TECHNICAL NOTE 7-1 ARCNET Cable Length vs. Number of Nodes for the HYC9088 in Coaxial Bus Topology By: Daniel Kilcourse May 1991 Page 1
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Introduction The original ARCNET star topology networks, those which make use of the LAND (Local Area Network Driver - HYC9068) module, are straightforward with respect to cable lengths and the number of nodes. Simply stated, the user is allowed a maximum distance of 2000 feet between any two devices containing a LAND module. Permissible distances become somewhat of a more complicated issue when dealing with the bus topology, made possible by the HIT (High Impedance Transceiver - HYC9088) module and worse when combining the two types of modules on a single cable. This technical note addresses the coaxial cable length vs. number of nodes issue for HIT only topologies, as well as HIT/LAND topologies. Twisted Pair issues do not fall within the scope of this technical note. Device/Module Identification Table 1 lists SMSC ARCNET products and indicates the type of module used in each product. DEVICE PC110 PS110 Active Hub-01 Active Hub-Opt PC210 Active Link Active Link-Opt Active Link-Opt 2 Twisted Pair Link Table 1 MODULE LAND LAND LAND (all ports) LAND (coax ports only) HIT HIT (both ports) HIT Only Topologies SMSC has specified that eight nodes may be connected to 1000 feet of RG62 coaxial cable. In fact, more nodes may be connected if the cable is shorter and fewer nodes are allowed if the cable is longer. A node, for the purposes of this discussion, is defined as any device containing a HIT and is connected to the cable. A HIT will output a signal 9 db over and above that required to drive its own receiver and insure proper signal to noise ratio for the remaining nodes. Each additional node has an insertion loss of 0.43 db and RG 62 cable has an attenuation of 0.6 db per 100 feet at 5 MHz (5 MHz is the frequency generated when transmitting a logical "one"). Based on this information, a chart may be made to show the maximum cable length for a given number of nodes. Refer to Table 2. Due to transmission line effects, SMSC does not recommend using more than 19 nodes on a single cable. Additionally, SMSC does not recommend the use of passive hubs with any HIT-type product. Page 3
Table 2 - Cable Length vs. Number of Nodes NUMBER OF NODES INSERTION LOSS REMAINING POWER MAXIMUM CABLE LENGTH 2 0.43 db 8.57 db 1428 feet 3 0.86 db 8.14 db 1356 feet 4 1.29 db 7.71 db 1285 feet 5 1.72 db 7.28 db 1213 feet 6 2.15 db 6.85 db 1141 feet 7 2.58 db 6.42 db 1070 feet 8 3.01 db 5.99 db 998 feet 9 3.44 db 5.56 db 926 feet 10 3.87 db 5.13 db 855 feet 11 4.30 db 4.70 db 783 feet 12 4.73 db 4.27 db 711 feet 13 5.16 db 3.84 db 640 feet 14 5.59 db 3.41 db 568 feet 15 6.02 db 2.98 db 496 feet 16 6.45 db 2.55 db 425 feet 17 6.88 db 2.12 db 353 feet 18 7.31 db 1.69 db 281 feet 19 7.74 db 1.26 db 210 feet HIT/LAND Bus Topology Although not recommended, the HIT and LAND modules may be mixed on a single cable. If you choose to mix the two modules, the following rules must be observed: 1. The cable must be RG62 type cable. 2. LAND modules may be connected ONLY at the ends of the cable. 3. Since the LAND provides the proper termination impedance, the terminator typically connected to the end of the cable in the bus topology must NOT be connected. 4. Use Table 2 to determine the maximum cable lengths for the number of desired nodes. In a mixed HIT/LAND environment, do NOT count devices containing LAND modules as nodes. 5. A device containing a LAND module must ALWAYS have power applied. If power is lost, the cable will no longer be terminated properly and may result in a non-functioning network. Once, again, the HIT/LAND Bus topology is NOT recommended but is presented above for those who find it necessary to use this topology in their application. Questions and Answers 1. Can a HIT-type device be connected to an existing star configuration which has 2000 feet of cable and a LAND-type device at each end? A HIT device presents an additional.43 db loss to the network. This is equivalent to approximately 72 feet of RG62 cable. A HIT device may be connected to a cable 1928 feet long (2000 ft. - 72 ft.) which has a LAND device on each end providing the distance between the HIT device and the furthest node is no greater than 1500 feet. Page 4
2. With respect to termination characteristics, how does the HYC9068 (LAND) differ from an HYC9088 (HIT) which is terminated externally with a 93Ω resistor? The HYC9068 (LAND) has a 93Ω impedance only when power is applied to the device. An HYC9068 must have power applied for it to function properly as a terminator. The HYC9088 (HIT) maintains a high impedance whether power is applied or not. This allows a HIT to be placed anywhere on the cable. When a HIT and a 93Ω resistor are connected to the end of a cable, it is the resistor which provides the actual termination. To summarize, with power applied, there is no difference between a LAND and a HIT with a 93Ω resistor. With power removed, a LAND will lose its terminating property while the HIT with a 93Ω resistor will maintain its terminating property. 3. Can a HIT be connected to a Passive Hub? No. Due to their heavy load and short cable length restrictions, the use of Passive Hubs with HITS is not allowed. 4. Can a HIT be used to directly drive Twisted Pair cable? Yes. The HYC9088 may directly drive either coaxial or twisted pair cable in a Bus topology. 5. Is RG62 cable required when connecting only HIT-type ARCNET products on a common cable? Other types of coax cable may be used to connect HIT-type ARCNET products providing the cable is terminated with a resistor of a value equal to the characteristic impedance of the cable. Keep in mind that the attenuation of other types of cable may be different from that of RG62 and, therefore, affect the maximum cable length for a given number of nodes. The frequency of concern is 5 MHz. RG62 will work with both HIT- and LAND-type devices while other type cabling is acceptable for the HIT but not the LAND. If your network topology is expected to change, and they do change from time to time, you should not consider cable other than RG62. 6. If given the choice of purchasing a LAND-type product or a HIT-type product, which should I buy? HIT-type products offer more flexibility. They can be connected to any point on the cable, providing they are at least six feet apart, with power applied or removed. LAND-type products may only be connected to the ends of the cable. The cable must be RG62 and power must always be applied. When in doubt, purchase the HIT-type product. Conclusion ARCNET maintains a rapid growth rate in the network environment. The HIT module, allowing a more flexible network architecture, is a primary reason for this rapid growth. Offering products which address Twisted Pair, the migration towards CMOS and higher levels of integration also contribute strongly to the wide acceptance of ARCNET. Standard Microsystems Corporation is committed to the ARCNET product line and will continue to offer reliable, state-of-the-art products. Page 5