National Instruments SCXI-1120 - Manual (Page 60)

Chapter 3 Theory of Operation
© National Instruments Corporation 3-13 SCXI-1120 User Manual
The SCXI-1120 output circuitry consists of a buffered-output multiplexer and channel-select
hardware. The channel-select hardware consists of a three-bit counter, MUXCOUNTER. This
counter is needed when the board is operating in the Multiplexed-Output mode. The counter
output is sent to the output multiplexer address pins to determine which of the eight channels is
to be connected to MCH0. In the Single-Channel Read mode, the MUXCOUNTER is loaded
with the desired channel number. In the Scanning mode, the counter is loaded with the first
channel to be read. During the scan, the counter is clocked by SCANCLK from the data
acquisition board, or TRIG0 from the SCXIbus, depending on the state of the CLKSELECT bit
in the Configuration Register. During scanning operations, the MUXCOUNTER is reloaded
with the channel value stored in the Configuration Register when SCANCON is high (inactive),
and will count upwards on each rising clock edge when SCANCON is low (active). In the
Parallel-Output mode, the MUXCOUNTER is disabled and its output indicates binary 00; thus,
amplifier Channel 0 is selected at the output multiplexer and is connected to MCH0. The seven
other channels are hardwired to MCH1 through MCH7 on the rear signal connector.
The output multiplexer multiplexes all eight amplifier outputs and the temperature-sensor
reading provided on the MTEMP line. To read the temperature sensor when it is multiplexed
with the other input channels, the RTEMP bit of the Configuration Register must be set high.
This measurement is only software controlled. For hardware control of the temperature sensor
reading, connect the temperature sensor to pin 18 on the rear signal connector as described in
Chapter 2, Configuration and Installation. The multiplexer output connects to the MCH0
±
and
is connected to the data acquisition board analog channel input. In the case of the MIO data
acquisition boards, MCH0
±
on the rear signal connector corresponds to ACH0 and ACH8.
Furthermore, the multiplexed output of the SCXI-1120 can be bused, via switches to AB0
±
on
the SCXIbus, to other modules. When you use multiple modules, it is possible to bus the module
output via AB0 to the module that connects to the data acquisition board. In this case, the AB0
switches of all the modules are closed, whereas the output multiplexer of all the modules, except
the one being read, are disabled. Refer to chapters 2 and 5 for further details on how to configure
and program multiple modules.
In addition to the Multiplexed-Output mode described in the previous paragraph, it is possible to
operate the SCXI-1120 in the Parallel-Output mode. In this mode, you need no software–other
than the software used with your data acquisition board–to control the scanning of the eight
channels or to perform a single read. To access the temperature sensor in this mode, configure
the temperature sensor in the DTS mode. At power up or reset, amplifier Channel 0 is selected
on the output multiplexer, and thus connects to MCH0. The other seven amplifier channels are
hardwired to the rear signal connector. Notice that when the Multiplexed-Output mode is
selected, pins 5 through 18 on the rear signal connector are still driven by the SCXI-1120. The
SCXI-1120 outputs on the rear signal connector are short-circuit protected.
Refer to the following Scanning Modes section for further details on how to scan the SCXI-1120
channels.
Scanning Modes
The SCXI-1120 has four basic types of scanning modes–single-module parallel scanning,
single-module multiplexed scanning, multiple-module multiplexed scanning, and
multiple-chassis scanning, which is possible only with the SCXI-1001 chassis. For additional
information, consult Chapter 2, Configuration and Installation, Chapter 5, Programming, your