Product specifications

Enhancements in Late Model CPUs
F-4
SIMATIC 545/555/575 System Manual
F.2 Fast PROFIBUS-DP I/O Update Times
The 555–1105/–1106 CPUs Release 5.2 or greater, the 545–1105/–1106
CPUs Release 5.1 or greater, and the 575–2105/–2106 CPUs Release 5.1 or
greater, support the new Fast PROFIBUS-DP I/O Update feature.
The CPUs listed above provide the ability to do faster PROFIBUS-DP I/O
updates using IORW box instructions, immediate contacts, and immediate
coils. Although these instructions existed previously in the CPU, they only
supported access to I/O points installed in the local 505 base. With this
release, these same instructions can now be used to access the memory bank
that is shared by both the main CPU and the PROFIBUS-DP master
coprocessor more often than once per RLL scan, which is how it was
previously.
The new Fast PROFIBUS-DP I/O feature provides the capability to do
multiple updates of analog and discrete PROFIBUS-DP I/O points per RLL
scan where a large program has increased the RLL scan to a point that it
can no longer provide fast enough control for certain operations connected
by PROFIBUS I/O. This is easily accomplished by placing these immediate
instructions in a cyclic task (Task 2) that is scheduled to operate at a faster
rate than the RLL executes.
To be effective, the fast PROFIBUS-DP I/O update feature requires the
PROFIBUS-DP channel update rate to be set sufficiently high. For example,
at 12 Mbaud, an asynchronous PROFIBUS-DP I/O channel will generally
update and place new I/O information in the shared memory buffer every 2
to 3 milliseconds.
Normally, a large RLL program may require 100 or more milliseconds to
execute. Even taking into consideration the small amount of time of most
I/O points to activate and the time for the PLC to make reasonable
calculations, the fast PROFIBUS-DP I/O feature provides the opportunity
for several I/O updates per RLL scan that could make a difference in
whether or not the remote process could be properly controlled.
To use fast PROFIBUS-DP I/O update, first set the PROFIBUS-DP mode to
“OPERATE ASYNCHRONOUS.” Then use any valid I/O point in the
PROFIBUS-DP configuration as a parameter in an Immediate instruction.
Upon a transition from Program to Run, the CPU’s RLL compiler
automatically searches the PROFIBUS-DP configuration when compiling
IORW box instructions, immediate contacts, and immediate coils and
generates special code if a match is found between an Immediate
instruction-operand and a DP-configuration entry. This code automatically
implements logic that allows immediate program access to/from the shared
DP buffer.
CPUs with this
Feature
Fast PROFIBUS-DP
I/O Updates
How it Works
Selecting Fast DP
Update Mode