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oxo
Joined: 13 Nov 2012 Posts: 219 Location: France
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Modbus TCP |
Posted: Tue Aug 18, 2015 12:57 am |
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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" |
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RF_Developer
Joined: 07 Feb 2011 Posts: 839
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Re: Modbus TCP |
Posted: Tue Aug 18, 2015 2:36 am |
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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" |
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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. |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19382
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Posted: Tue Aug 18, 2015 2:43 am |
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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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