Hardware Timed Scanning - NI SCXI-1127 User Manual

Chapter 2 Using the SCXI-1127
Note When using an NI-DMM with the SCXI-1127, synchronous mode is the only
mode in which the NI-DMM and the SCXI-1127 operate.
SCXI-1127 User Manual
There are two basic methods of scanning—hardware timed scanning and
software scanning. In hardware timed scanning, the measurement
instrument, such as a DMM, communicates via digital trigger signals to the
SCXI-1127. 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.
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 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.

Hardware Timed Scanning
In hardware timed scanning, the software downloads to the SCXI-1127 a
list of channels to scan, including each channels configuration information,
such as whether the channel is two-wire, one-wire, or four-wire. The
software then arms the SCXI-1127. Once armed, the SCXI-1127 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. Refer to the hardware timed section of the manual for the
instrument you are using. Figure 2-11 shows the hardware scanning
flowchart and may be useful in understanding the scanning/triggering
operation of the SCXI-1127.
2-12 © National Instruments Corporation