basp signal in siemens step5 135u cpu 928 plc


Thread Starter


basp signal is always on in the run mode therefore all outputs are inhibited. how to disable it for activating output signals?

thank you

Donald Pittendrigh

Remove all software and reset the PLC, see if BASP light is still on, if it still comes on, then check the bridges in the power supply, if it cannot be resolved then the problem probably lies with the interface cards to the I/O racks, disconnect all IO from the CPU, clear the CPU with a programmer and do a cold restart, (you must do this even though you have already erased the memory, there is a DB which is generated according to the IO present), put the IO racks and cards back one at a time and restart at each major step, until everything is working or you have found the faulty card. There is also a method for doing this with the ISTACK, but if you don't know how to do this, then the method I have described is better. Your PLC will probably benefit from being stripped and reassembled anyway.

If the problem is not hardware from the outset, and it probably isn't, then having deleted all software, and found the BASP light off, you can reload the program, and edit the first line of every OB to insert the statement BEU, this must make the light go off as no program will be running. Now start to move the BEU statement down the calling structure of the OB until you isolate the block that makes the light come on. When you have identified the block, you can open it and use the same tactic of inserting a BEU until you find the exact location of the problem statement. Once again there is a way to do this with the ISTACK but it is a bit involved and I am assuming if you had the ability to do this then you wouldn't be asking the question here, as you would already have the answer.

A few important considerations before you start with this, please switch off the power or otherwise disable the machine before you start the tests I have described. It is extremely dangerous to edit the program in this way on a live plant, it is also extremely dangerous to run the machine on 1/2 a program as you may enable the 1/2 that starts something, and the other 1/2 that takes care of stopping it or taking care of safety interlocks may be disabled, with obvious consequence.

If this does not solve your problem, then mail me at donaldp(at) and I will give you instructions on how to do more advanced testing to identify the problematic component.

Cheers DP

Erwin Libonati

There are differnt causes for the basp .

the most common mistake is that you you are looking for a I/O in your program that does not exixt in your rack .

For a compleate List of causes plase refer to the Systems Manual for the 928CPU . You can find it on the Siemens Web site.


From the 928B manual:

This indicates whether the S5 bus signal BASP (disable command output) is active: In the single processor mode, the CPU clears BASP when it changes to the RUN mode and sets BASP when it changes to the STOP mode. BASP is activated in the RESTART and in the STOP mode and in the first cycle following a warm restart. In the multiprocessor mode, the conditions for BASP are identical with those in the single processor mode, provided the switch on the coordinator is set to RUN. (See your System Manual (/2/ in Chapter 13) for more information on the "Test mode" special case.) Note If BASP is active, all digital outputs are disabled. If an AUTOMATIC or MANUAL WARM RESTART has been executed before the transition to the RUN mode, the BASP LED goes out only after the remaining cycle has been processed.

There is a reason the BASP is on. Try using ISTACK to see if processor is in STOP, then find out where in the program the the stop is coming from.

good luck!

Omar Gonzalez


Have you ever found erased EPROM in CPU 928B?
The CPU has three EPROM.
How can I get the copy of one of this EPROM?

Thank You

Donald Pittendrigh

I have not looked at a 928B for quite some time, but I only recall 2 eproms, one is the system eprom and looks like a credit card size memory module in the middle of the CPU, the other one is larger and plugs in at the top of the CPU module. The one in the middle you cannot program yourself, you would have to purchase a new one from Siemens, which raises the question of how you determined it is empty, and the other contains the user program, which you load onto it with a programmer or a PC with an external prommer, which one is empty in your case, and where is the third Eprom?

Donald Pittendrigh