System Synchronization Hinges on Signal Switching

In bus-based systems such as VXI, PXI, and SCXI you can conveniently share these synchronization signals among the several instruments. The SCXI platform, integrated into PXI systems as a hybrid chassis, brings the additional advantage that you can route synchronization signals as well as analogue signals on the built-in backplane.

Allow Settling Time

The purpose of a switching system used in conjunction with test instruments is to increase the channel count of each instrument. Typically, you want the switching system to go through a series of different configurations in order to connect various test signals to the instruments in a certain order.

Straightaway, the problem of settling time arises. Settling time is the period a signal input requires to stabilise before the instrument can make an accurate measurement. Settling times become particularly significant when inputs change rapidly. In practice, measurement range, cable properties, source impedance, and change in input level dictate input and instrument settling time. Ideally, you should use short cables with low capacitance and dielectric absorption, and also low output impedance sources.

Settling time becomes critical in scanning systems, when the scanner, or multiplexer, requires an additional settling time before you can take the measurement. It’s important to verify that a programmable delay exists between channels so that signals on both the measuring instrument (for example, a multimeter) and the scanner can settle.

Software or Hardware Triggers?

Your system can synchronize scanner switching by using software or hardware triggers. A software trigger is much easier to implement because the system requires no cabling. The triggering instruction for the scanner to switch to the next configuration exists simply as a step in the program running on the system controller PC. Although software triggers are easy to set up, timing precision is fairly low. For example, running a non-real-time operating system such as MS-Windows gives no certainty to trigger timing because of inherent system latency. For low-speed scanning applications (less than 0.2 Hz), software triggers give satisfactory results, but for anything faster you’ll have to employ hardware triggers.

Hardware triggers naturally require hardware additions to a system that software triggers do not, but they add microsecond or even nanosecond synchronization, as in the case of a PXI star trigger.

Figure 1. Most instruments synchonize within a system by accepting an “external trigger” to initiate a measurement and providing a “measurement complete” signal.

Figure 2. Instruments with no external trigger input work in synchronous mode, in which a timer internal to the measuring instrument starts a programmable delay that allows the scanner and instrument input signal to settle.

A common use of switching is in conjunction with a digital multimeter (DMM) and, as a result, most DMMs operate with the two I/O signals called External Trigger (EXT TRIG) and Voltmeter Complete (VMC). These signals enable DMMs to implement a handshake protocol with the switching system. In operation, a DMM outputs a pulse on the VMC line when it finishes taking a measurement on one channel of the scanner. This pulse instructs the switching system to advance to the next configuration. When the analogue circuitry of the scanner has settled on the new setting, the switching system sends a pulse to the DMM’s EXT TRIG input to initiate the next measurement cycle (see Fig. 1).

Sometimes external trigger symbols are not available or not required, and in this situation DMMs implement a so-called synchronous mode.

In synchronous mode, the VMC output from the DMM advances the scanner as before but the DMM receives no EXT TRIG signal in return (see Fig. 2). In this case, a programmable time delay within the DMM expires and internally re-triggers the DMM for the next measurement. You can program this delay to accommodate the settling times required for the scanner and DMM’s input together.

Check Memory Depth

As a switching system moves from configuration to configuration, it follows what is called a “scanning list”. As mentioned, because most PC systems don’t operate in real time, it’s important to divorce PC operation from your scanning list. In practice, a PC downloads a scanning list to memory on the scanner’s switching modules before each complete series of measurements or after any change to the main program. For this reason, it’s important to check that memory depth on the switching modules is sufficient to accommodate your scanning lists.

Typically, scanning lists for general-purpose switches are limited to straightforward open or close circuits and on-board memory is generally not required. However, scanning lists involving matrix and scanner switching may be more complicated.

External Box or Modular System?

One aspect to consider in building a signal switching system is the merits of using an external switching box versus a modular bus system such as PXI, VXI, or SCXI.

An external switching system, controlled via an IEEE-488 or a serial interface, has the advantage of being easily added to an existing IEEE-488-based ATE system. The system requires only two cables to connect the triggers to the box. The main disadvantage is that in addition to needing an IEEE-488 cable and signal wiring, you must also connect different switches together and externally route trigger signals.

Conversely, if your ATE system uses multiple instruments and several of them have to provide a hardware trigger, an integrated bus system (such as PXI, VXI, or SCXI) provides an elegant and convenient solution. These architectures feature a backplane that can carry all the trigger signals. For example, the PXI and VXI platforms have 8-TTL triggering lines that you can use in a multi-instrument system to synchronize the operations of the different devices.

Figure 3. A hybrid chassis such as the PXI-1010 or PXI-1011 embodies a SCXI bus that allows your system to transfer analogue signals as well as synchronization signals along its backplane.

In practice, a PXI switching module (such as the National Instruments Model 2503) in slot 2 can receive its trigger pulse from a data acquisition board sitting in slot 7, or from a DMM in slot 3. This flexibility allows you to easily implement even the most complicated triggering arrangement without routing a single external cable.

VXI and PXI digital buses allow you to route synchronization signals, but not analogue signals via the built-in backplane. Therefore, you need to install additional external cabling or accessories to connect analogue signals from the switching system to the instrument modules. SCXI buses complement PXI by providing an integrated multi-channel bus to transfer both synchronization and analogue signals (to 300 V).

You can capitalise on this feature by using a PXI/SCXI hybrid chassis (such as the PXI-1010 or the PXI-1011) in which the two buses share synchronization and analogue signals. The left-hand side of the system chassis contains the instrumentation — using the high-speed PXI bus for triggering and digital data transfer — and the right-hand side contains switching and routing — using the shielded analogue bus (see Fig. 3). This hybrid feature is useful in complex ATEs where the amount of cabling, particularly for analogue signals, can seriously push up the overall cost of a system. T&ME