interface with CPU 941 using step 5


Thread Starter



I am using Siemens PLC S5-11U and its hardware consists of power supply and CPU 941 plus I/O cards. I tried several times to interface online with the CPU 941 using PC loaded with step 5 software package but an error message "Undefined character from AS" comes up and in the mean time I do not have PG.

Can any one advice me how can I interface with CPU 941 using step 5 package. Please reply to "[email protected]", mailto:[email protected]

Thank you in advance for you help.
The error message you are getting is because of noise problem, check the cable for proper shielding, check the PC for proper earthing.

Hakan Ozevin

Place the CPU into STOP state. If you still have this problem, it must be a noice problem as stated earlier.

Andrew Hawdon


The problem could be due to a number of factors. Check the following to establish the problem

1. Check cable is the correct cable type. A TTY cable is required. The RS232 to TTY convertor is normally built into the cable in the D25.

2. What is your operating system and version of Step 5. For windows 95 you require at least ver 6.6. For 98 It is Ver 7.02 and for NT ver 7.12.

3. Step 5 must be the Basic version not the Mini version. Mini will not communicate to a 115. You can find this info under Help.

4. If all the above checks out then try running Step 5 in Dos mode as some PC's have problems running step 5 in Windows.


Hakan Ozevin

Dear Andrew, what you have written are correct for general connection problems, however you do not get a message as "illegal character from the AS" in those cases, except for case 3 which I am not sure, but you will get "PLC timeout" message. I met this illegal character message several times, if the port is not correctly configured (e.g. second port of S5 95U).
The message you are getting can be becauase of an incorrect version of S5 software. Are you trying to communicate using a normal PC with the S5 version for PG. Do you have the correct cable for communication because for a PC you need an adapter for your com port in order to communicate with the PLC using the S5 comm. cable.
Try the obvious thing first. Have you got your lead plugged into the right port on the CPU. Its the one thats the hardest to get to, set back from the front of the CPU.