Cabling an arcnet network

This section was rewritten by 
        Vojtech Pavlik     <>
using information from several people, including:
        Avery Pennarun     <>
 	Stephen A. Wood    <>
 	John Paul Morrison <>
 	Joachim Koenig     <>
and Avery touched it up a bit, at Vojtech's request.

ARCnet (the classic 2.5 Mbps version) can be connected by two different types of cabling: coax and twisted pair. The other ARCnet-type networks (100 Mbps TCNS and 320 kbps - 32 Mbps ARCnet Plus) use different types of cabling (Type1, Fiber, C1, C4, C5).

For a coax network, you "should" use 93 Ohm RG-62 cable. But other cables also work fine, because ARCnet is a very stable network. I personally use 75 Ohm TV antenna cable.

Cards for coax cabling are shipped in two different variants: for BUS and STAR network topologies. They are mostly the same. The only difference lies in the hybrid chip installed. BUS cards use high impedance output, while STAR use low impedance. Low impedance card (STAR) is electrically equal to a high impedance one with a terminator installed.

Usually, the ARCnet networks are built up from STAR cards and hubs. There are two types of hubs - active and passive. Passive hubs are small boxes with four BNC connectors containing four 47 Ohm resistors:

   |         | wires
   R         + junction
-R-+-R-      R 47 Ohm resistors

The shielding is connected together. Active hubs are much more complicated; they are powered and contain electronics to amplify the signal and send it to other segments of the net. They usually have eight connectors. Active hubs come in two variants - dumb and smart. The dumb variant just amplifies, but the smart one decodes to digital and encodes back all packets coming through. This is much better if you have several hubs in the net, since many dumb active hubs may worsen the signal quality.

And now to the cabling. What you can connect together:

Remember that you can not connect two passive hubs together. The power loss implied by such a connection is too high for the net to operate reliably.

An example of a typical ARCnet network:


           R                     S - STAR type card              
    S------H--------A-------S    R - Terminator
           |        |            H - Hub                         
           |        |            A - Active hub                  
           |   S----H----S                                       
           S        |                                            

The BUS topology is very similar to the one used by Ethernet. The only difference is in cable and terminators: they should be 93 Ohm. Ethernet uses 50 Ohm impedance. You use T connectors to put the computers on a single line of cable, the bus. You have to put terminators at both ends of the cable. A typical BUS ARCnet network looks like:


     B    B      B      B      B      B

  B - BUS type card
  R - Terminator
  T - T connector

But that isn't all! The two types can be connected together. According to the official documentation the only way of connecting them is using an active hub:

         |      B      B      B

The official docs also state that you can use STAR cards at the ends of BUS network in place of a BUS card and a terminator:

            B      B

But, according to my own experiments, you can simply hang a BUS type card anywhere in middle of a cable in a STAR topology network. And more - you can use the bus card in place of any star card if you use a terminator. Then you can build very complicated networks fulfilling all your needs! An example:

            B      B       B      |
                                  |       R
           |      B       B       |       |      B                         
           |   S                 BT       |                                 
           |   |                  |  S----A-----S
    S------H---A----S             |       | 
           |   |      S------T----H---S   |
           S   S             B    R       S  

A basically different cabling scheme is used with Twisted Pair cabling. Each of the TP cards has two RJ (phone-cord style) connectors. The cards are then daisy-chained together using a cable connecting every two neighboring cards. The ends are terminated with RJ 93 Ohm terminators which plug into the empty connectors of cards on the ends of the chain. An example:

          ___________   ___________
      _R_|_         _|_|_         _|_R_  
     |     |       |     |       |     |      
     |Card |       |Card |       |Card |     
     |_____|       |_____|       |_____|          

There are also hubs for the TP topology. There is nothing difficult involved in using them; you just connect a TP chain to a hub on any end or even at both. This way you can create almost any network configuration. The maximum of 11 hubs between any two computers on the net applies here as well. An example:


             |                 |
             PR                PR

    R - RJ Terminator
    P - TP Card
    H - TP Hub

Like any network, ARCnet has a limited cable length. These are the maximum cable lengths between two active ends (an active end being an active hub or a STAR card).

		RG-62       93 Ohm up to 650 m
		RG-59/U     75 Ohm up to 457 m
		RG-11/U     75 Ohm up to 533 m
		IBM Type 1 150 Ohm up to 200 m
		IBM Type 3 100 Ohm up to 100 m

The maximum length of all cables connected to a passive hub is limited to 65 meters for RG-62 cabling; less for others. You can see that using passive hubs in a large network is a bad idea. The maximum length of a single "BUS Trunk" is about 300 meters for RG-62. The maximum distance between the two most distant points of the net is limited to 3000 meters. The maximum length of a TP cable between two cards/hubs is 650 meters.