Thin Ethernet (10base2)

by Johannes Helmig [Published on 24 Jan. 1999 / Last Updated on 24 Jan. 1999]

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!

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 !)

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 !
The T-connector is put directly on the BNC-connector of the network board
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


Run the cable to the
system and then
run it back (watch out NOT to exceed the Maximum allowed
cable-length ) !

Solution 2


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.

Let me repeat that:
50 ohm terminators !(and not 75 or any other value !)


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 Termintor"?


No wire or chain : open terminator !

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.

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. On very large networks, it will be required to install switches to optimize network utilisation.


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.

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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