Timing is crucial in test and measurement (T&M) applications, which makes the IEEE 1588 Precision Time Protocol’s ease of use and high performance especially attractive to system designers. However, translating these advantages into useful features for test engineers can be challenging.
Although the IEEE 1588 specification’s flexibility has fostered its adoption in multiple industries, it can reduce interoperability and ease of use in some instances. Fortunately, the Lan eXtensions for Instrumentation (LXI) Standard builds on many of the time synchronization features specified in IEEE 1588 in ways that are useful to T&M system integrators. The base-level Class C instrument specification defines a consistent LAN implementation and a Web browser interface for setup, control, and data access. To this set of capabilities, Class A and B LXI instruments add a common sense of time using IEEE 1588 and peer-to-peer LAN messaging. Therefore, these instruments provide additional capabilities to optimize their use in configuring T&M systems:
• Measurement Data Time Stamping. When all of a system’s instruments share a common sense of time, data can be reliably correlated simply by comparing and sequencing time stamps.
• Synchronizing Measurement Triggers. These instruments allow initiating measurements or other actions at a specific time; they can also synchronize actions using peer-to-peer LAN messages. These synchronization capabilities are particularly valuable when system components are widely separated.
• Reducing or Avoiding System Latency Effects. These instruments can achieve better real-time trigger performance than hard-wired trigger systems by using time-based triggers to avoid external latencies and compensate for internal latencies.
One of the biggest challenges system builders face is integrating the time-based functions IEEE 1588 enables with other system functions. One simple but effective approach is to treat time-based triggers just like any other trigger source, such that any action that can be triggered by a command sent over via Ethernet or by a hard-wired trigger cable can also be triggered at a given time or in response to a LAN trigger message. Similarly, any internal function that can generate a trigger output, such as measurement complete or output settled, should also be able to generate an appropriate LAN message.
One recently developed instrument illustrates the advantages IEEE 1588 and Class B capabilities can offer T&M system builders. Keithley’s Model 3706 System Switch/Multimeter features an on-board Test Script Processor (TSP
) that allows loading short programs, or scripts, onto the instrument itself to perform tests independent of a separate controller. This instrument’s flexible event system allows a TSP script to run in response to a trigger and perform any desired combination of instrument functions. Both received LAN messages and time-based triggers are treated like any other triggers in the event system. Scripts can also send Class B LAN messages, so any instrument event can trigger a LAN message to trigger or control other Class B instruments. The Model 3706 also includes the standard Class B Web pages for configuring networking and IEEE 1588 functions, as well as pages for instrument control and script editing.
Perhaps most important from a system integration perspective, this use of IEEE 1588 and Class B capabilities boosts throughput significantly by eliminating controller and communications latencies. TSP provides a general-purpose scripting language with computation and program flow control capabilities extended with Instrument Control Library (ICL) commands for executing instrument functions. ICL commands can be sent to the instrument individually from a controller or multiple commands can be grouped into a script that can be executed with a single command.
More details on IEEE 1588 and LXI Class B instrument design are available in a new white paper available at www.keithley.com/IEEE1588.
Author: Paul Franklin, Keithley Instruments, Inc.
www.keithley.com
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