Rockwell Automation T3420AF ICS Regent Analog Input Modules - Manual (Page 7)

Analog Input Modules (T3420A, AF)
P D - 6 0 2 3 M a r - 0 6
7
Sampling Speed
The rate at which each of the analog input channels is
sampled depends upon the mode of operation. Each channel
is given a 225
m sec time slot during the multiplexing process.
During that time slot, the input voltage is connected to the
measuring system, the measuring system is allowed to settle,
the settled value is sampled (and held), and is then converted
to a 12-bit value by an analog-to-digital converter. The
channels are sampled in numerical order. In single-ended
mode, the 16 channels are sampled in 3.60 msec on a
continuous repeating basis. In differential mode, the eight
channel pairs are sampled in 1.80 msec on a continuous
repeating basis. Converted data are stored in the module’s
RAM and can be accessed by the controller. Data for a given
channel is available at the previously stored value until an
updated conversion replaces it.
Sampling speed is unrelated to internal access speed within
the Regent system. Programming and other features
determine how often the Regent system can access the
converted data. There are no “lockout times” when the Regent
system is prevented from reading the current stored values of
the converted data.
Single-Ended References
When single-ended input mode is selected, the ground
reference for all eight signals on the same input field wiring
terminal block is AREF1 on the top terminal block and
AREF2 on the bottom. This ground reference may be
different from the external I/O analog ground AGND also
available on the same connector. When used with the analog
input terminal block assemblies (T3325-XX), both AREF1 and
AREF2 are isolated from AGND by 1K Ohm resistors.
When the single-ended signals are measured, the appropriate
ground reference for the signal is selected by another
multiplexer on the board, and becomes the reference for the
measurement. In differential mode, AREF1 and AREF2 are
not used as references.