Chapter 2 Using the SCXI-1127/1128
© National Instruments Corporation 2-3 SCXI-1127/1128 User Manual
The SCXI-1127/1128 contains a safety interlocking mechanism that
prevents high voltages from appearing on the input connector when a
terminal block is not present. However, when a SCXI-1331 or SCXI-1332
is plugged into the front of the SCXI-1127/1128, the module detects its
presence and allows the module to close relays.
If you are using a custom terminal block, you must provide this interlocking
feature on your terminal block so that it is possible for the SCXI-1127/1128
to detect that a terminal block is present. Please refer to Appendix D,
SCXI-1127/1128 Front Connector, for information on how to do this.
2-Wire Channel Scanning Configuration
You can use the SCXI-1127/1128 as a 32-channel, 2-wire (differential)
multiplexer. Figure 2-2 shows the block diagram of the input switching
structure in 2-wire mode. You can connect up to 32 channels to the
SCXI-1127/1128 using the SCXI-1331 terminal block. The input signals
are switched using a 32 to 1 multiplexer. The output of the multiplexer is
available at the OUT0+ and OUT0- screw terminals on the SCXI-1331.
Note OUT0± is also referred to as the multiplexer common or COM0±.
Figure 2-2. 2-Wire Block Diagram
You can also connect the output of the SCXI-1127/1128 to the high-voltage
backplane using switch AB0 to make the connection. On power up, switch
AB0 is open, disconnecting the SCXI-1127/1128 from the high-voltage
bus. If you are using the module with a high-voltage backplane, you need
to close the AB0 switch to connect the multiplexer output to the
high-voltage analog backplane. You can do this through NI-SWITCH.
HVAB
Backplane
32-to-1
Multiplexer
AB0+
AB0
Switch
AB0–
COM0+
COM0–
Out0+
Out0–
CH0+
CH0–
CH31+
CH31–