Gated Replay

Bus: PCI Express, PCI-X
Instruments: Arbitrary Waveform Generators, High-Speed Digital I/O, Pattern Generators

The Gated Replay option allows you to start/stop the generators clock using an external TTL signal. Therefore signals are only output if the external gate signal is equal to the user programmed active level (TTL HIGH or TTL LOW).

Gated Replay can be used in either the Standard or FIFO (streaming) mode. Both operating modes are describes below. You may also want to consider the Multiple Replay option as an alternative.

How Gated Replay Works

For the sake of brevity, we will describe this option based on an Arbitrary Waveform Generator card. But the same information applies to the high-speed Digital I/O cards and Pattern Generator cards.

The user defines the signal generation parameters as usual including the amount of memory to allocate to the signal data. Unlike with Multiple Replay where the memory is divided into segments, with Gated Replay the memory is one continuous space.

In addition, when using the Standard mode it is possible to define the number of loops where the content of the memory will be output multiple times. If the number of loops is set to 1 the card stops immediately as soon as the total amount of data has been replayed. In that case the last gate segment is ended by the expiring memory size counter and not by the Gate end signal. If loops is set to zero the Gated Replay mode will run in a continuous loop until explicitly stopped by user. If the replay reaches the end of the programmed memory it will start again at the beginning with no gap in between. If you set the loops to a number larger than 1, this number of complete gates will be replayed and then the card will stop automatically. See the diagram below:

When the Gate becomes active the signal generation starts and when the Gate becomes inactive the sampling clock stops and the output stops. Next time the Gate goes active, the output continues. The process repeats until the card has output all the samples defined by the users’ memory allocation and the number of loops.

Depending on the sample rate and number of channels, this option can also be used in the FIFO mode so that data is continuously streamed to the host PC. This allows more data to be recorded that the available card memory. In FIFO mode, there is no need to specify the memory allocation or the number of loops as the card will continue to accept the transferred data and output this until stopped by software. If you want loops, you can create these in the host PC software.

The Gate re-arm time between the end of replay event and the next start of output, is less than 4 samples so potentially millions of events can be generated in a second. However, bear in mind that if your Gate inactive time is less than 4 samples then the card won’t have time to re-arm without losing a few samples of data. There is also the possibility that if you set the Gate to detect an edge instead of a level, that the event would be missed.

Useful Calculations

Please refer to the links below:

Digitizer Operating Modes > /learn/ waveform-generator-cards-operating-modes.html
Digital I/O Operating Modes > /learn/ digital-io-cards-operating-modes.html