The entire frame is only one second long. IRIG-B has a synchronization precision of tens of nanoseconds.įigure 6 shows how time is sent using IRIG-B. This data frame presents time information in seconds, minutes, and days and provides a status byte. The signal is similar to PPS, but instead of a single pulse once a second, IRIG-B sends coded bits that make up a data frame that is one second long. IRIG-B – This protocol is used to transmit time data. The pulse width is generally 100 ms, but many receivers allow the user to specify the pulse width, as long as it is less than one second. It does not contain information about the specific time of day or year it outputs a pulse only once a second. Pulse per second (PPS) – PPS, the simplest form of synchronization, is a signal that outputs a high logic level once a second. Time protocols are the tools you use to transport time information across large distances. The same conveyance of time information takes place in time-referenced systems. If each person looks at the clock tower and resets, or synchronizes, his or her wristwatch to the clock tower time reference, then everyone has the right time and arrives at work on time. But each person’s wristwatch could be different, and there might be confusion over the correct current time. and they all have their own wristwatches. Everyone in the town must be at work at 8:00 a.m. Think of the master node as being the clock tower and the slave nodes as being the people in a small town. Time-Referenced Synchronization SystemĪnother method of visualizing this transfer of time data via time protocols is the clock tower/wristwatch analogy. Figure 3 shows a time-referenced synchronization system.įigure 3. You can use a future time event, which is when an action starts after a defined time is reached, to trigger an action across all nodes simultaneously. This time information is used by each system node to determine the present time and create a clock based on that reference. In a time-referenced system, protocols such as GPS are used to convey time information across greater distances than are possible with cabling. The blue portion of the precision versus distance graph shows the precision achievable by time-referenced synchronization. This method, examined later in this tutorial, is called time-referenced synchronization. You need another method of conveying the clock and trigger signals from the master node to the other slave nodes in the system. The precision versus distance graph shows that as the distance between nodes increases past a certain point, you cannot physically connect the clock and trigger lines for each node together anymore. ![]() Figure 1 shows the precision versus distance graph for physically connected and time-referenced synchronization systems.įigure 1. ![]() ![]() If you cannot, then you must use a time reference to relay the clock domain information. Based on this, you must decide if you can successfully transmit the clock and trigger signals over this distance without too much degradation. You may have a single node, a group of nodes in one location, or multiple nodes that are spread out over a greater distance. In most systems, you know the distances you must design for. This trade-off between precision and distance presents a problem: to have a high precision of synchronization, you must have a clock with high frequency and accuracy, which can degrade as the distance between chassis, or nodes, increases. System designers must account for the limitations created by these variables because as transmission distance increases, it is more difficult to share signals between systems to keep them synchronized. The two most important variables in designing a timing and synchronization scheme are synchronization precision and the distance between the system nodes.
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