The Frequency Output (or FREQ OUT) signal is a digital pulse train that can be programmed on E, M and X Series devices without the use of any onboard counters. The signal uses the 10 MHz (the 20 MHz Timebase divided by 2) or 100 kHz Timebase, which can each be divided down by an integer divisor from 1 to 16. This gives the capability of producing one of 32 distinct digital pulse trains ranging from 6.25 kHz to 10 MHz. It is important to note that this a limitation of using this method to output a frequency signal vs. using a traditional counter output. With this method there isn't an option to have a custom frequency, it has to be one of the 32 distinct digital pulse trains and can't be varied from that.
By default, the Frequency Output signal will be generated on Pin 1 of the connector for E, M, and X Series devices. For E Series devices, Pin 1 is a dedicated pin for the FREQ OUT signal. On M and X Series devices, Pin 1 corresponds to PFI 14, but the FREQ OUT signal can be routed to any PFI or RTSI line, as well as the DO or DI Sample Clock. The Frequency Output signal may also be accessed internally by referencing /<NI-DAQmx Device Name>
You can find more information about the Frequency Output signal in the X Series User Manual
and M Series User Manual
under Counters»Counter Output Applications»Frequency Generation
or in the E Series User Manual
under Counters»Counter Timing Signals»Frequency Output Signal
While the Frequency Output signal is useful when a counter is not available, it is recommended to use a counter when possible. The Frequency Output has the following disadvantages when compared to the counters. This list is not all inclusive.
- As stated previously, the Frequency Output can only produce 32 distinct digital pulse trains. Counters divide down the Timebase by an integer value to produce the pulse train. The frequency of the pulse train must meet the following requirement:
Timebase Rate / Counter Maximum Count <= Frequency <= Timebase Rate / 4
The Counter Maximum Count is determined by the resolution of the counter. See Related Links for more information.
- When using the Frequency Output, you can only use either the internal 100 kHz Timebase or the 10 MHz Timebase. When using counters, you can use any of the internal Timebase clocks or specify an external Timebase.
- Frequency Output does not support triggering.
- Counters support the following trigger types:
- Arm Start Trigger
- Start Trigger
- Pause Trigger