to
put in "junctions" , to connect for example 3 PC's in a
Y-configuration).
At the end of the cable, the coax-cable MUST be terminated, using
a BNC 50 ohm terminator.
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Let me repeat that:
50 ohm
terminators !
(and not 75 or any other value ! ) |
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There are 2 different type of terminators:

The original Thin-Ethernet specifications state, that the
cable has to be terminated at one end with an "open
terminator" and on the other side with a "grounded
terminator", a wire or small chain, which has to be
connected to a metal-part on the back of the PC to get a
grounding.
I know, that a lot of documentation shows only 2 "open"
terminators,and that "grounded" terminators are
sometimes difficult to find. One small networks, it even works
"somehow", but not reliable and not at top-speed.
I have been asked several times:
"How do I recognize an OPEN and a GROUNDED Terminator ?"
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No wire or chain : open
terminator ! |
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A wire or chain: a Grounded
Terminator !
Don't forget to connect it to a metal part of the
computer box,
otherwise it is NOT grounded ! |
The need for the terminators require to shutdown the complete
network, when having to add a new system to the network (see Twisted Pair versus Coax: Reliability /
Advantages ).
Limitations:

each connection to an Thin-Ethernet RG-58 cable is called a
"node", which can be a system like a
PC, a UNIX-workstation or a Fileserver, but also anything else
connecting via a BNC-connector counts a a node (network printers,
repeaters,....).
- maximum 30 nodes on one Thin-Ethernet segment
- minimum 0.5 meter distance between nodes
- maximum total cable-length of 185 meter
If more than 30 nodes need to be connected or if the total
cable-length needs to be longer
than 185 meters, a repeater is the
solution:

Multi-Segment network:
For each cable-segment, you need to be within the
10base2 limitation (max. 185 meter, 30 nodes),
but this box called "Repeater"
connects now the 2 (or even more) segments (some
Multi-port repeaters can connect 4 or even 8 cables),
which in view of the users extends the cable-length
beyond the 185 meters. More than 1 repeater can be used in a
network, but there are limits (see Large Networks: 5-4-3 Rule). On very large networks, it will be required to
install Switches to optimize network
utilisation.
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You want a "healthy" network ?
you need one "open" terminator at one end of
the cable
and one "grounded" terminator at the other end
of the Thin Ethernet cable !
Without proper "termination", the network may
not work,
it may work, but then unreliable or slow.
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This need for a GROUNDED terminator is not
mentioned often in other documentation,
but I am sure about this due to my own experience and also due to
the feedback, which
I am getting back from my visitors, example (received
Feb.21,99):
"I found your emphasis on grounding one end of thin-net
coax extremely helpful.
I had a working network, except for one machine. We had
occasional slow-downs.
I finally got the one machine on the net after I grounded one end
of the cable.
I think maybe the slow-downs will stop also, we will see.
Note: As I remember, I have seen nothing mentioned about
grounding of the thin-net cable
any where else. "
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