Siemens SIMATIC 505 - Introduction; Overview of the Advanced Functions; Advanced Software Functions

Advanced Function Programming
3-2
SIMATIC 505–2556 Installation and Operation Guide
3.1 Advanced Software Functions
As PLC control systems become more complex, the need for real-time
processing of analog signals is needed at the I/O level. Current
implementations using the 505 controllers utilize analog alarm blocks
and/or special function programs within the controller . The SIMA TIC
505–2556 analog input module from Siemens Energy & Automation, Inc.,
can reduce the program complexity and scan time by performing this signal
processing in the module.
Scaling, alarming, peak/valley hold, digital filtering, and averaging are
available on a per
-channel basis and are selected through a simple PLC
configuration routine. When these advanced functions are enabled, the
module logs in as 16X / 16Y / 32WX / 32WY . A jumper on the module selects
the standard 16WX login or the high-density advanced function interface.
Each of these functions can be selected on a per -channel basis, and each
channel can have any function in any combination, e.g. alarming on a scaled
value which is digitally filtered and set for peak hold. (See Section 3.4 for
timing considerations.)
Scaling Each channel can be configured with low and/or high scale value.
A flowmeter that outputs 0 mA @ 5 cfm and 20 mA @ 50 cfm would have a
low scale of 5 and a high scale of 50. An operator interface attached to the
controller could then read the analog values directly in engineering units
without having to run a Special Function program to scale the input.
Alarming Each channel can be assigned a low and/or high alarm value.
No analog alarm blocks are needed in the controller. Alarming occurs
real-time as the signal is processed by the module. T wo WX words are used
to indicate high and low alarm conditions (bit 1 = channel 16, etc.). A third
WX word is the logical OR of the high and low alarms.
Peak/valley hold The peak or valley of a rapidly changing analog signal
has been impossible to detect unless an external circuit was used. The
SIMA TIC 505–2556 makes possible the detection of a peak or valley and
holds that value until reset by the controller . The peak/valley measurement
is available to the controller at the same time as the currently measured
analog value.
Averaging This option is used to “clean up” a signal that is at a steady
state, e.g., a sensor riding on a liquid tank with riplets. The user specifies
how many signal scans to average and this value is presented to the
controller.
Digital filtering This has the effect of a moving average operation
(actually it is an Infinite Impulse Response filter), and is useful to smooth
out the high frequency noise on a changing analog signal. See Section 3.4.
Introduction
Overview of the
Advanced
Functions