National Instruments NI USB-621x 156

	Glossary
digital trigger	A TTL level signal having two discrete levels—A high and a low level.
DIO	Digital input/output.
DMA	Direct Memory Access—A method by which data can be transferred to/
	from computer memory from/to a device or memory on the bus while the
	processor does something else. DMA is the fastest method of transferring
	data to/from computer memory.
DMA controller chip	Performs the transfers between memory and I/O devices independently of
	the CPU.
driver	Software unique to the device or type of device, and includes the set of
	commands the device accepts.
E
E Series	A standard architecture for instrumentation-class, multichannel data
	acquisition devices.
edge detection	A technique that locates an edge of an analog signal, such as the edge of a
	square wave.
EEPROM	Electrically Erasable Programmable Read-Only Memory—ROM that can
	be erased with an electrical signal and reprogrammed. Some SCXI modules
	contain an EEPROM to store measurement-correction coefficients.
encoder	A device that converts linear or rotary displacement into digital or pulse
	signals. The most popular type of encoder is the optical encoder, which uses
	a rotating disk with alternating opaque areas, a light source, and a
	photodetector.
EXTCLK	External clock signal.
external trigger	A voltage pulse from an external source that causes a DAQ operation to
	begin.
EXTREF	External reference signal.

G-7