INTRODUCTION

Twisted-pair telephone wire appears to be an idealmedia for local area network (LAN) cabling. It isinexpensive, simple to terminate and, mostimportantly, frequently found in abundance in mostcommercial, industrial and institutional buildings.There would be no need to run additional cabling fora new LAN system if spare telephone pairs could beused.

The problem with twisted-pair cabling is its poorperformance compared to coaxial cable which isusually the specified media for high-speed LANs.This situation is no different with ARCNET®.ARCNET systems originally specified RG-62/ucoaxial cable for interconnecting the variousnetwork interface modules (NIMs) and hubs withinthe network. However, the popularity of twisted-paircabling demanded workable solutions for using thismedia with ARCNET; and, indeed, there aresolutions.

The Use of BALUNs

BALUNs, acronym for BALance-UNbalance, canbe used to link coaxial cabling with twisted-paircabling. For example, a television antenna systemusually consists of a dipole antenna connected to300 ohm “twin lead.” Twin lead has an impedanceof 300 ohms between conductors, but the impedancefrom any one conductor to ground is the same or“balanced.” On the other hand, coaxial cable canhave an impedance from 0 to 95 ohms between thecenter conductor and the outer braid; however, theimpedance to ground from either the centerconductor and the other braid is different or“unbalanced.” To couple these two media togetherrequires a transformer which matches the differingimpedances by way of the turns ratio of thetransformer, while adapting to the balance orunbalance characteristics of the media. Thistransformer, along with any passive filtercomponents, is termed a BALUN and is used intelevision sets as well as local area networks.

APPLICATION NOTELong-Haul Twisted-Pair Interface

Networking and Control Systems for Industry

AN-202

Conventional ARCNET Twisted-PairCabling

As mentioned before, ARCNET originally specifiedRG-62/u coaxial cable for the media; and, therefore,the compatible transceiver used on ARCNET NIMswas unbalanced. In order to drive twisted-paircabling, BALUNs have to be used—one located ateach end of a point-to-point segment. AtContemporary Controls (CC), we built-in theBALUN circuitry onto the network interface moduleand hub expansion modules and label the transceiveras a -TPS for twisted-pair star. The connectors areRJ-11 style consistent with twisted-pair wiringpractice. The distance limitation, however, is 100meters or 330 feet which is far shorter than the 2000feet that can be achieved with coaxial cabling. Toextend the 100 meter distance requires a differentapproach.

Extending Twisted-Pair Distances

One of the main problems with twisted-pairtelephone wiring is that it was not intended to carryhigh-speed digital signals. Cable attenuation issignificant at higher frequencies, and there is noconvenient way to compensate for this problemexcept to reduce ARCNET’s transmission rate of 2.5MB/sec. However, to do this would defeatARCNET’s compatibility with existing equipment.If a general purpose long-haul twisted-pair design isto be implemented, it needs to operate at standardARCNET speed.

If we could reduce the data rate, we would increasethe cable distance. CC has developed a novelapproach to increasing twisted-pair cablingdistances to 4000 feet, with only the cost ofincreasing the twisted-pair requirements from onepair to four.

MXP-4TP EXPANSION MODULE

The MXP-4TP is a double-wide expansion module(occupies two slots) that is installed into either aMHP-S or MHP-L MOD HUBplus Modular ActiveHub. The MXP- 4TP accepts the backplane signalsfrom the MOD HUBplus and generates four “mini-ARCNET” signals, each transmitting at a 625 Khzrate (¼ the rate of ARCNET). Each twisted-paircontains information from the original 2.5 MB/sARCNET signal. The four signals are recombined atthe other end of the twisted-pairs by anotherMXP-4TP which logically “ORs” the signals,thereby reconstructing the original ARCNET signalwithout diminishing the effective throughput ofARCNET.

The MXP-4TP contains two modules attached in apiggyback fashion. One module is called thetransmitter and the other the receiver. In a modularhub, such as the MOD HUBplus, ARCNET signalssignifying a logic “1” are present on the backplane ofthe hub as pulses P1 and P2. Both of these signalsare 100 ns wide with P2 delayed from P1 by 100 ns.A logic “0” does not generate either a P1 or P2. Thetransmitter of the MXP-4TP receives the P1 signaland generates an ARCNET dipulse, but at ¼ thespeed, onto one of the twisted-pairs. When the nextP1 is received, a second dipulse is generated downthe second twisted-pair. This sequence continues in a“Gatling gun” fashion (four barrels firing insuccession) every time a P1 is received. If a logic “0”occurs, there is no P1 activity; and the Gatling gunremains cocked at the next barrel position awaiting tobe fired. To implement these mini-ARCNET signalsrequires four separate timing generators (using delaylines) and four separate transformer coupled dipulsegenerators.

Likewise, the receiver module requires four separatereceivers consisting of an adjustable gain amplifierand a matched filter tuned to the low-speedARCNET signal. The gain of the amplifiercompensates for the varying distances encountered inan installation and must be set once. The four mini-ARCNET signals are received and logically ORedtogether in order to reconstruct the originalARCNET signal. This recombined signal is thenapplied back onto the MOD HUBplus backplane forretransmission to other ARCNET nodes.

In theory, everything should work; however, twisted-pair cabling introduces several problems due to itspoor high-speed performance. Significant bit jitter isexperienced during the encoding/decoding processthat propagates through conventional hubs resultingin unreliable operation by accumulating the jitter.

What is needed is a “reclocking hub” thatsynthesizes a new ARCNET signal, without bit jitter,from the jittery signal sent via the twisted-pair cable.The MOD HUBplus enclosure contains thenecessary reclocking timing generator at the expenseof more sophisticated electronics.

MOD HUBplus

The MOD HUBplus active hub was designed toaccommodate extended range ARCNET applicationsover non- conventional media which, by their nature,generate excessive bit jitter for conventional hubs.Conventional hubs simply reproduce the signalreceived on one of its ports to all remaining ports.The MOD HUBplus incorporates anti-jitter circuitrywhich captures the incoming signal in a short first in-first out (FIFO) register and retransmits the datausing a jitter-free clock. In this way, induced bit jitteris eliminated at each hub preventing its accumulationwithin the network, therefore, making specializedcabling applications possible. The downside to thisapproach is that hub delay in reclocking hubs is morethan conventional hubs. CC specifies a MODHUBplus hub to have a 1200 ns worst case delayversus 250 ns for a standard MOD HUB. Thisincrease is usually insignificant but should beconsidered in networks approaching 20,000 feet inend-to-end network distances. Another feature of theMOD HUBplus is that it has built-in two coaxial star(-CXS) ports which can accept one or two NIMconnections or one to two cascade hub connections.MOD HUBplus enclosures are available in two sizes:a four slot MHP-S and a 12 slot MHP-L. Rememberthat each MXP-4TP requires 2 slots and, therefore, aMHP-S can accommodate up to two MXP-4TPswhile the MHP-L can handle up to six.

TOPOLOGY

To connect two NIMs with long-haul twisted-paircabling requires two MOD HUBplus enclosures eachwith one MXP- 4TP installed. Connect each NIMwith a short length of coaxial cable to one of the twoports on the MOD HUBplus. Then connect twisted-pairs between each MXP- 4TP. If the NIMs havecoaxial bus transceivers (-CXB), then connect theMOD HUBplus to one end of the bus segmentwithout a tee connector and terminator.

MXP-4TPs must be wired in either a star ordistributed star configuration. No bus connectionsare allowed on the twisted-pair segment. Forexample, to connect four distant NIMs using twisted-pair cabling in a star configuration use a MHP-S andMXP- 4TP near each NIM. Connect the NIM to theMHP-S with coaxial cable. In the center, use aMHP-L and four MXP-4TPs. Connect the twisted-

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pairs between a MXP- 4TP near the NIM to one ofthe MXP-4TPs located in the center. If the remainderof the network uses conventional coaxial cabletechnology, it is unnecessary to use more MODHUBplus hubs. Simply use conventional MODHUBs and a coaxial cable connection to a coaxial-star port on either the MHP-S or MHP-L.

Cabling

We specify the use of two types of cable. Either useBelden's #9566 or AT&T's #AK-186BKMA-100.Other cables will work, but it is difficult to test allvarieties. The common situation is that the cablealready exists and the building owner wants sparetelephone pairs to be used. Follow these rules whenspecifying cable:

— Segment length cannot exceed 4000 feet.

— Telephone pairs are to be in the same bundle.

— No stubs allowed. Connections must be point-to-point since unused stubs cause reflections.Disconnect unused stubs.

Wiring

The connection rules are simple. Correspondinglyconnect each wire of each pair to the same point oneach MXP-4TP. The field connector is removable.Insert a small screwdriver into the side of theconnector, in order to open the jaws of the connector,and simply push the stripped wire into the frontopening. Removal of the screwdriver releases thejaws onto the wire. Connections are marked as pairs1 through 4 with a (+) and (–) sign. Do not invert thesignals. The (+) connection on any one pair must bemade to the (+) connection at the other end. Wire tothe DATA side of the connector.

NOTE: The system will work even if two pairs wereswapped at any one side, as long as no phaseinversion occurred. However, for maintenance sake,we recommend that cable pairs be connected tocorresponding terminals.

All four cable pairs must be made. A loss of onecable will render the system useless.

Distance Setting

With the power off, remove the MXP-4TP. It isnecessary to know the approximate distance of therun in order to set the amplifier gains. There are fourbanks of four-position DIP switches. Each switchbank MUST be set the same as the others on bothsides of a segment. Table 1 has been provided forconvenience referencing the distance of the segment.

Pick the entry equal to, or lower than, the estimatedlength of the segment. For example, if the segment isestimated to be 2100 feet long, pick the entry under2000 feet (OCCO).

With the MXP-4TP removed and orientated with theface plate on your right, look at the row of DIPswitch banks. The word OPEN appears on top of theswitch (although upside down) and the numbers 4through 1 are on the bottom (although upside down).The chart is organized so that switch settingscorrespond to the numbers and the location of theswitch positions on the DIP switch bank. In ourexample, we want to set the range to 2000 feet(OCCO). From the chart, 3 and 2 are to be closedand 4 and 1 opened. Using a pointed object, pushrocker switches 4 and 1 at the OPEN side in order toopen the switches. Do the opposite for 3 and 2 sincethey are to be in the closed position. Do the same forall four switch banks.

If you are not sure of the actual distance, estimate thebest you can. The switch settings are very broad atthe longer distances and capable of spanning a widerange. (At the end of this application note, wediscuss the laboratory results of a test conducted todetermine the sensitivity of gain setting for variouslengths of cable.) Once the switches are set, insertthe module and power-up the enclosure. Make sureyou set the switches the same on the MXP-4TPlocated at the other end of your segment.

DIP Switch Bank Settings

Distance (feet) 4 3 2 1300 O O O O600 C O O O900 O C O O

1200 C C O O1400 O O C O1700 C O C O2000 O C C O2300 C C C O2500 O O O C2800 C O O C3100 O C O C3400 C C O C3600 O O C C3900 C O C C

*4200 O C C C*4500 C C C C

Notes: O = open C = close. Select the setting thatis equal to or just below the estimatedsegment distance.

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* Although the MXP-4TP is specified tooperate up to 4000 feet, greater distanceshave been achieved with good results butwith reduced margins.“

Table 1—DIP Switch Bank Settings

Indicator Lights

There are indicator lights present on the timingmodule and MXP-4TP that can be quite helpful tothe installer. The timing module indicators, when lit,are defined as follows:

RECON—A network reconfiguration is taking place.(Note: The RECON detection is only valid if theTIMING light is on.)

+PWR—The +5 volt power supply within the hub isfunctioning properly.

–PWR—The –5 volt power supply within the hub isfunctioning properly.

TIMING—A valid ARCNET signal is present fromeither one of the two coaxial ports or from thetwisted-pair cable; and the internal timing circuitrywithin the hub is properly regenerating the signal.

In addition to the above indicators, the timingmodule has two unmarked indicators adjacent to theBNC connectors. These are port activity indicatorsthat light when the port is “receiving” data from adevice connected to the port.

In addition to the timing module, the MXP- 4TP hasfour LED indicators—one for each twisted-paircable. The uppermost light corresponds to DATApair 1.

With no cables connected to either of the coaxialports or twisted-pair ports, the RECON light shouldbe on as well as the +PWR and –PWR lights. TheTIMING light should be off. Connect a validARCNET signal to one of the coaxial ports, and itscorresponding activity light should come on as wellas the TIMING light. Notice that the MXP-4TPlights remain out. This is because these lightsindicate received data and not transmit data. TheMXP-4TP is, however, transmitting at this time.Connect the twisted-pair cable to the MXP-4TP. If avalid ARCNET signal exists on the other end of thecable, the two ARCNET nodes should link up,thereby extinguishing the RECON light. The fourLEDs on the MXP-4TP should be solidly lit. This isthe proper state of the LED indicators.

If one of the MXP-4TP lights is out, that pair is notcommunicating. If the lights flicker, the gain settingmay be improper. First, check your wiring; second,attempt setting the gain to a different value.

Piggy-Backing Onto Telephone Circuits

When no twisted-pair cables are available, we couldshare existing telephone circuits if a private branchexchange (PBX) is available. Telephones function ina star topology with individual telephone circuitsattached to a central switch. As part of our optimizedtwisted-pair transceiver, a crossover network isimplemented with a high-pass filter for ARCNETsignals and low-pass filter for the voice circuit andring signal. Using this approach, no additionaltwisted-pair circuits are needed beyond the onesalready dedicated for telephone use.

The sharing of telephone circuits with ARCNET isnot normally recommended due to theunpredictability of the installation and the potentialfor objectional levels of cross-talk that may occurwhen four telephone circuits pass through the MXP-4TP. If spare twisted-pairs are available, werecommend dedicating them to ARCNETcommunications only. If there are simply no unusedpairs available, use active telephone circuits insteadby breaking the connection between the telephoneand the PBX in two places and by installing a MHPwith MXP-4TP at each of the two locations. Towardsthe PBX side, connections from the PBX are made toa PHONE pair marked (+) and (–) on one of theMXP-4TPs. At the telephone end, make the sameconnection to the corresponding pair marked (+) and(–) without signal inversion to the other MXP-4TP.Between the two MXP-4TPs, connect their DATApairs together to corresponding terminals. In thiscase, it is important to make correspondingconnections. Complete the installation by setting thegain switches as previously discussed.

Using Phone Connections forCheckout

If piggybacking onto phone circuits is not needed, itis still possible to utilize the PHONE connections toassist in checkout. If two inexpensive wire-basedintercoms are available, connect one set to a PHONEpair on one MXP-4TP and connect the other tointercom to the corresponding PHONE pair on theother MXP-4TP. Connect the two MXP-4TPstogether using the DATA connections as previouslydiscussed, and two installers can communicate withone another while ARCNET traffic is being passed.

This could be helpful when “buzzing out” thecircuits, except the intercom is the buzzer. Move theintercom connections down each pair on both sidesto test for wiring integrity communicating to oneanother before moving connections. This could speedinstallation and checkout.

LABORATORY RESULTS

CC performed a test in its laboratory in order toascertain the sensitivity of gain settings with respectto the actual distances involved. From a large reel ofAT&T's #AK-186BKMA-100 cable, segments werecut in 500, 500, 500, 500 and 2750 foot lengths.Using a patch panel, overall lengths of 500, 1000,1500, 2000, 2750, 3250, 3750, 4250 and 4750 feetcould be selected. At each distance, the gain dip

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switches were changed until an ARCNET confidencetest would fail due to either too high or too low of asetting. The test was repeated for increasing lengthsof cable and the results plotted. For example, at 2000feet a gain setting from 900 to 2000 feet wouldfunction correctly while all other settings would not.Use Table 2 to determine actual switch settings. Itwas determined that gain setting is much morecritical at the lower distances than at the higherdistances. Since most applications involve distancesgreater than 1000 feet, gain setting accuracy becomesless of an issue. Use the accompanying chart as aguide, but remember that this was developed usingAT&T cable. Results should be similar for othercables; however, the chart may become skewed ifattenuation varies from that of the AT&T cable.

PT4-PXMYTIVITISNESGNITTESHCTIWSNIAG

TEEFNIECNATSID

0 005 0001 0051 0002 0572 0523 0573 0524 0574

003

006

009

0021

0041

0071

0002

0032

0052

0082

0013

0043

0063

0093

0024

0054

D

I

P

S

W

I

T

C

H

S

E

T

T

I

N

G

S

SHADED AREAS INDICATE OPERATING RANGE OF EACHDIP SWITCH SETTING FOR A GIVEN DISTANCE

Table 2—Gain Switch Setting Sensitivity

Contemporary Control Systems, Inc.2431 Curtiss Street l Downers Grove, Illinois 60515 l USATEL +1-630-963-7070 FAX +1-630-963-0109 E-MAIL info@ccontrols.comWWW http://www.ccontrols.com

ARCNET is a registered trademark of Datapoint Corporation. ARCControl and Contemporary Controls are registered trademarks ofContemporary Control Systems, Inc. Other product names may betrademarks or registered trademarks of their respective companies.Specifications are subject to change without notice. © CopyrightContemporary Control Systems, Inc. October 2002

CC also learned additional information when makingthe test. An attempt was made to use two cable pairsfrom one cable and another two cable pairs fromanother cable. Both cables were thought to be 500feet in length. Unreliable performance wasexperienced regardless of gain setting. CC speculatedthat additional bit jitter was introduced due to thefact that the cable lengths may not have been equallymatched. The MXP-4TP relies upon the fact thatsignals sent down each of the four cable pairs can berecombined at the other end without any additionalphase shift or time delay. Unequal delays in the fourcable pairs result in unequal spacing of logic “1”signals which results in data errors.

Customers have also reported to CC that if any stubexists in the cabling run, unreliable operation willoccur. Frequently, it is difficult to determine if a stubexists. Make sure that all punch down panels areexamined for stubs and disconnect them accordingly.Also, examine the punch down connectionsthemselves or reterminate the connections.Connections result in slight impedance mismatchesthat can effect the high speed data being sent.

It has been our experience that once a MXP-4TP isproperly installed, the link has proven highlyreliable.

The MXP-4TP card makes it possible to connectARCNET segments from 300 feet up to 4000 feet.