linux-2.4.0内核源代码

----------------------------------------------------------------------------- 1) This file is a supplement to arcnet.txt. Please read that for general driver configuration help. ----------------------------------------------------------------------------- 2) This file is no longer Linux-specific. It should probably be moved out of the kernel sources. Ideas? ----------------------------------------------------------------------------- Because so many people (myself included) seem to have obtained ARCnet cards without manuals, this file contains a quick introduction to ARCnet hardware, some cabling tips, and a listing of all jumper settings I can find. Please e-mail apenwarr@worldvisions.ca with any settings for your particular card, or any other information you have! 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 the advantages and disadvantages (mostly the advantages) of ARCnet networks, you might try the "ARCnet Trade Association" WWW page: http://www.arcnet.com 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 (or something else if you don't have the right ones) terminators. (Avery's note: oops, I didn't know that. Mine (TV cable) works anyway, though.) 3. A card to an active hub. Here 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 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 | | 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: 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 ty