Omega 208H, DAQP-208, 308 user manual High byte of the almost full threshold 15 2K option

5.2.6.3 Flush Data FIFO Command

The data FIFO should be flushed before data acquisition is initiated by the trigger/arm command, but not until after the scan list has been configured. The flush command may also be followed by FIFO threshold programming. After the FIFO is flushed, the FIFO empty flag will be set to “1” and the almost full and full flags reset to “0”. Anytime the data FIFO is flushed, the default threshold setting will be restored (7 bytes to full) by the hardware. The data FIFO threshold should always be programmed after flushing if the required threshold is different from the default one.

5.2.6.4 A/D Stop Command

Once data acquisition is initiated by the trigger/arm command, it can only be stopped by receiving the A/D stop command. The A/D stop command should be issued as soon as the required data points are collected to prevent data FIFO overflow. Data FIFO overflow is the only flag that indicates lost data during the acquisition process. Without the stop command, the A/D can continue to run, filling the data FIFO. When the FIFO is full, it will ignore data samples coming from the A/D converter.

5.2.6.5 Data FIFO Program/Access Control

The A/D data FIFO has two programmable thresholds, almost empty and almost full, and two associated flags. The almost empty threshold and flag are not used. By default, the thresholds are set to 7 bytes (7 to full and 7 to empty) when reset, powered up or anytime the FIFO is flushed. It can be programmed to any value between 1 and FIFO size - 1 (in bytes).

To program the FIFO threshold, make sure the A/D has been stopped. Set this bit to “0” by writing an all “0” byte to the auxiliary control register. Then send an A/D FIFO flush command with the same bit setting by writing a byte of 40H (hex 40) to the same register. This will put the FIFO into program mode. The following read/write operation will be directed to the threshold registers instead when accessing the data FIFO at base +1. The 4 byte threshold setting should be written into the data FIFO by doing four consecutive write operations. Optionally, the threshold setting can be read back for verification by doing four consecutive read operations. Table 5-17 lists the 4 byte threshold setting format.

Table 5-17. Data FIFO Threshold Setting

Byte	Definition	Valid Range
0	Low byte of the almost empty threshold	0..255
1	High byte of the almost empty threshold	0..15
2	Low byte of the almost full threshold	0..255
3	High byte of the almost full threshold	0..15 (2K option)

DAQP-208/208H/308 Users Manual 52