Modbus Connection Problem - S7-1500

Having some issues initiating a connection to a S7-1500 PLC over Modbus. I have an OB created with a MB_SERVER block and a configured port (Port 505). I downloaded ModScan and was able to connect to IP address and Port 505; I could both read/write values.

When I later re-opened ModScan, I could no longer make a connection without any PLC changes.

Anyone know why it connects intermittently?

Not sure about the 1500's, but the 1200's will stop communication over Modbus if the connection is idle for a period of time. I can only get them going again with a full download of the program. FYI, the usual default port for Modbus is 502.

One possibilirty

I assume you are talking about an Ethernet link...

If its during a continuous session, you could have filled some buffers. Either the outgoing buffer from your client or the incoming buffer from the server side.

When this happens, STOP the client requests and go have a coffee and 5 minutes later IF the conversation cannot be re-established, then its not it.

Their also is some very good video on YouTube about setting up such com.


If its serial MB the SNOOP the com port, you should see what is going on since the com stream is very easy to figure out when you see one...

In your MB block you should also have an error code.


COM setup is also important... 8N1... or what?
Cable and wires is also a source of error...


RSVP so we can figure this out... should no be that big of a problem...