Matrox Helios Series Installation And Hardware Reference page 72

72 Chapter 4: Matrox Helios hardware reference
Trigger format
Exposure
Quadrature decoder
When received in TTL format directly, the signal must have a maximum
amplitude of 5 V; when received in LVTTL format directly, the signal must have
a maximum amplitude of 3.3 V. A signal over 2 V is considered high, while
anything less than 0.8 V is considered low; the transition of 0.8 V to 2 V is
considered to be the rising edge.
If using the trigger to start acquisition, the trigger signal's pulse width must be
greater than two pixels; if using the trigger to start the exposure timer, the trigger
signal's pulse width must be greater than two clock periods of the timer. To
determine the timer period, take the inverse of the pixel or timer's clock frequency,
respectively. For example, if the pixel frequency is 12.27 MHz, the minimum pulse
width is 2 x 1/12.27 MHz (approximately 164 nsec).
A trigger signal connected to external I/O connector 1 of the adapter board, passes
through an opto-coupler. The voltage difference across the positive and negative
components of the signal must be between 3.15 V and 6.45 V for logic high, and
between -5.0 V and 0.8 V for logic low.
Each PSG has two exposure timers. These timers can generate two periodic
exposure output signals to control the exposure time and other external events
related to the video source. Using auxiliary output signals, you can output either
exposure signal in TTL or LVDS format.
The exposure timers can use a clock that is internally generated or one from an
external source. In the latter case, you can define an auxiliary signal as the clock
source (timer clock input).
Each PSG features a quadrature decoder that can decode input from a rotary
encoder with quadrature output. A rotary encoder (sometimes called a quadrature
encoder) is a device that provides information about the position of a rotating
shaft. The encoder outputs a two-bit code (also known as gray code) on two pairs
of LVDS wires in a precise sequence: 00 01 11 10. The sequence determines the
position of the rotating shaft and the direction of rotation.
The quadrature decoder can decode gray code and update a 16-bit internal
counter. You can read the counter at different stages of the grab or trigger a grab
based on the value of the counter. The quadrature decoder can support a maximum
encoder frequency equal to the pixel clock frequency for the camera.