Passive monitoring of ModBus networks

Sometimes when implementing facility automation it's necessary to hook existing, working ModBus-based SCADA systems to our monitoring platform.

Since ModBus is a single-master network, where only a single client is permitted to query devices for their current data, connecting to existing infrastructure needs to be non-intrusive.

The good news is that since we can see all the traffic on the bus, we can match up the queries and the responses, pair them up and decode the data.

However, since there is nothing on the network that tells you which kind of device is on which address, you need to pre-configure with the same knowledge of the ModBus network as the existing controller has.

This means, for each meter we need the following information:

• Serial speed (baudrate)
• Serial Parity
• Serial stop bits
• Unit id (address)
• Device model (brand & model)

The Device Model then requires a mostly complete set of register information in order to properly decode the data on the wire. For most ModBus units, this is available in the form of documentation on their homepage or similar. See our article about Supporting a new ModBus device for details.

Further limitations apply, in that while a controller may change settings on the network (baudrate, stopbits, parity), a passive monitor needs all these settings to be the same. ( At least in our current implementation. If you have such mixed networks that you wish to monitor, contact us ).

Passive ModBus monitoring is a new feature in Release 3.4 of the Modio product line.