Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 117)

Performance and Theory of Operations
Chapter 7
7-12
Figure 7.1
Remote I/O Scan and Program Scan Timing Loops
a b
Adapter
Adapter
Adapter
IOT (x)
IIN (y)
x
y
Rack 3
Rack 2
Rack 1
Housekeeping
Scan
Logic
Data
Exchange
Immediate I/O ➀
Data Exchange
I/O Image
Table Remote I/O
Buffer
Program Scan TimingLoop Remote I/O Scan Timing Loop
➀ The processor responds to immediate input (IIN) and immediate output (IOT) requests during the
logic scan. The logic scan is suspended at the request for immediate input/output data. The logic scan
resumes after obtaining the data and fulfilling the request.
IIN and IOT data transfer directly to and from I/O modules in extended-local I/O chassis.
With remote I/O, only the remote I/O buffer is updated.
For more information, see the instruction quick reference in chapter 22.
Update
I/O Image
a
write outputs
b
read inputs
Extended
Local
I/O Rack
During the housekeeping portion of the program scan, the remote I/O
buffer is updated. Remember that the I/O scanner is constantly updating
the remote I/O buffer asynchronously to the program scan.
PLC-5/VME processors can transfer discrete data and block data to/from
processor-resident local I/O, extended-local I/O chassis, and/or remote
I/O chassis.
Transferring Discrete Data
The remote I/O system is scanned in a separate and asynchronous scan to
the program scan. The remote I/O scan takes output data from the remote
I/O buffer to output modules and puts input data into the remote I/O buffer
from input modules. The remote I/O scan time can take 3, 6, or 10 ms per
one rack in a chassis on the remote I/O link, depending on baud rate. The
PLC-5/VME processor then exchanges the input and output image table
data with the remote I/O buffer during the I/O-update portion of
housekeeping.
Discrete and Block Transfer
I/O Scanning