Modbus TCP Exception Code 06 - Slave Device Busy

[harryting]

Well, probably have to take a road trip for this but I'm not understanding something more fundamental about Modbus TCP. I thought it can handle multiple Master (client) polling at the same time, right?

So situation is this. We have 3 of the same Modbus TCP devices that talks to kepware. Now trying to setup a new Kepware system while the old Kepware is still running. The new Kepware can't get comm to the 3 devices. I have the local guy try using a plain Modbus scanner on the new machines and got the exception code 06-Slave Device Busy for all 3 devices.

This is something unexpected, again, since I thought Modbus TCP server should have no problem answering to multiple clients?

 

[OkiePC]

That is going to be specific to the device. I believe that, for example, a Click PLC can only handle up to 3 Modbus TCP clients.

 

[harryting]

Thanks, didn't know that.

The manual for the device says there's a max of 4 modbus TCP "connection" limit. As far as I know, there are only at most 2 other kepware server talking to them. I guess I can do a tap and wireshark the thing if I get out there.

 

[Lare]

There was setting on kepware for how many simultaneous connection kepware can open to one slave. (max sockets per device)
You should check it if one kepware is takin all sockets for speeding up communication.
On some devices sockets are also on different TCP ports so you have to use different port for every connection.

 

[lfe]

A Modbus TCP slave could theoretically reply the requests in asynchronous mode, that is, the master sends new requests before waiting to receive the reply from the previous ones.

In Modbus TCP the transmission incorporates a transaction number so the master can identify to which request a reply corresponds.

But this is not supported by all Modbus TCP slaves and some do not admit a new request before having sent the previous reply ... maybe this is your problem

 

[harryting]

Lare, thanks, I will have the tech check the socket setting.

Ife, thanks, I think that maybe it.

This particular device has always been a bit flaky with it's Modbus implementation so I wouldn't be shocked if it can't handle multiple masters.