Gustafsson, B

Gutíerrez, P

Marchet, F B

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THE VLT TIME REFERENCE SYSTEM: A MICROSECOND-ACCURATETIME/SYNCHRONIZATION BUS FOR DISTRIBUTED CONTROLSYSTEMSF. Biancat Marchet, B. Gustaffson, ESO, Garching B. Muenchen, GermanyP. Gutierrez, ESO Paranal, ChileAbstractThe demanding timing and synchronisationrequirements of the VLT (Very Large Telescope) array oftelescopes operating in interferometric mode, have led tothe development of a dedicated fibre-optic bus, the TimeBus, distributing accurate timing to more than 100 LocalControl Unit computers  (LCU) scattered on a wide area.The UTC  (Universal Time Coordinated ) is receivedfrom the Globalstar GPS (Global Positioning System)satellites, processed and distributed through the Time Busto the Local Control Computers with accuracy in therange of few microseconds. Means for processing,backing up and relay the time information as well as forinterfacing the Local Control Computers with the TimeBus, have been developed and integrated. Softwaredrivers have also been developed providing means tosynchronise processes running on different localcomputers to the absolute time from better than 10microsecond accuracy at interrupt level to schedulingprocesses, or sending messages with millisecondaccuracy. Tests have been performed to evaluate theperformances both at ESO  (European SouthernObservatory) headquarters and at the VLT observatory onCerro Paranal, Chile, where the system is now in regularoperation. The compliance with leap seconds, Y2K andGPS week rollover has been investigated.1  THE VLT OBSERVATORYThe VLT observatory, located on Cerro Paranal in theAtacama Desert, northern Chile, is composed by fourtelescopes equipped with 8.2m diameter main mirror. Thefour telescopes can be operated independently or in acombined way to reach a total collected power of a 16-mequivalent telescope.  In addition, with the help of three re-locatable 1.5-mauxiliary telescopes, the telescopes can be operated in aninterferometric mode to attain very high angularresolution imaging.At this time the first telescope, named Antu (after themapuche name of the sun) is under scientific operation,the second one (Kueyen, the moon) is beingcommissioned and the other two, Melipal (the SouthernCross) and Yepun (Sirius) are under test.All the four units are planned to be fully operationalshortly after the turn of the century.More than 100 Local Control Unit computers (LCU)implement a distributed real time control system: accuratesynchronisation among the LCU’s operating in a singletelescope as well as in different telescopes is a key issue.In addition, the absolute time information is needed inorder to perform astronomical observation.The Time Reference System (TRS) has been developedin order to supply each LCU and workstation with theabsolute time information and guarantee thesynchronisation among all the LCU's.2  THE TIME REFERENCE SYSTEMThe main purpose of the TRS is to supply a uniqueclock with suitable accuracy to all the LCU’s andworkstations throughout the VLT observatory.Since the LCU’s implement high-speed control loopswith real time constraints, the timing accuracy required isbetter than 10 microseconds.On the order hand, the workstations have high-levelsupervising/monitoring purposes and the accuracyrequired is in the range of 10 milliseconds.Two different means have been used to distribute thetime to workstations and LCU’s.The workstations can access a “time server” thatsupplies the time through the Local Area Network (LAN).The LCU’s, instead, are connected through a “TimeInterface Module”  (TIM) to a “Time Bus” (TB)dedicated fibre carrying high accuracy time information.Although astronomical coordinates are based on thestandard UT1, the standard Universal Time Coordinated(UTC) has been selected to represent the time informationin the VLT system.The difference between UT1 (based on the mean solarday) and UTC (based on a fixed duration of second) isalways kept less than 1 second, and as soon as itapproaches the second, a unit, called leap second, is addedor subtracted from UTC.The difference between the two, as well as notice ofleap second occurrence, is supplied in advance by theInternational Earth Rotation Society (IERS).The VLT control system needs this information inorder to compute UT1 starting from UTC.It is composed by the following sub-systems:• Central Time Standard• Time Server• Time Bus Encoder• Time Bus Distribution Boxes• Time Interface ModulesInternational Conference on Accelerator and Large Experimental Physics Control Systems205International Conference  Accel rator and L rge Experim ntal Physics Control Systems, 1999, Trieste, ItalyThe architecture of the TRS is shown in fig. 1Figure 1: TRS architecture2.1 Central Time StandardThe Central Time Standard (CTS) retrieves the UTCtime from the GPS satellites.This information is used to discipline a local high-stability rubidium clock.The rubidium clock acts as a backup in case of failureof the GPS, which, although highly reliable, is not underESO control. The rubidium clock guarantees at least onenight at full time accuracy. The synchronisation betweenthe computers, which is of main concern for the operationof the observatory, is kept indefinitely.The CTS is a commercial unit manufactured by DatumInc., type ExacTime GPS Time Code and FrequencyGenerator (part number 9390-6000) [1].2.2 Time serverThe Time Server (TS) distributes the time informationthrough the LAN.It gets the information from the CTS as an IRIGB(Inter Range Instrumentation Group) coded signal andmakes it available to all the workstations according to theNetwork Time Protocol (NTP) [2].Due to the network delays, the timing accuracy is inthe order of 10 milliseconds.The Time Server is also made by Datum Inc., typetymServe 2100 [3].2.3 Time Bus EncoderThe time output from the CTS is also fed to the TimeBus Encoder (TBE). It converts the time into the time bussignal: a PWM modulated 1MHz signal.This has several advantages against the traditionalIRIGB code, from which it has been derived.The IRIGB is a 1KHz amplitude modulated code.Its low frequency does not allow reaching the requiredtiming accuracy of 10 microseconds.In addition, being analogue, it is sensitive toenvironmental noise and is not suitable for thetransmission through fibre optic.The Time Bus code is based on a high frequencycarrier on which a simple PLL (Phase Locked Loop) canlock with accuracy better than 1 microsecond.It is a digital signal, as consequence the transmissionthrough fibre is easy.The modulation scheme, represented in figure 2,consists in a low level PWM encoding in which thelogical one is represented by a pulse having duty cyclelarger than 50% (500ns) and the logical zero by a pulsehaving duty cycle smaller than 50%.The low-level encoding is used to generate the high-level 10ms bit stream (which has the same structure as thestandard IRIGB).In this bit stream a high-level “zero” bit is encoded as atrain of low level ones longer than 5ms (but shorter than8ms) followed by a train of low level zeroes up to 10 ms.A high-level “one” bit is encoded as a train of lowlevel ones shorter than 5ms followed by a train of lowlevel zeroes up to 10 ms. Figure 2 shows the encodingscheme.Figure 2: Encoding schemeAn additional symbol is encoded to mark the beginningof a message word, it is coded as an 8ms train of low levelones, followed by low level zeros up to 10ms.The fibre used for the distribution is a popular 62.5/125microns multi-mode fibre, the same type normally usedfor LAN’s.The time bus encoder is a custom device, made byLange elektronik GmbH.2.4 Time Bus Distribution BoxThe signal generated by the TB encoder is converted tooptical and then distributed by a tree made up of TimeBus Distribution Boxes (TBDB) connected through fibre.The time distribution box can accept an electrical oroptical input and delivers four or more optical outputs.Only the first TBDB, the one connected to the TBencoder, uses the electrical input, all the others use theoptical input.Each TBDB contains a receiver that converts theincoming signal to electrical and feeds one or moretransmitters, each having four optical outputs.The conversion from optical to electrical and back tooptical introduces both phase distortion and a widthNSTBE TBDB TBDBTBDBTBDBTIMCPULCUVMETIMCPULCUVMECTSLANTBTB (fibre) TB (fibre) TB (fibre)1 MHz duty cycle<50%             >50%High level “0”High level “1”mark10 ms (bit frame)206distortion in the signal. These two effects limit to four themaximum number of TBDB’s that can be cascaded.The timing error introduced by the TBDB is <100ns.The time bus distribution boxes are custom devicesproduced by ESO.2.5 Time Interface ModuleA VME card has been developed at ESO to interfaceeach LCU with the time bus, the Time Interface Module(TIM) [4].This card contains the logic needed to decode the timebus signal and let the CPU’s connected on the bus accessthe time information.Whenever the TB is not available, the TIMautomatically switches to a low-accuracy local oscillatorthat keeps the functionality of the TIM, although withdegraded accuracy.The card contains also six user-configurable timers,which can be programmed to generate frequency signalsor cyclic interrupts.Since the timers are clocked synchronously with thetime bus, timers on different LCU’s count synchronously.An additional "next 100 ms" interrupt can be generatedat the edge of the next 0.1 second of UTC time. Thisfeature helps synchronising to absolute time the tasksrunning in the LCU.A software driver has been developed under VxWorksreal-time operating system to give the user software fullaccess to the card.The timing error introduced by the TIM is<1microsecond.A new release of the card is under test, which has someadditional features:• Robust time code check to avoid noise in the timebus signal or sporadic CTS/GPS malfunctions affectthe time.• Alarm interrupt to generate an interrupt at a specifieddate/time• Battery backed up real time clock to keep (lowaccuracy) time information after power off, whendisconnected from TB3 TESTSTests have been performed for several years both atESO headquarters in Europe and at the VLT site in Chile,to qualify all the components of the TRS.Particular conditions like the year 2000, leap yearshandling and the GPS week rollover have been checkedeither by simulation or at real occurrence.4  POSSIBLE EXTENSIONSAlthough the TRS is performing satisfactorily, someextensions are envisaged that could improve theperformances of the system or make it suitable for otherapplications.4.1 Fibre delay compensationThe largest contribute to the timing error is due to thepropagation of the light through the fibre. Since it is aconstant parameter related to the location, it would bepossible modify the TIM or the TBDB to be configurableto compensate for the propagation delay.4.2 Local generation of TBIf the time bus fails, the TIM automatically switches tolocal mode using the inaccurate on board clock.This has two consequences: the LCU’s cannot get anylonger an accurate absolute time and they loosesynchronisation. To limit the impact of a TB failure, theTBDB could be designed in such a way that it cangenerate the TB basing on a low accuracy local clock,when the incoming signal fails. This obviously doesn’timprove the accuracy of the absolute time, but at leastkeeps the synchronicity among all the LCU’s fed by therelevant sub-tree.4.3 Extension of supported busesThe local computers used in the VLT are based on theVME bus architecture. As a consequence the TIM hasbeen developed for such a bus.The possibility to develop TIM’s for different standardbuses could be considered.5 CONCLUSIONThe presented central time reference system, based onthe GPS system, provides an accurate timing source withbackup capabilities.It supplies more than 100 local control computersdistributed on an area of about 300m across with a timeinformation having a measured absolute accuracy betterthan 2.5 microseconds.Means for accurate synchronisation between tasksrunning in separate computers have been developed.It also feeds time information with accuracy of 10milliseconds to several workstations.6 REFERENCES[1]  “ExacTime GPS Time Code and FrequencyGenerator User’s Guide”, DATUM Inc., August, 1997[2] E.g. D. Mills,  "Internet Time Synchronization: TheNetwork Time Protocol", IEEE Transactions onCommunications, Vol. 39, No 10, Oct. 1991.[3] “TymServe 2100 Network Time Server User’s GuideRev. B”,  DATUM Inc. [4] M. Ravensbergen, “VLT TRS Time Interface ModuleTechnical Manual”,  ESO document VLT-MAN-ESO-17300-473 Issue 2, July 1995207

The VLT Time Reference System: A Microsecond-Accurate Time\/Synchronization Bus for Distributed Cointrol Systems

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