1.. SPDX-License-Identifier: GPL-2.0 2 3=============== 4ARCnet Hardware 5=============== 6 7:Author: Avery Pennarun <apenwarr@worldvisions.ca> 8 9.. note:: 10 11 This file is a supplement to arcnet.rst. Please read that for general 12 driver configuration help. 13 14Because so many people (myself included) seem to have obtained ARCnet cards 15without manuals, this file contains a quick introduction to ARCnet hardware 16and some cabling tips. If you have any other information, do not hesitate to 17:ref:`send an email to netdev <arcnet-netdev>`. 18 19 20Introduction to ARCnet 21====================== 22 23ARCnet is a network type which works in a way similar to popular Ethernet 24networks but which is also different in some very important ways. 25 26First of all, you can get ARCnet cards in at least two speeds: 2.5 Mbps 27(slower than Ethernet) and 100 Mbps (faster than normal Ethernet). In fact, 28there are others as well, but these are less common. The different hardware 29types, as far as I'm aware, are not compatible and so you cannot wire a 30100 Mbps card to a 2.5 Mbps card, and so on. From what I hear, my driver does 31work with 100 Mbps cards, but I haven't been able to verify this myself, 32since I only have the 2.5 Mbps variety. It is probably not going to saturate 33your 100 Mbps card. Stop complaining. :) 34 35You also cannot connect an ARCnet card to any kind of Ethernet card and 36expect it to work. 37 38There are two "types" of ARCnet - STAR topology and BUS topology. This 39refers to how the cards are meant to be wired together. According to most 40available documentation, you can only connect STAR cards to STAR cards and 41BUS cards to BUS cards. That makes sense, right? Well, it's not quite 42true; see below under "Cabling." 43 44Once you get past these little stumbling blocks, ARCnet is actually quite a 45well-designed standard. It uses something called "modified token passing" 46which makes it completely incompatible with so-called "Token Ring" cards, 47but which makes transfers much more reliable than Ethernet does. In fact, 48ARCnet will guarantee that a packet arrives safely at the destination, and 49even if it can't possibly be delivered properly (ie. because of a cable 50break, or because the destination computer does not exist) it will at least 51tell the sender about it. 52 53Because of the carefully defined action of the "token", it will always make 54a pass around the "ring" within a maximum length of time. This makes it 55useful for realtime networks. 56 57In addition, all known ARCnet cards have an (almost) identical programming 58interface. This means that with one ARCnet driver you can support any 59card, whereas with Ethernet each manufacturer uses what is sometimes a 60completely different programming interface, leading to a lot of different, 61sometimes very similar, Ethernet drivers. Of course, always using the same 62programming interface also means that when high-performance hardware 63facilities like PCI bus mastering DMA appear, it's hard to take advantage of 64them. Let's not go into that. 65 66One thing that makes ARCnet cards difficult to program for, however, is the 67limit on their packet sizes; standard ARCnet can only send packets that are 68up to 508 bytes in length. This is smaller than the Internet "bare minimum" 69of 576 bytes, let alone the Ethernet MTU of 1500. To compensate, an extra 70level of encapsulation is defined by RFC1201, which I call "packet 71splitting," that allows "virtual packets" to grow as large as 64K each, 72although they are generally kept down to the Ethernet-style 1500 bytes. 73 74For more information on ARCnet networks, visit the "ARCNET Resource Center" 75WWW page at: 76 77 https://www.arcnet.cc 78 79 80Cabling ARCnet Networks 81======================= 82 83This section was rewritten by 84 85 Vojtech Pavlik <vojtech@suse.cz> 86 87using information from several people, including: 88 89 - Avery Pennraun <apenwarr@worldvisions.ca> 90 - Stephen A. Wood <saw@hallc1.cebaf.gov> 91 - John Paul Morrison <jmorriso@bogomips.ee.ubc.ca> 92 - Joachim Koenig <jojo@repas.de> 93 94and Avery touched it up a bit, at Vojtech's request. 95 96ARCnet (the classic 2.5 Mbps version) can be connected by two different 97types of cabling: coax and twisted pair. The other ARCnet-type networks 98(100 Mbps TCNS and 320 kbps - 32 Mbps ARCnet Plus) use different types of 99cabling (Type1, Fiber, C1, C4, C5). 100 101For a coax network, you "should" use 93 Ohm RG-62 cable. But other cables 102also work fine, because ARCnet is a very stable network. I personally use 75 103Ohm TV antenna cable. 104 105Cards for coax cabling are shipped in two different variants: for BUS and 106STAR network topologies. They are mostly the same. The only difference 107lies in the hybrid chip installed. BUS cards use high impedance output, 108while STAR use low impedance. Low impedance card (STAR) is electrically 109equal to a high impedance one with a terminator installed. 110 111Usually, the ARCnet networks are built up from STAR cards and hubs. There 112are two types of hubs - active and passive. Passive hubs are small boxes 113with four BNC connectors containing four 47 Ohm resistors:: 114 115 | | wires 116 R + junction 117 -R-+-R- R 47 Ohm resistors 118 R 119 | 120 121The shielding is connected together. Active hubs are much more complicated; 122they are powered and contain electronics to amplify the signal and send it 123to other segments of the net. They usually have eight connectors. Active 124hubs come in two variants - dumb and smart. The dumb variant just 125amplifies, but the smart one decodes to digital and encodes back all packets 126coming through. This is much better if you have several hubs in the net, 127since many dumb active hubs may worsen the signal quality. 128 129And now to the cabling. What you can connect together: 130 1311. A card to a card. This is the simplest way of creating a 2-computer 132 network. 133 1342. A card to a passive hub. Remember that all unused connectors on the hub 135 must be properly terminated with 93 Ohm terminators (or something else if you 136 don't have the right ones), although the network may work without 137 terminators. 138 1393. A card to an active hub. Here there is no need to terminate the unused 140 connectors except some kind of aesthetic feeling. But, there may not be 141 more than eleven active hubs between any two computers. That of course 142 doesn't limit the number of active hubs on the network. 143 1444. An active hub to another. 145 1465. An active hub to passive hub. 147 148Remember that you cannot connect two passive hubs together. The power loss 149implied by such a connection is too high for the network to operate reliably. 150 151An example of a typical ARCnet network:: 152 153 R S - STAR type card 154 S------H--------A-------S R - Terminator 155 | | H - Hub 156 | | A - Active hub 157 | S----H----S 158 S | 159 | 160 S 161 162The BUS topology is very similar to the one used by 10BASE2 Ethernet. The only 163difference is in cable and terminators: they should be 93 Ohm. 10BASE2 Ethernet 164uses 50 Ohm impedance. You use T connectors to put the computers on a single 165line of cable, the bus. You have to put terminators at both ends of the 166cable. A typical BUS ARCnet network looks like:: 167 168 RT----T------T------T------T------TR 169 B B B B B B 170 171 B - BUS type card 172 R - Terminator 173 T - T connector 174 175But that is not all! The two types can be connected together. According to 176the official documentation, the only way of connecting them is using an active 177hub:: 178 179 A------T------T------TR 180 | B B B 181 S---H---S 182 | 183 S 184 185The official docs also state that you can use STAR cards at the ends of a 186BUS network in place of a BUS card and a terminator:: 187 188 S------T------T------S 189 B B 190 191But, according to my own experiments, you can simply hang a BUS type card 192anywhere in middle of a cable in a STAR topology network. And more - you 193can use the bus card in place of any star card if you use a terminator. Then 194you can build very complicated networks fulfilling all your needs! An 195example:: 196 197 S 198 | 199 RT------T-------T------H------S 200 B B B | 201 | R 202 S------A------T-------T-------A-------H------TR 203 | B B | | B 204 | S BT | 205 | | | S----A-----S 206 S------H---A----S | | 207 | | S------T----H---S | 208 S S B R S 209 210A completely different cabling scheme is used with Twisted Pair cabling. Each 211of the TP cards has two RJ (phone-cord style) connectors. The cards are 212then daisy-chained together using a cable connecting every two neighboring 213cards. The ends are terminated with RJ 93 Ohm terminators which plug into 214the empty connectors of cards on the ends of the chain. An example:: 215 216 ___________ ___________ 217 _R_|_ _|_|_ _|_R_ 218 | | | | | | 219 |Card | |Card | |Card | 220 |_____| |_____| |_____| 221 222 223There are also hubs for the TP topology. There is nothing difficult 224involved in using them; you just connect a TP chain to a hub on any end or 225even at both. This way you can create almost any network configuration. 226The maximum of 11 hubs between any two computers on the net applies here as 227well. An example:: 228 229 RP-------P--------P--------H-----P------P-----PR 230 | 231 RP-----H--------P--------H-----P------PR 232 | | 233 PR PR 234 235 R - RJ Terminator 236 P - TP Card 237 H - TP Hub 238 239Like any network, ARCnet has a limited cable length. These are the maximum 240cable lengths between two active ends (an active end being an active hub or 241a STAR card). 242 243 ========== ======= =========== 244 RG-62 93 Ohm up to 650 m 245 RG-59/U 75 Ohm up to 457 m 246 RG-11/U 75 Ohm up to 533 m 247 IBM Type 1 150 Ohm up to 200 m 248 IBM Type 3 100 Ohm up to 100 m 249 ========== ======= =========== 250 251The maximum length of all cables connected to a passive hub is limited to 65 252meters for RG-62 cabling; less for others. You can see that using passive 253hubs in a large network is a bad idea. The maximum length of a single "BUS 254Trunk" is about 300 meters for RG-62. The maximum distance between the two 255most distant points of the net is limited to 3000 meters. The maximum length 256of a TP cable between two cards/hubs is 650 meters. 257 258 259Setting the Jumpers 260=================== 261 262 - Every ARCnet card has its own "unique" network address from 0 to 255. 263 Unlike Ethernet, you can set this address yourself with a jumper or switch 264 (or on some cards, with special software). Since it's only 8 bits, you can 265 only have 254 ARCnet cards on a network. DON'T use 0 or 255, since these 266 are reserved (although neat stuff will probably happen if you DO use them). 267 By the way, if you haven't already guessed, don't set this the same as any 268 other ARCnet device on your network! 269 270 - There may be ETS1 and ETS2 settings. These may or may not make a 271 difference on your card (many manuals call them "reserved"), but are 272 used to change the delays used when powering up a computer on the 273 network. This is only necessary when wiring VERY long range ARCnet 274 networks, on the order of 4km or so; in any case, the only real 275 requirement here is that all cards on the network with ETS1 and ETS2 276 jumpers have them in the same position. Chris Hindy <chrish@io.org> 277 sent in a chart with actual values for this: 278 279 ======= ======= =============== ==================== 280 ET1 ET2 Response Time Reconfiguration Time 281 ======= ======= =============== ==================== 282 open open 74.7us 840us 283 open closed 283.4us 1680us 284 closed open 561.8us 1680us 285 closed closed 1118.6us 1680us 286 ======= ======= =============== ==================== 287 288 Make sure you set ETS1 and ETS2 to the SAME VALUE for all cards on your 289 network. 290 291LED Indicators 292============== 293 294Many cards have red and green LEDs, which have the following meanings: 295 296 =============== =============== ===================================== 297 Green Red Status 298 =============== =============== ===================================== 299 OFF OFF Power off 300 OFF Short flashes Cabling problems (broken cable or not 301 terminated) 302 OFF (short) ON Card init 303 ON ON Normal state - everything OK, nothing 304 happens 305 ON Long flashes Data transfer 306 ON OFF Never happens (maybe when wrong ID) 307 =============== =============== ===================================== 308