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diff --git a/Documentation/networking/arcnet-hardware.rst b/Documentation/networking/arcnet-hardware.rst new file mode 100644 index 000000000..37c016cee --- /dev/null +++ b/Documentation/networking/arcnet-hardware.rst @@ -0,0 +1,307 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +ARCnet Hardware +=============== + +:Author: Avery Pennarun <apenwarr@worldvisions.ca> + +.. note:: + + This file is a supplement to arcnet.rst. 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 +:ref:`send an email to netdev <arcnet-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: + + https://www.arcnet.cc + + +Cabling ARCnet Networks +======================= + +This section was rewritten by + + Vojtech Pavlik <vojtech@suse.cz> + +using information from several people, including: + + - Avery Pennraun <apenwarr@worldvisions.ca> + - Stephen A. Wood <saw@hallc1.cebaf.gov> + - John Paul Morrison <jmorriso@bogomips.ee.ubc.ca> + - Joachim Koenig <jojo@repas.de> + +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) + =============== =============== ===================================== |