Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 119)

Performance and Theory of Operations
Chapter 7
7-14
Figure 7.3
PLC-5/V40L Timing Loops for Discrete Data Transfer
Adapter
Adapter
Adapter
Housekeeping
Rack 3
Rack 2
Rack 1
Remote I/O
Buffer
Logic
Scan
Remote I/O Scan
Timing Loop Program Scan
Timing Loop
IOT (x)
IIN (y)
Immediate I/O
I/O Image
Table
Data
Exchange
Update
I/O Image
Data Exchange
a
b
a
write outputs
b
read inputs
x
y
Extended
Local
I/O Rack
Immediate I/O
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 transfers 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.
Transferring Block Data
The exchange of block-transfer data and the logic scan run independently
and concurrently. The following sections explain block-transfer for
extended-local I/O and then for remote I/O.
Extended-Local I/O
Requests for block-transfer data occur during the logic scan. Concurrent
with the execution of the logic, block-transfer requests are forwarded to the
appropriate 1771-ALX adapter module(s) and data is transferred. A
1771-ALX adapter module may start block-transfer operations to multiple
slots and have block-transfer data transactions on-going in parallel within
the 1771 chassis.