Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 122)

Calculating Program Timing
Chapter 9
9-5
A Classic PLC-5 processor can transfer discrete data and block data
to/from processor-resident local I/O, extended-local I/O chassis, and
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 processor then exchanges the input and output image table data
with the remote I/O buffer during the I/O-update portion of housekeeping.
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 processor-
resident I/O and 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 paragraphs explain block transfer for
extended-local I/O and then for processor-resident I/O and remote I/O.
I/O ScanningĊDiscrete
and Block Transfer