Understanding M Files - Allen-Bradley 1747-SN User Manual

4–12 Scanner Configuration and Programming

Understanding M Files

Publication 1747 6.6 - July 1996
M Files Overview
The scanner provides RIO device control and status information
through the M0 and M1 files. The M0 file is a control file. The M1
file is a status file
The SLC processor does not automatically update M file data during
the end of the program scan as it does I/O scans. Instead, M file
values act as interrupts and are immediately read from or written to
upon the execution of the ladder logic instruction in which they are
used. When M-file data (bits or words) is addressed in the ladder
program, the processor stops scanning the program to read or write
the M-file data to/from the scanner module. M-file bits/words in the
ladder program will, therefore, impact the ladder scan time. If scan
time is critical, it is better to set binary file bits and copy them all at
once to the M0 file, or copy a portion of the M1 file to a binary file
and then address the binary file in the program. Refer to the ladder
example that follows. For more information on M files, refer to
appendix B. You can find M file information relating to Block
Transfer operations in chapter 5, Block Transfer.