Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 49)

Chapter
3
3-1
I/O Communication
This chapter describes timing and programming considerations when using
the scanner for I/O communication and helps you learn how:
I/O scan affects ladder program execution
to calculate block-transfer times
to program and calculate times for a peer-to-peer or backup
communication channel
You must remember that in scanning the ladder program and the I/O:
input changes do not instantly appear in the input image table. The
length of time between an input change and the update of the input table
depends on the input module delay, the signal propagation time,
backplane access time, and the I/O scan time.
the I/O scan is not synchronized with the ladder program scan.
Therefore, the same inputs can be in different states at different times
during the program scan.
Calculating the I/O Scan Time
The amount of time that the scanner takes to scan all the I/O on a given
channel depends on the communication rate and the number of:
entries in the I/O chassis scanning sequence list
active channels on the scanner
block transfers
retries required due to noise
Table 3.A shows the scan times for one I/O adapter on each active channel
with no block transfers.
Chapter Objectives
Effect of I/O Scan on
Program Execution