A low-cost time-of-event recorder

N U C L E A R I N S T R U M E N T S AND METHODS

I26 (I975) 5 9 1 - 5 9 3 ;

A LOW-COST

TIME-OF-EVENT

© N O R T H - H O L L A N D P U B L I S H I N G CO.

RECORDER

COLM T. O'SULLIVAN, MICHAEL C. SPILLANE and PATRICK J. TWOMEY

Department of Physics, University College, Cork, Ireland Received 15 April 1975 A system is described which enables the Universal Time of occurrance of certain events to be recorded. Hourly synchronization of the system by means o f standard radio VLF timing signals eliminates the need for expensive clocks.

In certain astronomical, astrophysical, and geophysical experiments an accurate knowledge of the time of occurrence of particular incidents is required. The instrument to be described here was designed and built for the purpose of recording, to an accuracy of 1 ms, the Universal Time at which certain cosmic-ray events occur. The system is initialized once an hour using a pulse derived from the 60 kHz signal broadcast by the U . K . National Physical Laboratories' MSF transmitter at Rugby, England. The accuracy required

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The hour-pulse generator The receiver used was a war-surplus National H.R.O. with a plug-in coil set covering 6tY'kHz. The B.F.O. on the receiver was adjusted to give a beat frequency of 6 kHz. The normal signal from MSF, Rugby is interrupted in the six seconds preceding each hour for station identification. The station call sign

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can thus be achieved using a clock with a drift of less than three parts in l0 T.

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Fig. 1. The circuit for the generation of the hour pulse comprising two 74123 ICs (dual retriggerable monostable multivibrators). The width of the output pulse (tw), and hence the retriggering time, of each monostable is chosen to give the outputs indicated in the timing diagram.

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.Time of Event Recorder

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A LOW-COST TIME-OF-EVENT RECORDER

(MSF) in morse is transmitted twice by positive keying of the carrier. Fig. 1" illustrates schematically how the higher frequency of the carrier interruptions during this interval is detected. The signal at the secondary of the output transformer of the receiver is passed, via a diode, to provide positive half cycles of 3 V amplitude, to the first of the series of four retriggerable monostable multivibrators. An output (HOUR) from the final stage will appear on the first interruption in the carrier after 59 rain 59.8 s, of every hour, that is, at the epoch of each Universal Time hour. The timing accuracy is limited by the decay time of the carrier interruption, which is 0.5 ms for decay from 90% to 10% of the original intensity'). Correction must of course be made for the distance of the system from the transmitter.

The recording system Fig. 2* is a schematic diagram of the time-of-event recorder. The H O U R pulse, generated as described above, is fed to the START input which sets the counters (18-23) and the memory address register (M.A.R.) to zero, puts the control flip-flop (43) in the read-in state, and enables the EVENT and C L O C K inputs. The appearance of an event pulse causes the contents of the counters to be loaded into the memory after which the M.A.R. is incremented and the counters reset to zero. Thus the time of occurrence of the first event after the hour and the time between each subsequent event are recorded in memory (to within 1 ms). At approximately three seconds before each hour a pulse is received at the G A T E input from an external gate generator which has been started by the H O U R * Detailed circuit diagrams available on request from the authors.

593

pulse. The appearance of the G A T E pulse causes the number of events recorded in memory to be latched to the B-inputs of the comparator (45), puts the system, via flip-flop (43), in the read-out mode, disables the C L O C K and EVENT inputs, and finally resets the M.A.R. to zero. The contents of each memory location are read out through the parallel-to-serial converters (36-39) and punched out on a Data Dynamics 1132 paper-tape punch in ASCII code. The comparator (45) ensures that only those events recorded in the previous hour are punched out. After the time corresponding to each event has been punched on the tape they are followed by the characters for carriage return ( " C / R " ) and line feed ("L/F"). After the last character of the last event has been punched the A > B output of the comparator (45) is used to generate the ASCII character '° ?" to indicate the end of data. The system then awaits the arrival of the next H O U R pulse to initiate another cycle. The cost of the whole system, excluding the papertape punch, was less than £120. The authors wish to thank Mr Jim Fleming, formerly of the department of Electrical Engineering, U . C . C . , for his help in the design of the recording system. We also wish to thank the staff of the National Physical Laboratories at Teddington, Middlesex, England, for making all the relevant information available to us. Our thanks are also due to Dr J. J. Lennon, Department of Physics, U.C.C. for many very helpful discussions. Reference 1) U.K. National Physical Laboratory, Teddington, Middlesex, Standard frequency and time signal transmissions for M S F RUGBY.