IEEE T4an6action6 on Nucteaw Science, VoL.NS-25, No.l, Febwaxy 1978

THE MEASUREMENT OF SERIAL DATA TRANSMISSION CHARACTERISTICSOF CABLES FOR CAMAC SERIAL SYSTEMS

P. N. Clout and C. G. Ratcliffe

Daresbuxy Laboratory, Science Research Council, Daresbury,Warrington WA4 4AD, Cheshire, U.K.

When implementing CAMAC serial systems the choiceof-cable used is governed by both the speed requiredand the length between each transmitter and receiver.We find that the manufacturer's specifications do not,in general, give attenuation figures for balanced modeoperation and in single-ended mode only give an attenu-ation figure for one frequency. Our requirement was toestablish the information that enables us to selectfrom a small range of cables the particular cable for agiven job. At one extreme is the maximum speed of5 MHz and the maximum point-to-point distance at ourLaboratory, 600 metres. At the other extreme is thecheapest twisted-pair wire available.

with a UK cable manufacturer has enabled us to specifyand order a multi-pair cable which will successfullyoperate at 5 MHz bit rate over 600 metres, the longestpoint-to-point distance on site. Another cable whichhas to be measured is one which can be connected to aCannon connector, whilst at the extreme end, the cheap-est twisted pair cable available has been tested. Thehighest performance cable, although quite inexpensive(about $1 a metre) is only suitable for installedcables, smaller cable is needed between junction boxesand the CAMAC system.

The Attenuation Measurements

The attenuation characteristics of some twistedpair cables have been measured, and a large cable manu-facturer has supplied a 600 metre length designed tooperate successfully at 5 MHz. The cost of this cableis less than $1 per metre. Further cables were supp-lied by the same manufacturer which were both flexibleand of a size which could just terminate in the appro-priate standard connector. The results of data trans-mission tests on these cables for a CAMAC serial datalink will be presented. The data link uses the samedrivers and receivers as the serial highway but beingasynchronous is, if anything, more error prone.

Introduction

When implementing CAMAC serial systems1 the choiceof cable used is governed by both the speed requiredand the length between each transmitter and receiver.The data given by manufacturers (at least in the UK) isinadequate to make this choice as it is usually a meas-urement of attenuation at one frequency in single-endedmode, that is with one wire of a pair earthed. What isrequired is attenuation figures at a range of frequenc-ies up to 10 MHz so that cables for a particular systemcan be chosen from a small range stocked. Cooperation

The voltage attenuation measurements were made onthe cable samples using standard techniques. In orderto obtain some idea of the usefulness of manufacturerssingle-ended attenuation data, one cable was measuredwhen driven single-ended and in balanced mode.

The results of the measurements for attenuationare shown in fig. 1. The definition of the cables isgiven in table 1. Cable A, the cheapest twisted paircable obtainable, was measured in both modes, and itwill be seen that the measurements are vezy similar atlower frequencies but balanced mode operation has abetter attenuation at higher frequencies. The otherinteresting aspect of these measurements is that cableD has better high frequency attenuation than cable C,whilst at lower frequencies the attenuation is similar.

Cable D was an attempt by BICC to improve on thehigh frequency performance of cable C by using thickerconductors. This improvement was not as great as ex-pected because the inter-pair capacitance was found tobe higher than calculated.

'able 1

Definition of cables tested

tBICC - BICC General Cables Limited, Helsby, Warrington WA6 ODJ, U.K.*Smaller wire used for centre pairs, larger wire for outer pairs.

0018-9499/78/0200-0520$00.75 (©) 1978 IEEE520

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A Balanced >>

10-

20-58 30-

0

40-

50

60

70-

2 3 4 5 6 7 8 9 10

Frequency (MHz)

Fig. 1 Attenuation as a function of frequencyfor the six cables tested.

Maximum Data Transmission Rate Versus CableLength Determination

In order to translate these measurements to cablelength - maximum information transmission rate graphs,all the line characteristics and the frequency spectrumof the signal have to be taken into account. In asearch of the literature no help was found and so itwas decided to carry out some further measurements. Tomake these measurements, a small computer system withCAMAC was used. Two CAMAC serial data link modules(Sension Limited Type 0376) were used to transmit datadown various lengths of the cable. Some lengths wereobtained by looping back pairs in a multi-pair cable.These serial data links use drivers and receivers tothe same specification as CAMAC serial crate control-lers type L-2 and CAMAC serial highway drivers but workasynchronously. From the mode of asynchronous opera-tion one would expect either similar or worse perform-ance than the serial highway. The data link moduleswere modified so that a continuously variable externalclock could be used.

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1 2 3 4 5 6 7 8 9 10

Serial bit frequency (MHz)

Fig. 2 Maximum Zength for error free operationpZotted against frequency for the sixcables tested.

Error free operation was assumed if every numberfrom 0 to 65535 was sent ten times down the line with-out error. The program increased the data sent tenfold if any error was detected and then printed out thepattern sent, the pattern received and the percentageerror. The results are shown in fig. 2. It will beseen that cable D has a very similar performance tocable C with a slight improvement at higher bit rates.No account has been taken in this work of externallyinduced noise.

Conclusion

The main conclusion of the work so far is that themanufacturer's data is not adequate for the selectionof cables for use with balanced drivers such as areused with the CAMAC serial highway.

References

1. CAMAC Serial Highway System and Serial CrateController, EUR 6100e, Office for OfficialPublications of the European Communities, PO Box1003, Luxembourg and also IEEE Std. 595, IEEE,345 E47 Street, New York, NY 10017.

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