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High-speed digital I/O

What’s high-speed Digital I/O?

High-speed Digital Input and Output instruments are electronic test equipment devices that are used to record and generate electrical digital signals or patterns. These patterns can be either repetitive or single-shot (once only) in which case some kind of triggering source is required (internal or external).

High-speed Digital I/O instruments can also be used as an alternative to Digital Pattern Generators.

Applications

The most common applications for the Strategic Test high-speed Digital I/O cards include recording or generating bus signals from devices under test both electronics systems and new semiconductor designs such as ADC and DAC component testing.

In some cases the Digital I/O cards are used as an alternative to Logic Analysers. These are devices used to record digital signals from equipment under test so that the design can be tested. However, these devices usually include software running on the logic analyzer that can convert the captured data into timing diagrams, protocol decodes, state machine traces, assembly language, or correlate assembly with source-level software. Engineers who need to record or generate long patterns use Strategic Test’s high-speed Digital I/O cards instead of Logic Analyzers as these can be configured with up to 4 GByte memory of depth.

Handshaking Operation

Some high-speed Digital I/O devices provide the capability for hand-shaking. This means that one of the channels (sometimes referred to as bits), is able to change direction from input to output while the instrument is operating. This is typically used in communication applications when data needs to be sent on one of bits to a device and then record the device response.

The high-speed Digital I/O cards available from Strategic Test do not have a handshaking capability. They operate only as digital recorders or generators where all bits are set in the input or output direction at the start of the test and remain in this configuration until the card is stopped and the direction changed.

Simultaneous Input and Output

Some of the Strategic Test Digital I/O card can be used for stimulus and verification tests. Cards with 32 or 64 bits can be configured so that one half Word of these are recording while the other half Word is generating. This means that these cards can generate a stimulus to the device under test and simultaneously record the response.

The term ‘high-speed’ is used to differentiate this class of product from low-speed digital I/O cards that are used for industrial equipment control. These typically have maximum clock rates in the region of 50 kHz, compared with Strategic Test’s high-speed Digital I/O devices that can operate with clock rates from 1 kHz up to 125 MHz.

The patterns to be generated can be derived from previously recorded signals, or can be created by defining these as a series of voltage amplitudes on a time reference – in the same way as we constructed simple x/y graphs at school. Today the most popular method to create new waveforms is to use computer software such as MATLAB or LabVIEW.

A high-speed Digital I/O instrument consists of five main sections:

  • A digital input and output section to record or generate TTL level signals. Different Strategic Test cards are available with 16, 32 or 64 bits.
  • A memory section used to temporarily store the incoming or outgoing digital pattern data. To generate digital output signals the pattern data file is transferred from the host PC into the Digital I/O card memory and can then be output once, or continuously until stopped. Similarly if recording, the incoming data can be transferred from the inputs to the card memory. Once the recording has stopped the data can be transferred to the host PC. When the data transfer is made when the card is not recording or generating this is referred to as the Standard Mode of operation on the Strategic Test. Clearly, the amount of memory on the card determines the maximum duration for recording or generating. However, longer signals can be recorded or generated if the signal data is continuously transferred while the recording or generation process is ongoing. This method is known as the FIFO Mode or Streaming Mode on the Strategic Test high-speed Digital I/O cards.
  • A bus section that is the electrical interface to the host computer. Its purpose is to transfer commands to the Digital I/O card to set up the card for recording and/or generation. The bus also transfers the data to be generated from the PC to the card or data that has been recorded to the host PC.
  • A clock section that generates a clock signal that determines the recording or generation speed. It also generates internal signals for internal bus synchronization and refreshing the onboard memory devices.
  • A control section. This is used to control the previous four sections and transfer the data from bus to the onboard memory.
High-Speed Digital I/O – Modular Instruments

High-speed Digital I/O instruments are available as standalone instruments and as electronic cards that can be integrated with standard PC’s or cards that require an external industrial chassis based on the PXI, CompactPCI, VXI, VME or CAMAC bus standards.

Strategic Test supplies instrument cards based on the PCI Express, PCI-X, PXI and CompactPCI standards.

What about Multiple Instrument Card Synchronization?

High-speed Digital I/O cards, such as those from Strategic Test are available with a choice of 16, 32 or 64 bits (channels). Each group of 16 bits is fed with the internal sample clock to ensure that all of the inputs or outputs operate synchronously i.e. without time-skew errors between the bit inputs or outputs. Of course, each bit is able to generate totally different signal but the clock rate must be identical.

What would be the situation if more bits were required to test a certain system and multiple high-speed Digital I/O cards had to be used? If the Digital I/O instruments are PXI cards then these have the natural advantage that clock and trigger synchronization signals are contained on the bus for multiple card synchronization. However, the PCI Express, PCI-X and CompactPCI buses were not originally designed for instrumentation applications and so don’t have this feature.

Strategic Test solves this problem with the Star Hub option. This consists of a small daughterboard that is added to one of the PCIe, PCI-X or cPCI cards, which feeds the master clock and trigger synchronization signals to all the other cards via small flat cables. Star Hub enables up to 16 cards to be perfectly synchronized in one PC or chassis, while the System Star Hub option that is only available on the PCIe and PCI-X cards can also synchronize up to 17 Star Hub enabled PC’s equipped with up to 271 instrument cards.

What Are the Advantages of PCI Express, PCI, PXI or CompactPCI Cards over Standalone Instruments?

If you need the highest sample rates possible, then only standalone Logic Analysers have the capability to operate at clock rates to 8 GHz and beyond. At slower rates up to 125 MHz modular instrument cards based on one of the standard bus formats will usually be more cost-effective or the preferred choice when flexibility, memory depth, number of channels and system size are determining factors.

For example, consider configuring a mixed-signal system consisting of a Digitiser with 4 channels, an Arbitrary Waveform Generator also having 4 channels and a 64 bit Digital I/O instrument. Strategic test could supply three PCIe cards that were synchronized with the Star Hub option that would fit in any low-cost good performance PC that can also be used to analysis and report the measured data.

The alternative standalone system would consist of three large chassis that may have synchronization difficulties and would most likely cost significantly more and have less memory depth to record or generate signals. In addition, standalone instrument tend to use older embedded processors that offer no possibility to upgrade and have a limited use compared to the versatility of a PC.