> For the complete documentation index, see [llms.txt](https://docs.axsun.com/axsun-knowledge-base/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.axsun.com/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/fpga-register-reference.md). # FPGA Register Reference Although the user has access to all FGPA registers, only those which provide user-accessible functionality are documented in this manual. Most FPGA registers in the full register map are reserved for Axsun internal purposes. FPGA registers with which a user might interact via the API are listed here: | **Register#\[bit]** | **Description** | **Notes (“0x” prefix indicates a hexadecimal value)** | | ------------------- | -------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | 2\[2] & 19\[15] | Imaging control (Ethernet interface only) |

both bits = 1 for Live Imaging

= 0 for Imaging Off

(control PCIe operation via axImagingCntrlPCIe() in AxsunOCTCapture)

| | 2\[9, 11] | Image\_sync input selection | See [Selecting the Image\_sync Input](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#selecting-the-image_sync-input) | | 20\[4] | Raw data source selection |

= 1 for Test Vector raw data source

= 0 for ADC raw data source

| | 20\[5, 13] | Channel select | See [Selecting which ADC Channel to Transmit](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#selecting-which-adc-channel-to-transmit) | | 20\[6] | Windowing function load applies to both channels | | | 20\[14] | Real/Imaginary select for windowing function load | See [Dispersion Compensation](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#dispersion-compensation) | | 23\[15..0] | Offset | See [Offset and Gain](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#setting-gain-and-offset-for-dynamic-range-compression) | | 24\[15..0] | Gain | See [Offset and Gain](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#setting-gain-and-offset-for-dynamic-range-compression) | | 25\[15..0] | Windowing function | Write 2048 consecutive U16 values representing the windowing function (Real or Imaginary depending on Reg 20 \[bit 14]). See [Setting the Apodization Window Function](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#setting-the-apodization-window-function) | | 26\[15..0] | Test Vector | Write 2048 consecutive U16 values representing the desired test vector. (Enable Reg 20\[4] to turn on Test Vector raw data source) | | 30\[15..0] | Pre-FFT Background Subtraction | Write 2048 consecutive I16 values representing the background signature. See [Background Subtraction](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#background-subtraction). | | 31\[0] | Burst Record control (Ethernet interface only) |

= 1 for Burst Record imaging ON

= 0 for Burst Record OFF

(control PCIe operation via axImagingCntrlPCIe() in AxsunOCTCapture)

| | 33\[15..0] | Burst Record # of images (Ethernet interface only) | See [Operational Modes](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/basic-operation.md#live-imaging-burst-record) | | 37\[15..0] | Post-FFT Background Subtraction | Write 1024 consecutive U16 values representing the background signature. See [Background Subtraction](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#background-subtraction). | | 60\[15..0] | Sub-sampling Factor | The desired Subsampling Factor minus one (M-1). See [Raw Data / Bypass Modes and A-line Subsampling](/axsun-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation.md#raw-data-bypass-modes-and-a-line-subsampling) | | 61\[15..0] | Bypass Select | Write two consecutive values according to [Bypass Modes table](https://axsun.gitbook.io/axsun-technologies-knowledge-base/ref-manual/gigabit-ethernet-daq-board/advanced-operation#raw-data-bypass-modes-and-a-line-subsampling) |