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Modbus TCP

 
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oxo



Joined: 13 Nov 2012
Posts: 219
Location: France

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Modbus TCP
PostPosted: Tue Aug 18, 2015 12:57 am     Reply with quote
Hi guys,

In the CCS modbus code, there is a 5 minute timeout in <modbus_app_layer_tcpip.c> in ModbusTCPTask(), case MODBUS_TCP_STATE_CONNECTED.

Any ideas why this is there? It contravenes the Modbuf Implementation guide http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf

Quote:
page 10 "It is recommended to keep the TCP connection opened with a remote device and not
to open and close it for each MODBUS/TCP transaction"
RF_Developer



Joined: 07 Feb 2011
Posts: 839

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Re: Modbus TCP
PostPosted: Tue Aug 18, 2015 2:36 am     Reply with quote
oxo wrote:
Hi guys,

In the CCS modbus code, there is a 5 minute timeout in <modbus_app_layer_tcpip.c> in ModbusTCPTask(), case MODBUS_TCP_STATE_CONNECTED.

Any ideas why this is there? It contravenes the Modbuf Implementation guide http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf

Quote:
page 10 "It is recommended to keep the TCP connection opened with a remote device and not
to open and close it for each MODBUS/TCP transaction"


Contravene is rather a strong word in this context. You can't contravene a recommendation. Also unless the specification specifies otherwise, I'd take the term "transaction" in the modbus context as being a command and response pair, not an open-ended connection, which I would term a "session", but I doubt Modbus, which was originally developed pre-TCP/IP, has such a concept.

I didn't design or write the CCS Modbus code, and it certainly has a few quirks and non-conformances, but the idea that any TCP connection, once opened, is there forever come what may, is nonsense. An inacticvity timeout, which is what a five minute timeout sounds like, sounds a very sensible precaution to me. Furthermore, if there really hasn't been any activity after that time, a transaction can hardly be said to be still taking place.

Given that, I'd suggest keeping the connection alive with an occasional "heartbeat" or "keepalive" command. Some read of a status register or something, every ten seconds or so maybe.

I certainly won't clamour for a change to the driver to remove what I feel to be a pretty sensible precautionary timeout.
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Tue Aug 18, 2015 2:43 am     Reply with quote
and in fact, I don't think the timeout even disagrees with the recommendation.

The recommendation is saying not to have a system that makes a connection, does one transaction, and then breaks the connection. Disconnecting if nothing at all is happening, does not disagree with this....
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