User manual

RT USB3000 Technical Description and User Manual Rev. 4.1. 9 of 18
4. Synchronization
4.1. Start synchronization
By default the device starts the acquisition (from ADC or from digital inputs) since after issuing the
command “Start” from a PC.
This command can be executed within a few milliseconds. The exact execution time of this
command under OS Windows (that is not a real-time OS) is impossible to be learnt in advance.
For the cases when it is necessary to bind the start of the data acquisition to any external event
with high precision one can use the external start synchronization mode. In this mode for the
beginning of the acquisition after the issuing the “Start” command it is necessary to give a negative
digital pulse (logical “1” - “0” - “1”) to the "SYN" contact of the
analog port of the device. The data
acquisition begins right after 430±14 ns after negative pulse edge (logical “1” to “0”) arrives. The
duration of the SYN pulse (i.e. of the logic “0”) must be at least 50 ns. The “SYN” line has an internal
pull-up resistor, so one can just short the “SYN” line to “ground” to generate the required pulse.
Switching between start synchronization modes of data acquisition is software-selectable.
4.2. Acquisition synchronization
The RT USB3000 device has a integrated clock generator operating on the frequency of 36 MHz.
By default the ADC of the device operates on the frequency received from the frequency division of
this clock generator. Also the scanning of digital inputs in the "Logic analyzer" mode operates with the
frequency received from the frequency division of this clock generator.
In some cases it can be inconvenient. For example, if several RT USB3000 devices operate
together. As each device has its own clock generator, after a while after the start of acquisition the
data from each device will start "creeping away" in the course of time as the clock signal generators
are not perfect and they have some error of frequency.
Special mode is provided for such cases, when the data acquisition (the ADC operating or reading
of digital inputs) clocks from an external source. External impulses of required frequency should be
supplied to the ADC_EXT contact of the device’s
analog port. Any external generator could be such
an impulse source. The RT USB3000 in-built clock generator could be such an impulse source, too. In
this case the desired impulse sequence is taken from the ADC_CONV contact of device’s
digital port
and is supplied to on the ADC_EXT pin of the analog ports of other RT USB3000 devices. Thus, ADC
operating (scanning of digital inputs) on all the devices is strictly simultaneous.
Switching between acquisition synchronization modes of data acquisition is software-selectable.