National Instruments 1128 用户手册

SCXI-1127/1128. In its simplest form, hardware-timed scanning uses one 
triggering signal to advance the multiplexer to the next channel in the scan. 
This is called synchronous scanning.

When using a DMM with the SCXI-1127/1128, synchronous mode is the only mode 

in which the DMM and the SCXI-1127/1128 operate.

Another hardware-timed scanning method is called handshaking. While 
synchronous scanning requires just one signal from the instrument to the 
multiplexer, handshaking requires an additional signal connected from the 
multiplexer to the instrument. This additional trigger, SCANADVD, 
notifies the measuring instrument that the multiplexer has settled and the 
next measurement can be made. This method is useful when using an 
SCXI-1127/1128 with external instruments to guarantee that both the 
measurement instrument and the multiplexer have settled before taking 
the next measurement.

The second type of scanning, software scanning, is a method by which 
channels are selected through the software. This method requires no trigger 
connections. Software scanning does not offer the time accuracy of 
hardware-timed scanning.

In hardware-timed scanning, the software downloads to the 
SCXI-1127/1128 a list of channels to scan, including each channels 
configuration information, such as whether the channel is 2-wire, 1-wire, 
or 4-wire. The software then arms the SCXI-1127. Once armed, the 
SCXI-1127/1128 advances to the next channel when it receives a trigger 
from the DMM. In multi-module scanning, the chassis arbitrates between 
modules. Each module in turn drives its output onto the analog bus and on 
the OUT (COM) terminals. Figure 2-11 shows the hardware scanning 
flowchart and may be useful in understanding the scanning/triggering 
operation of the SCXI-1127/1128.