FIFO Mode

FIFO mode is used to record signal data continuously to or from the PC memory (RAM) and hence onto the hard disk. It has a cost advantage as PC RAM and drives are of a relatively low cost. Below is a description of the mode, we will concentrate on signal capture, but the operation with signal generation cards is much the same but in the opposite direction

image showing approximate card data transfer rates to PC RAM and hard drive image showing data transfer path to PC RAM and hard drive

Method: During data transfer the whole of Spectrum M2i / M3i cards on-board memory is available as a transfer FIFO (first in first out) buffer ensuring reliable data movement, automatically controlled from the driver for best performance. In a continuous operation a section of this transfer ("FIFO") buffer is emptied to the PC whilst another section is filled by the card. This operation can work alongside the processing the data in your software application. Transfer starts with a trigger event, running until the user stop command, or until user defined number sample segments have been collected. Utilising the scatter-gather DMA technique transfer rates are dependent on PC component performance and the type of slot the card is plugged into. The rates illustrated in the diagram above are for capture into PC RAM and achievable by many PC's. Some variation on the absolute maximum is to be expected.

Real-time capture to the serial (SATA) hard disk drive has the great advantage of storage space and works well for most applications, but note that it is slower than PC RAM, a typical rate for a hard-disk drive might be 100MBytes/sec read speed. Most recent developments have seen serial Solid State Drives (SSD) become more common and cost effective, these offer at least double the transfer rate of a disk drive, hence the fastest solution of all. With signal capture operations, any signal data the hard disk cannot deal with is temporarily stored in PC RAM, but this can result in data backing-up, filling the PC RAM then the cards own memory in extreme cases, resulting in a data over-run message, at which point the Spectrum card driver will stop the card. Note that with signal generator cards the over-run process will not occur where data starts from the hard disk, as this is the slowest component in the data transfer chain. For more information about increasing read/write speed to HDD / SSD drives click here for more information on RAID systems.

FIFO mode may be combined with features such as Multiple Recording / Replay for greater capture control, this way data is only captured (or generated) when an event of interest comes along, thus making best use of the available PC's PCI/PCI Express bus bandwidth. FIFO mode works under Linux™ or Windows™. The amount of PC RAM data storage available under 32 bit Windows is limited to about 3.2 Gbytes with 4 GBytes fitted into the PC, as some space is always required for the operating system. These limits do not apply Windows 64 bit and the user can fit as much memory as the motherboard can hold!

A final important note! If you wish to capture or generate signals for longer periods at greater rates than the PC will handle, then consider upgrading the cards considerable on-board memory, up to 2GBytes are available as an option for the M2i and M3i card series.


DataQuest Solutions Ltd. | Phone: 01526 557171 | Email: info@dqsolutions.co.uk