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Enhancements in Late Model CPUs

F-2

SIMATIC 545/555/575 System Manual

F.1 Enhancements to Error Handling

For the 555–1103/–1104 CPUs Release 4.2 or greater, 555–1105/–1106 CPUs

Release 5.2 or greater, 545–1105/–1106 CPUs Release 5.1 or greater, and

575–2105/–2106 CPUs Release 5.1 or greater, the following changes in the

firmware have been implemented to minimize problems caused by excess

noise, as described below.

A few SIMATIC 505 customers have experienced an occasional CPU fatal

error (typically several months or more between failures on a specific CPU)

that usually results in a TISOFT or SoftShop message such as “Program

Memory Diagnostic Error.” These fatal errors generally occur in an

environment that includes various equipment adjacent to the PLC such as

UPS systems, variable speed drives and battery chargers. These types of

equipment often contain switching power supplies that can be sources of

high frequency electrical noise spikes, which may occasionally induce such a

fatal memory error condition.

While it is preferred that this electrical noise be eliminated through

suppression or dissipation in an adequate grounding system, this may be

difficult to effect in many cases. At the same time, there are many

customers who place a high value on “up time” and cannot justify a fatal

error unless it has a direct effect on the CPU operation.

The changes described in this section should provide a better solution for

affected customers, since this may reduce or even eliminate the problem for

them without the need to make changes to their systems. However, for most

of the customers who do not have this type of environment and have never

experienced a Program Memory Diagnostic Error, there will be no noticeable

change.

When the SIMATIC 505 CPU diagnostics were originally written, it was

assumed that any memory error found was an indication of a hard memory

chip failure, and the conservative approach of declaring a fatal error and

shutting down the CPU was taken, even for “soft” memory errors caused by

noise spikes. In addition, the fatal error response was applied to all program

memory, whether or not it was being used to store program data.

Therefore, to improve up time in the event of any soft memory errors,

Siemens has made the following changes in the SIMATIC CPUs:

1. The checksums that are used to determine if there is an uncommanded

memory change problem will be calculated only on the L (ladder) and

CL (compiled ladder) memory that is actually used. Errors discovered

in unused portions of memory, whether configured or unconfigured, will

be ignored. (Refer to the SIMATIC 545/555/575 Programming

Reference User Manual for a more detailed discussion on SIMATIC 5X5

CPU memory architecture.)

CPUs with

Enhanced

Firmware

Fatal Error

Conditions

Changes in

Memory Error

Handling

1 2 ... 302 303 304 305 306 307 308 309 310 311 312 ... 321 322

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