Analog And Timing Circuitry; Analog Input Channels - National Instruments SCXI-1121 User Manual

Theory of Operation Chapter 3
The Module ID Register connects to MISO on the SCXIbus. The Module ID Register is an 8-bit
parallel/serial-in serial-out shift register and an SPI communication adapter. The contents of the
Module ID Register are written onto MISO during the first four bytes of transfer after SS* has
been asserted low. Zeros are written to MISO thereafter until SS* is released and reasserted.
The SCXI-1121 module ID is hex 00000002.

Analog and Timing Circuitry

The SCXIbus provides analog power (±18.5 VDC) that is regulated on the SCXI-1121 to
±15 VDC, a guard, an analog bus (AB0±), and a chassis ground (CHSGND). AB0± buses the
SCXI-1121 output to other modules or receives outputs from other modules via the SCXIbus.
Refer to the Calibration section later in this chapter for more information. The guard guards the
analog bus, and can be connected via jumper W33 to the analog ground reference or can be left
floating (a connection can be made by another board).
The data acquisition board analog input and timing is the interface between the SCXI-1121
output and the data acquisition board. This is fully described in the following section.

Analog Input Channels

Figure 3-5 is a diagram of the analog input block.
SCXI-1121 User Manual 3-8 © National Instruments Corporation