Siemens TD 390


Thread Starter


Hi all.

I am using a Siemens S5-115U PLC with a 941CPU, and I decided to use a TD 390 text display to display error messages coming in from a plant. It is the first time that I used the TD 390, and I wonder if anyone can please help me with some information.

At first I wrote my whole program (Consisting out of 4 PB's of about 33 segments each, as well as OB1,21,22). Then I put in the error conditions in PB5. It worked, but the strange thing is that as soon as a error code was to be displayed, it took about ten seconds before the message was displayed, and then it also showed on the screen for only about 2 seconds. I then swopped PB1 and PB5, and the problem was solved. The error messages was displayed immediately, and they stayed on the screen until the error condition went false. Can anyone tell me why this is so ? (Note : The scan time on the TD was set to 2 seconds).

Michael Griffin

I'm not familiar with the TD 390 in particular, but I have used a lot of the Siemens OP/TD text series.
I would guess from your description that you are using "events" to display messages. If this is the case, then provided there is nothing else with a higher priority currently being displayed, you should see the event being displayed immediately.
I would guess that you have a bug in your PLC program. The fact that it dissappeared after you swapped two PBs suggests that the word(s) you are using for events are getting over written by something else. The 10 second delay probably is due to the OP occasionally (at random) seeing the event before it gets over written again.
Something to note here is that the FB drivers for OP/TD interface supplied by Siemens do use a few words in the scratch flags (FW200 to FW254) area for temporary storage. Siemens defines this memory area as to be used as a a scratch memory area only - not for permanent storage. If your program is using any of these, this might be the cause of some of your problems.

Michael Griffin
London, Ont. Canada
[email protected]