Thin Ethernet (10base2)

	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 Terminator ?"

	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 (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.

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. "