Rockwell Automation 1747-DSN DISTRIBUTED I/O SCANNER - Manual (Page 11)

Chapter 1
Introduction
1–4
The SLC processor scan consists of an input, program, and output scan.
During the input scan, the scanner input file (which contains the on/off input
status of all configured I/O blocks) is read into SLC processor memory.
During the SLC program scan, the input information is used by your
application program. An SLC output file, based on the logic of your
program, is then written to the scanner during the output scan.
The scanner’s scan consists of reading and writing data to all I/O blocks. For
example if three I/O blocks are attached, the scan consists of three read/write
cycles.
The SLC processor scan and the scanner scan operate asynchronously, or
independent of each other. The SLC processor reads the scanner input file
during its input scan and writes the output file to the scanner during the
output scan. The scanner continues reading inputs and writing outputs to the
scanner input file, independently of the SLC processor scan.
Depending on your network and application program size, it is not
uncommon for the scanner to complete a number of scans before the SLC
processor reads the scanner’s input file.
The figure shown below illustrates the asynchronous operation of the SLC
processor and scanner.
Output 3
Output 2
Output 1 Input 3
Input 2
Input 1
Output Input
I/O block 1 I/O block 2 I/O block 3
SLC Processor Scanner
SLC Processor Scan Scanner Scan
Program
The scanner updates its input file
each time it scans an I/O block. The
scanner may scan all three of its
configured I/O blocks many times
before the SLC Processor reads the
scanner’s input file. The SLC processor reads the
scanner input file into the SLC input
file, processes it and creates an SLC
output file. The SLC output file is
transferred to the scanner during the
output scan.
SLC Input File
Scanner Input File
SLC Output File
Scanner Output File
How the Scanner Interacts
with the SLC Processor