ARCnet Hardware

Author:

Avery Pennarun <apenwarr@worldvisions.ca>

Note

This file is a supplement to ARCnet. Please read that for general driver configuration help.

Because so many people (myself included) seem to have obtained ARCnet cards without manuals, this file contains a quick introduction to ARCnet hardware and some cabling tips. If you have any other information, do not hesitate to send an email to netdev.

Introduction to ARCnet

ARCnet is a network type which works in a way similar to popular Ethernet networks but which is also different in some very important ways.

First of all, you can get ARCnet cards in at least two speeds: 2.5 Mbps (slower than Ethernet) and 100 Mbps (faster than normal Ethernet). In fact, there are others as well, but these are less common. The different hardware types, as far as I’m aware, are not compatible and so you cannot wire a 100 Mbps card to a 2.5 Mbps card, and so on. From what I hear, my driver does work with 100 Mbps cards, but I haven’t been able to verify this myself, since I only have the 2.5 Mbps variety. It is probably not going to saturate your 100 Mbps card. Stop complaining. :)

You also cannot connect an ARCnet card to any kind of Ethernet card and expect it to work.

There are two “types” of ARCnet - STAR topology and BUS topology. This refers to how the cards are meant to be wired together. According to most available documentation, you can only connect STAR cards to STAR cards and BUS cards to BUS cards. That makes sense, right? Well, it’s not quite true; see below under “Cabling.”

Once you get past these little stumbling blocks, ARCnet is actually quite a well-designed standard. It uses something called “modified token passing” which makes it completely incompatible with so-called “Token Ring” cards, but which makes transfers much more reliable than Ethernet does. In fact, ARCnet will guarantee that a packet arrives safely at the destination, and even if it can’t possibly be delivered properly (ie. because of a cable break, or because the destination computer does not exist) it will at least tell the sender about it.

Because of the carefully defined action of the “token”, it will always make a pass around the “ring” within a maximum length of time. This makes it useful for realtime networks.

In addition, all known ARCnet cards have an (almost) identical programming interface. This means that with one ARCnet driver you can support any card, whereas with Ethernet each manufacturer uses what is sometimes a completely different programming interface, leading to a lot of different, sometimes very similar, Ethernet drivers. Of course, always using the same programming interface also means that when high-performance hardware facilities like PCI bus mastering DMA appear, it’s hard to take advantage of them. Let’s not go into that.

One thing that makes ARCnet cards difficult to program for, however, is the limit on their packet sizes; standard ARCnet can only send packets that are up to 508 bytes in length. This is smaller than the Internet “bare minimum” of 576 bytes, let alone the Ethernet MTU of 1500. To compensate, an extra level of encapsulation is defined by RFC1201, which I call “packet splitting,” that allows “virtual packets” to grow as large as 64K each, although they are generally kept down to the Ethernet-style 1500 bytes.

For more information on ARCnet networks, visit the “ARCNET Resource Center” WWW page at:

Cabling ARCnet Networks

This section was rewritten by

Vojtech Pavlik <vojtech@suse.cz>

using information from several people, including:

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
   R
   |

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:

  1. A card to a card. This is the simplest way of creating a 2-computer network.

  2. A card to a passive hub. Remember that all unused connectors on the hub must be properly terminated with 93 Ohm terminators (or something else if you don’t have the right ones), although the network may work without terminators.

  3. A card to an active hub. Here there is no need to terminate the unused connectors except some kind of aesthetic feeling. But, there may not be more than eleven active hubs between any two computers. That of course doesn’t limit the number of active hubs on the network.

  4. An active hub to another.

  5. An active hub to passive hub.

Remember that you cannot connect two passive hubs together. The power loss implied by such a connection is too high for the network 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        |
                |
                S

The BUS topology is very similar to the one used by 10BASE2 Ethernet. The only difference is in cable and terminators: they should be 93 Ohm. 10BASE2 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:

  RT----T------T------T------T------TR
   B    B      B      B      B      B

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

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

    A------T------T------TR
    |      B      B      B
S---H---S
    |
    S

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

S------T------T------S
       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:

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

A completely 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:

RP-------P--------P--------H-----P------P-----PR
                           |
  RP-----H--------P--------H-----P------PR
         |                 |
         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.

Setting the Jumpers

  • Every ARCnet card has its own “unique” network address from 0 to 255. Unlike Ethernet, you can set this address yourself with a jumper or switch (or on some cards, with special software). Since it’s only 8 bits, you can only have 254 ARCnet cards on a network. DON’T use 0 or 255, since these are reserved (although neat stuff will probably happen if you DO use them). By the way, if you haven’t already guessed, don’t set this the same as any other ARCnet device on your network!

  • There may be ETS1 and ETS2 settings. These may or may not make a difference on your card (many manuals call them “reserved”), but are used to change the delays used when powering up a computer on the network. This is only necessary when wiring VERY long range ARCnet networks, on the order of 4km or so; in any case, the only real requirement here is that all cards on the network with ETS1 and ETS2 jumpers have them in the same position. Chris Hindy <chrish@io.org> sent in a chart with actual values for this:

    ET1

    ET2

    Response Time

    Reconfiguration Time

    open

    open

    74.7us

    840us

    open

    closed

    283.4us

    1680us

    closed

    open

    561.8us

    1680us

    closed

    closed

    1118.6us

    1680us

    Make sure you set ETS1 and ETS2 to the SAME VALUE for all cards on your network.

LED Indicators

Many cards have red and green LEDs, which have the following meanings:

Green

Red

Status

OFF

OFF

Power off

OFF

Short flashes

Cabling problems (broken cable or not terminated)

OFF (short)

ON

Card init

ON

ON

Normal state - everything OK, nothing happens

ON

Long flashes

Data transfer

ON

OFF

Never happens (maybe when wrong ID)