Rockwell Automation 1746-NI16V SLC 500 Analog Input Modules User Manual - Manual (Page 80)

Publication 1746-UM001A-US-P
7-2 Application Examples
In contrast, Class 3 mode supports up to 32 I/O image words. Class 3
mode can be utilized with SLC 5/02, 5/03, 5/04, 5/05 and also in a
remote ControlNet chassis (1747-ACN, ACNR). No multiplexing of data
is required, making configuration and reading of status and analog
data more straightforward.
The following ladder program enters subroutine File 3 at power-up.
File 3 contains logic to configure all 16 analog channels as well as
receive and store the 16 status words for each channel. The status
words contain success/failure error codes as well as reflecting back
the same configuration information contained in the configuration
words for each channel. These status words for 16 analog channels
are stored in data table words N7:66 through N7:81 and are
bit-mapped. Therefore, to understand them, they must be viewed
in the Binary Radix. Refer to Chapter 5 for an explanation of each
bit in these words.
File 3 configures analog channels 0 to 7, then as each of these channel
configurations complete, it configures analog channels 8 to 15. When
all 16 channels are configured, subroutine File 3 is no longer scanned.
Rungs 2:2 and 2:3 are then alternately enabled to first read the analog
data from channels 0 to 7, then from channels 8 to 15.
The program continues to alternate between reading analog data
from the first 8 channels, then the last 8 channels, and then storing the
data in 16 consecutive words beginning with N7:50. The 16 analog
words from N7:50 through N7:65 are in decimal and for this
0 to 10V dc example are represented by the decimal range 0 to 32767.
The reason for the fairly complex ladder logic to configure, read status
and then read analog data from the NI16 module in Class 1 mode is
due to the need to multiplex 16 channels with 8 I/O words.