Problem with OP7 and 95U


Thread Starter


Hi Mr. Griffin,

I'm in a real emergency condition in which I can't get connected with my OP7. I get the alarm: "PLC not present" I have read all articles about using fb51 and ob21, 22, DB51. But I don't have FB51. Even I load the PLC functions, but it only has 240 to 252. How do I can find or how can I program it. Why every one don't explain it full. Is it a legacy??? Please help me!

I want to monitor some analog values and order some bits and analogs to PLC. I have written the PLC program and OP program and just remains interface.

marc sinclair

Yes the 95U and the OP7 are 'not design types' Siemens don't use the word obsolete, they just keep pushing the prices up until designers get the hint. you could use an s7-200 and a TD and save money and you wouldn't need the silly S5 disc tokens

Marc Sinclair

Michael Griffin

FB240 to FB252 are built in to the PLC. FB51 isn't. You have to get it from Siemens and load it into your program. It may have come with your OP software (along with FB52, FB53, etc.). There are different FB5? blocks for different methods of communications, and versions of FB51 for different models of S5 PLC, and you have to use the correct one for the model you are using.

Since you said the OP7 was already connected to the previous S5-95U, there should already be a call to an FB5? in the program already. You just need to either replace the current FB5? with FB51, or if it is already using FB51, change the way it is called. If I recall correctly, it should look something like the example below.

L KY51, 0 JU FB51

The KY51 value is the interface DB (DB51 in this example), but needs to match what is actually in your program (most people use a DB which matches the FB number). If you change the DB number, you have to create the new DB and also change any other parts of the program which read or write to it (e.g. for initialisation). The ",0" though should be as written.

Also check the OP to see what communications method it is using. You can usually change this through the built in menus (unless the person who programmed it erased the set-up menus). You should check what the default is set to in the OP program as well (using the OP programing software).

Michael Griffin

Further to marc sinclair's reply: The problem being addressed is a repair situation, where an S5-95U is being replaced with a similar, but not identical model. Replacing it with an S7-200 would require a new program and replacement of all I/O. This requires a good deal more expense and especially downtime.

However, if someone does decide to replace an S5-95U with an S7-200, I did a study a few years ago which indicated the two were equivalent in capability, except for the S5-95U's on-board analogue I/O. In addition, the S5-95U does have a bigger variety of special I/O modules available, although these were in practise seldom used.

The S5-95U does have more RAM than the S7-200, allowing a "larger" program (as measured in bytes). However, the instruction set of the S7-200 allowed an equivalent program to be written with fewer instructions than is required for an S5. The end result (for the types of applications that I studied) was that an S7-200 program was approximately half the size of an equivalent S5 program, cancelling out the S5's larger memory.

Overall, unless the application requires special I/O modules which are not available for the S7-200 (or which can't be replaced by the S7-200's on-board I/O), the S7-200 is at least as capable as the S5-95U.