Method and apparatus for synchronising the local time of a plurality of instruments

Abstract

A method of determining the downstream propagation time of signals from a USB Host Controller across one or more USB cables and one or more USB Hubs to a SuperSpeed USB device, including locking a clock of the SuperSpeed USB device to information that includes a first timestamp, transmitting a plurality of signals to the USB Host Controller, each of the signals containing a second timestamp indicative of a local time of the SuperSpeed USB device when the respective signal was generated by the SuperSpeed device; the USB Host Controller creating a third timestamp indicative of a time of reception from the SuperSpeed USB device; determining a time period from one or more respective time differences between corresponding second and third timestamps, the time period being indicative of a sum of a downstream propagation time and an upstream propagation time; and determining the downstream propagation time from the time period.

1 . A method of determining a downstream propagation time of signals from a selected point in a USB network across one or more USB cables and one or more USB Hubs to a first SuperSpeed USB device of a plurality of SuperSpeed USB devices, the method comprising:
 ensuring that said first SuperSpeed USB device is in link state U 0 ;   ensuring that said SuperSpeed USB devices other than said first SuperSpeed USB device are not in link state U 0 ;   ensuring that no request is made to change the link state of any links in a USB network branch spanned by said selected point and said first SuperSpeed USB Device; and   performing a plurality of round trip propagation time measurements for said first SuperSpeed USB device.   
     
     
         2 . A method as claimed in  claim 1 , wherein said selected point in a USB network is a USB Host Controller. 
     
     
         3 . A method as claimed in  claim 1 , wherein said selected point in a USB network is a USB hub. 
     
     
         4 . A method as claimed in  claim 1 , wherein no traffic other than propagation time measurement traffic for the first SuperSpeed USB device is present on the USB network when measuring the propagation time to minimize potential hub retransmission delays. 
     
     
         5 . A method as claimed in  claim 2 , wherein no traffic other than propagation time measurement traffic for the first SuperSpeed USB device is present on the USB network when measuring the propagation time to minimize potential hub retransmission delays. 
     
     
         6 . A method as claimed in  claim 3 , wherein no traffic other than propagation time measurement traffic for the first SuperSpeed USB device is present on the USB network when measuring the propagation time to minimize potential hub retransmission delays. 
     
     
         7 . A method of determining a downstream propagation time of signals from a selected point in a USB network across one or more USB cables and one or more USB Hubs to a first SuperSpeed USB device of a plurality of SuperSpeed USB devices, the method comprising:
 ensuring that said first SuperSpeed USB device is in link state U 0 ;   ensuring that said SuperSpeed USB devices other than said first SuperSpeed USB device are not in link state U 0 ;   ensuring that no request is made to change the link state of any links in a USB network branch spanned by said selected point and said first SuperSpeed USB Device;   performing a plurality of round trip propagation time measurements for said first SuperSpeed USB device;   using the plurality of round trip propagation time measurements to determine the downstream propagation time; and   controlling traffic on the USB network so that no traffic other than propagation time measurement traffic for the first SuperSpeed USB device is present on the USB network when measuring the round trip propagation time to minimize potential hub retransmission delays.