Acquiring with Line Scan Cameras

Some Cognex vision boards and video modules, such as the MVS-8120/CVM11 (which is not supported any longer), MVS-8600, and MVS-8600e support line scan cameras. These cameras acquire a single row of pixels at a time, building a complete image as the subject passes in front of the camera on a stage or conveyor. An encoder keeps track of the position of the subject as it travels in front of the camera, and communicates this information to the video module to let it know when to start and stop acquisitions. (Detailed information on the MVS-8600 and MVS-8600e frame grabbers can be found in Acquiring with Camera Link Cameras).

The major difference between line scan acquisition and area scan (normal) acquisition in CVL is that you need to specify line scan-specific properties, such as the number of encoder steps that correspond to a single image line.

There are significant differences between the use of line scan cameras with MVS-8120/CVM11, and with the MVS-8600 and MVS-8600e frame grabbers, as outlined in the table below.

Line scan differences between MVS-8120/CVM11 and MVS-8600/MVS-8600e
	MVS-8120/CVM11	MVS-8600/MVS-8600e
Maximum image size	32 megapixels	Limited by available memory.
Maximum lines per image	8192	64000
Supports continuous line scan	No	Yes (FreeRun trigger model only)
Supports strobed acquisition	Yes (not tested)	No
Supports encoder-triggered acquisition	Yes	MVS-8601/MVS-8602: No MVS-8602e: Yes
Steps-per-line setting for test encoder	Settable	Ignored. Camera free runs at maximum rate allowed by exposure setting.
Directional test encoder	Yes	No
Supports fractional steps-per-line	Yes	No
Encoder types supported	RS-422	RS-422, LVDS or TTL (open collector)
Uses positiveAcquireDirection() setting	At start of acquisition.	For every line
Supports useSingleChannel()	No	Yes (useful when direction of motion is irrelevant)
Encoder count behavior	Continuous	Available only when acquisition is in progress
Supports zeroCounter()	Yes	Yes (only to reset backwards count)
Supports zeroCounterWhenTriggered()	Yes	No
Supports currentEncoderCount()	Returns instantaneous encoder counter value.	Only valid during acquisition; returns estimated value relative to start of acquire, based on currently acquiring line number.
Supports encoderPort()	Yes	Yes
How to offset first acquired line from trigger	zeroCounterWhenTriggered() encoderOffset()	Not available
Selectable encoder resolution	No. Fixed encoderResolution of 4x	Fixed encoderResolution of 1x for MVS-8601/MVS-8602. Selectable encoderResolution of 1x, 2x, or 4x for MVS-8602e
Encoder overruns	Always reported	Never reported
Ignoring backward motion between acquires	Use zeroCounterWhenTriggered()	Use zeroCounter()

To understand how the two platforms use the positiveAcquireDirection() setting, see Detecting Encoder Direction.
To understand line scan acquisition with MVS-8120/CVM11, see positiveAcquireDirection() with the MVS-8600 and MVS-8600e.
To understand line scan acquisition with MVS-8600 and MVS-8600e, see Line Scan Acquisition with MVS-8600 and MVS-8600e.