[collectd] Modbus support

[Matt Baker]

Hi Florian,

If you have a look at http://en.wikipedia.org/wiki/Modbus you can see the difference in frame format.
Basically there are three variants:

Modbus RTU		- modbus over serial interface, includes using CRC calculations for error checking

Modbus RTU over TCP - sends the RTU over a TCP/IP connection to a remote serial interface. Still uses RTU frame format including CRC calculations

Modbus/TCP		- has a slightly different frame format and doesn't use CRC calculations.


Unfortunately the last two variants are often confused :-)



libmodbus appears to currently support Modbus RTU and Modbus/TCP.



Regards,

Matt


> Hi Matt,
> 
> On Fri, Jan 07, 2011 at 10:33:34PM +1030, Matt Baker wrote:
>> Unfortunately though I discovered that the device I am using to do the
>> Modbus conversion supports Modbus/TCP for interfaces connected
>> directly, but provides a Modbus RTU over TCP for other external Modbus
>> serial devices.
> 
> I thought that's what Modbus/TCP was, the Modbus protocol encapsulated
> in a TCP stream rather than written to a serial interface. How is this
> "RTU over TCP" thing different? Is there an additional protocol layer?
> 
>> Before I start going into this too far I was wondering if you had any
>> thoughts on how difficult it would be to modify collectd to do this?
> 
> I'd actually implement this for "libmodbus" first, then adapt collectd /
> the Modbus plugin if necessary. Author and maintainer of libmodbus is
> Stéphane Raimbault who is very likely a much better help with anything
> Modbus related than I am ;)
> 
> Best regards,
> —octo
> -- 
> Florian octo Forster
> Hacker in training
> GnuPG: 0x0C705A15
> http://octo.it/