Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 116)

Performance and Theory of Operations
Chapter 7
7-11
I/O Scanning
The remote I/O scan cycle is the time that it takes for the processor
(configured as a scanner) to communicate with all of the entries in its rack
scan-list once. The remote I/O scan is independent of and asynchronous to
the program scan.
The scanner processor keeps a list of all of the devices connected to each
remote I/O link. An example system would look like this:
PLC-5/V40
Rack 1
Rack 2
Rack 3
Ch 1A
Ch 1B
Ch 1B Scan List
Rack
Address Starting
Group Rack
Size
Range
1
2
3 0
0
0 Full
1/2
Full 010-017
020-023
030-037
In this example, channel 1B continually scans the three racks in its scan list and
places the data in the remote I/O buffer in the processor. The processor updates its
own buffer and the I/O image table. During housekeeping, the two buffers are
updated by exchanging the input and output data with each other.
Important: The remote I/O scan for each channel configured for scanner
mode is independent and asynchronous to the remote I/O scan for any
other channel.
Figure 7.1 shows timing loops for discrete data transfer in a
PLC-5/VME processor.