Timing and synchronization are crucial in building test and measurement (T&M) systems, which makes the IEEE 1588 Precision Time Protocol’s ease of use and high performance especially attractive to system designers. This paper offers an overview of the benefits IEEE 1588 brings to T&M systems, especially when paired with LXI Class B instrument capabilities, and illustrates how these capabilities can be integrated into instruments that can serve as powerful building blocks for creating high performance test systems.
IEEE 1588 overview
IEEE 1588 specifies a Precision Time Protocol (PTP) that may be used to synchronize clocks in a T&M system. When a T&M system implements PTP, each instrument, computer, or other controller in the system contains a clock. PTP allows synchronizing all these clocks and keeping them synchronized. PTP requires data communications between all devices; in T&M systems, this is typically implemented using Ethernet LAN. Other time synchronization methods have been used in T&M applications, including Network Time Protocol (NTP), Global Positioning Satellite (GPS) based systems, and hard-wired distribution of reference oscillator signals.
However, when compared with these alternatives, IEEE 1588-based systems provide more precise and accurate synchronization while offering the benefits of standard Ethernet LAN networking connections. Also, there are signs that IEEE 1588 will become widely used outside the T&M industry, leading to wider availability and lower costs for IEEE 1588 systems based on greater economies of scale.
LXI Instruments and IEEE 1588
LXI overview: The LXI (LAN eXtensions for Instrumentation) Consortium is made up of members from most major T&M companies. Since its formation several years ago, the Consortium has developed and released standards for the use of Ethernet networking in instrumentation. The Consortium has recognized the potential of the IEEE 1588 protocol, so the LXI Standard requires that support for it be included in several classes of LXI instruments. Additional information on the LXI Consortium is available at www.lxistandard.org.
LXI classes: The LXI Standard defines three classes of devices. The baseline class, Class C, defines a consistent LAN implementation, and a Web browser interface for setup, control and data access; it also requires an IVI instrument driver for a programmatic interface. Class B builds on Class C by requiring a common sense of time using IEEE 1588 and peer-to-peer LAN messaging. Class A builds on both Classes C and B by adding a high performance, hard-wired trigger bus for applications with stringent timing requirements. Although this white paper refers specifically to LXI Class B, LXI Class A devices also incorporate all Class B capabilities.
LXI class B details: The IEEE 1588 specification includes provisions for configuring many aspects of PTP to suit a wide variety of applications. It was intentionally designed to be flexible and to meet the needs of many industries, not just T&M. Furthermore, this specification only standardizes how the timing functions work, not how they are used in any given application nor what meaning is given to the timestamps they provide. Although this flexibility is helpful in fostering wider adoption of IEEE 1588 in multiple industries, it can be a barrier to interoperability and ease of use in specific industries. Therefore, the LXI Class B specification provides additional direction for the configuration and use of IEEE 1588. These additional specifications were chosen to tailor and optimize IEEE 1588’s operation and use in T&M systems, ensuring compatibility and interoperability between all LXI Class B and A instruments.
The LXI Class B specification defines:
Benefits of IEEE 1588 and LXI Class B for T&M system design
- A standard implementation of IEEE 1588, including tailoring for T&M applications
- A standard meaning for the timestamps provided by IEEE 1588
- A standard peer-to-peer LAN messaging protocol incorporating timestamps
- A standard configuration mechanism for connecting events, triggers, and actions
- A standard logging mechanism for recording important events with timestamps.
Many (if not most) T&M systems can benefit from a precise sense of time common to all system components. The variety of T&M instruments and their applications is very broad and so is their need for precision clock synchronization. The particular capabilities needed and the degree of precision required vary from one instrument or application to the next, but the following examples illustrate a broad cross-section of test system needs.
Measurement data time stamping:
Time stamping measurement data (that is, associating a time with each data point) is widely used in T&M applications. For example, time stamps are useful for maintaining quality control and satisfying regulatory requirements. However, they are only useful for such purposes if their accuracy and precision are known and reliable.
Through the use of the IEEE 1588 PTP protocol, LXI Class B instruments can be synchronized to a single time source of sufficient quality to meet the application’s requirements, eliminating the problems caused by out-of-sync clocks, as well as the labor, inaccuracy, and risk of error associated with manually synchronizing clocks in multiple instruments and controllers.
When all the instruments in a T&M system share a common sense of time, data from multiple instruments can be reliably correlated after the fact simply by comparing and sequencing time stamps. There’s no need to check that data is transferred from the instruments quickly and in the correct sequence because the time stamps can be used to put the data in the proper sequence when necessary.
System troubleshooting is a particularly useful application of this correlation capability. Many modern instruments can record important events, such as measurement triggers and error conditions, and these events can be time stamped like any other data. By collecting and correlating these event records using the time stamps, the exact sequence of events can be reconstructed, even when multiple instruments are involved, for verifying correct system operation and tracking down the source of any problems that occur.
Synchronizing measurement triggers:
Time stamping data isn’t the only use of synchronized clocks. LXI Class B instruments allow users to initiate measurements or other actions, such as sourcing a voltage, at a specific time. Used in this way, these IEEE 1588 capabilities allow coordinating the test system’s operation without the need for hard-wired trigger cables or precise timing of commands sent from a central controller. LXI Class B instruments can also synchronize actions using peer-to-peer LAN messages. A message sent from one LXI device to another can trigger an action either immediately upon receipt (analogous to a hard-wired trigger cable) or at a future time. These synchronization capabilities are particularly valuable when the components of the system are so widely separated that it’s impractical to connect them with physical trigger cables.
Reducing or avoiding system latency effects:
One potential drawback of using LAN to interconnect T&M systems is the variability in latency and timing of LAN when compared with other communications busses. Although this is not an issue in many applications, systems with stringent timing requirements must account for LAN timing characteristics. Using time to coordinate and synchronize triggers and actions eliminates this consideration in most applications.
LXI Class B instruments can achieve better real-time trigger performance than hard-wired trigger systems by compensating for internal latencies. For example, if an instrument requires 10 milliseconds to prepare to take a measurement, then the measurement taken with that instrument will always lag behind the actual trigger by 10 milliseconds. Using time-based triggers, however, allows the instrument to compensate by starting the preparation 10 milliseconds before the time specified for the trigger, so the measurement will occur precisely at the trigger time, not 10 milliseconds later.