A recurring question among engineers who program National Instruments' DAQmx counters has exposed a subtle but consequential limitation in how retriggerable pulse trains behave. The issue involves the Initial Delay parameter in the "Gen Digital Pulse Train Finite Retriggerable" example: on hardware such as the 6602 counter/timer board and certain E Series cards, that delay only applies to the very first trigger received after the task starts. Every subsequent trigger fires the pulse train immediately, with no delay at all.
For engineers building automated test systems, this is not a cosmetic detail. Many applications depend on a consistent, repeatable delay between a trigger event and the start of pulse generation, whether for synchronizing instruments, staggering signal paths, or protecting sensitive components from timing collisions. Discovering that the delay silently disappears after the first cycle can introduce subtle errors that are difficult to trace, especially in systems where timing accuracy is treated with the same rigor that privacy-conscious users apply when evaluating, say, what a no-logs policy actually means for a service provider - the behavior promised on paper has to match what actually happens in practice, every single time, not just once. what a no-logs policy actually means
Why the Delay Only Happens Once
The explanation lies in how the counter hardware interprets the Initial Delay property. It is applied strictly to the first pulse generated after the task is armed, not to every retrigger event that follows. This is the documented, intended behavior of the counters on both the 6602 and E Series devices, not a software bug. Once the first pulse has fired, the counter reverts to its steady-state timing, meaning the "low time" parameter governs the gap before each subsequent pulse rather than the originally specified delay.
This distinction matters because retriggerable finite pulse generation requires only two counters, a meaningful advantage for boards with limited counter resources, such as E Series cards that offer just two. Any workaround that reintroduces a true per-trigger delay tends to consume additional counters, which forces engineers to weigh hardware economy against timing precision.
Two Practical Workarounds
One solution uses two counters cooperatively: one counter generates a retriggerable single pulse whose low time equals the desired delay and whose high time matches the pulse train period, while a second counter runs a continuous pulse train that is paused by the first counter's output. This reproduces a delayed, retriggerable finite pulse train using only two counters, making it usable on hardware with minimal counter resources.
A second approach, better suited to applications where the number of pulses is itself a controlled parameter, adds a dedicated counter purely to generate the delay as a single retriggerable pulse, whose rising edge then triggers the existing finite pulse generator. This is conceptually simpler but requires three counters total, since finite pulse train generation already consumes two. Boards like the 6602 or 6601, which offer eight counters, accommodate this comfortably, while two-counter E Series cards cannot.
A Software-Level Alternative
For developers working directly against the NI-DAQmx driver in C or C++, there is a more direct route. The driver exposes trigger properties, specifically StartTrig_Delay and StartTrig_DelayUnit, that let a program specify an explicit wait time after a start trigger before generation or acquisition begins. Setting these properties through functions such as DAQmxSetStartTrigDelay and DAQmxSetStartTrigDelayUnits gives programmers finer control than the graphical example VIs allow, without necessarily requiring extra counters, depending on how the application is structured.
The broader lesson is a familiar one in instrumentation design: example code demonstrates typical use, not every edge case. Engineers who need guaranteed, repeatable timing behavior across every trigger cycle have to look past the default examples and into the underlying counter architecture and driver properties. National Instruments has acknowledged user requests to add a dedicated option - effectively a "use Initial Delay for all triggers" setting - as a possible future enhancement, though no such feature exists yet. Until it does, the workarounds described here remain the established methods for achieving consistent delayed, retriggerable pulse generation.