Siemens S5 RAM backup when loaded with EPROM

  • Thread starter PVCInstrument HZ
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PVCInstrument HZ

We are facing a peculiar problem in Siemens S5 115U CPU943D. When we load the program from the backup which we have in our PC, the program runs and even after power failure it reads from the RAM which is battery backed up. But while we
load through EPROM, it runs but after power failure and EPROM in unload condition it did not take from the RAM which is normally done.

V G Karthikeyan.

Hakan Ozevin

This is the normal behaviour of 115U CPU's (except for CPU945 I think). The CPU creates a list about where the blocks are loaded from (EPROM or RAM) and expects that media. So you should not remove the EPROM. Contrary to 90/95/100U series, 115U EPROM module is not regarded as a back up media that can be removed, it should always be plugged in.

Hakan Ozevin

D. Pittendrigh

Hi All

There are a few possibilities. I f you run in RAM, all is as it should be. If you run off an EPROM, there are some issues, if you load your DB's in the EPROM then the PLC program cannot change them, so you have to generate them in RAM on startup. This is done in OB21 and 22 one of which is used for a warm restart using the keyswitch, and the other cold restart after a power failure. If you are already doing this, then you may be using the wrong OB to generate the blocks. In either case if you have the battery in the DB's will not be lost after a restart and you should not re-generate them, this
is a simple test you do in the DB generate program and is described in the manuals.

The command to generate the blocks is G DB??, there are some numbers that have to be loaded into the accumulators to define the DB length and number before executing the command, look this up and try it out, if it doesn't solve the problem get back to me at email:[email protected]

Good luck
Donald Pittendrigh

Hakan Ozevin

What you wrote is another possibility for the problem, since the author did not write the symptoms in details.
I want to note that in some cases the user does not want to generate a DB, since it will be full of zeros, thus the latest variables will be lost. In such a case, he should write all blocks except DB's into the Eprom and transfer the DB's to the RAM via the PG port. The disadvantage of this solution is, the program is not really backed up. If the DB's are lost due to lack of power and battery, CPU stops when a DW from the DB is called. As an intermediate solution, he can generate a DB and load-transfer some default values for the variables in OB21 and OB22, if he has enough memory available.
This behaviour of 115U PLC's are the worst part that makes servicing difficult. I think that the designer had cost concerns where RAM's for the PLC's were expensive (especially compared to EPROM's) and was trying to make the system cheap for very basic usage, but upgradable (The first CPU, CPU 941 had/has 2 kB RAM and upgradable with 16 kB RAM or EPROM. That was year 1984-1985).

By the way, format for generating a DB is:
L KF x
G DB y
which creates DB y with length x. If x=0, that DB is deleted.

Hakan Ozevin