CoaXPress Host To Device Trigger

The CoaXPress Host to Device Trigger transmitter can be sourced from:

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The Camera Trigger output of the associated Camera and Illumination Controller

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Any input-capable General Purpose I/O

The trigger source is indirectly controlled through the CameraControlMethod GenICam feature.

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When CameraControlMethod is set to RG or RC, the trigger source is the Camera Trigger output of the associated Camera and Illumination Controller.

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When CameraControlMethod is set to EXTERNAL:

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The trigger source is the line source of a dedicated LIN tool of the I/O Toolbox: LIN1 for Device0, LIN2 for Device1, LIN3 for Device2, and LIN4 for Device3.

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Any input-capable GPIO line can be used as trigger source by configuring the line source – LineInputToolSource – of the dedicated LIN tool.

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The polarity of the external trigger signal can be controlled with the LineInverter setting of the selected I/O Control block.

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The time constant of the glitch-removal filter can be adjusted through the LineFilterStrength setting of the selected I/O Control block.

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When CameraControlMethod is set to NC, the Host to Device Trigger transmitter is disabled.

The Coaxlink implementation of the CoaXPress Host to Device Trigger transmitter complies with the requirements of the CoaXPress 1.0 and 1.1 standards for a low-speed CoaXPress Host to Device Trigger when it is configured with the default settings:

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CxpTriggerMessageFormat = Pulse

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CxpTriggerAckTimeout = 20.0

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CxpTriggerMaxResendCount = 3

The transmitter initiates a trigger transaction on both edges of the trigger source signal:

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It computes a delay value allowing the receiving device to recreate the event with a fixed latency. For more information, refer to: .

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It inserts a high-priority "trigger packet" on the low-speed host-to-device connection at the next character boundary.

Then, the transmitter waits for the acknowledgment from the Device (camera):

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If the acknowledgement is received before the expiration of the timeout, the transaction terminates normally.

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If no acknowledgement is received within the 20 µs timeout, the transmitter performs a retry: it resends the trigger packet and initiates a new waiting period for the acknowledgement.

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If no acknowledgement is received after 3 times, the transaction terminates abnormally.

The transmitter doesn't initiate a new transaction while the previous one is not completed.

Trigger message transactions using default settings
Case 1 and case 2: the camera acknowledges each message as expected
Case 3: no acknowledgment from camera. Abort after 3 retries

Events Reporting

The transmitter reports the following events:

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CxpTriggerAck: Received acknowledgement for CoaXPress Host to Device trigger packet.

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CxpTriggerResend: Resent CoaXPress Host to Device trigger packet.

The transmitter can be customized:

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To send trigger messages only on the rising edge of the source signal using the Message Format Control

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To configure the acknowledge timeout and the number of retries using the Message Acknowledge Control

Trigger Accuracy

The Host to Device trigger packets are transmitted over the low-speed (20.833 Mbps) connection of the CoaXPress Link. The transmission of trigger packets can only start at the boundary of a character. This introduces a jitter of 480 nanoseconds corresponding to one character transmission time.

To minimize trigger jitter, the time between the trigger event and the trigger packet being sent is encoded into the trigger packet as a delay value expressed in units of 2 ns (1/24th of the bit period). The receiver (camera) can then use this value to recreate the trigger event with a fixed latency. It compensates the transmission jitter by delaying the decoded message by the remaining fraction of one character time.

CoaXPress Camera Trigger transmission Timing

CIC Camera Trigger to Sensor Exposure timing diagrams

The above diagram shows the time delay required to propagate Camera Trigger events from the frame grabber up to the camera through the CoaXPress Link using Host to Device CoaXPress Trigger messages.

The above diagram assumes that:

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CameraControlMethod is set to RG.

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The camera properly acknowledges the trigger messages and effectively initiates a new exposure.

The delay from the rising (or the falling) edge of the Camera Trigger signal (inside the Coaxlink card) up the CoaXPress link is composed of:

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A variable delay of 0-480 ns corresponding to the time delay until the next character boundary on the low-speed CoaXPress connection.

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A fixed delay of 1.92 µs corresponding to a 4-character pipeline delay in the Trigger Transmitter implementation.

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A fixed delay of 2.88 µs corresponding to a 6-character message transmission time.

The delay from the CoaXPress Link to the effective start (or end) of exposure is camera-dependent. In the above drawing, this delay is assumed to be N character times (with N=4).

Jitter-compensated cameras

When the camera implements the CoaXPress jitter compensation, the one-character jitter (480 ns) introduced by the transmitter can be entirely compensated.

The overall latency is fixed but it remains camera-dependent: the lowest possible latency with Coaxlink products is 11 x 480 ns, i.e. 5.28 µs.

The residual jitter after compensation can be as low as 4 ns.

Jitter-Uncompensated cameras

When the camera doesn't implement the CoaXPress jitter compensation, the one-character jitter (480 ns) introduced by the transmitter remains.

The overall latency is variable and camera-dependent: the lowest possible latency with Coaxlink products is (10 ~ 11) x 480 ns, i.e. (4.80 ~ 5.28) µs.