Lynn's Industrial Protocols over IP: Modbus Report-By-Exception over Cellular IP

Summary: While traditionally serial Modbus has been considered unable to use Report-by-Exception, when combined with IP networks Modbus Report-by-Exception becomes very natural and effective.

Modbus/TCP is inherently peer-to-peer
People using Modbus/TCP over Ethernet or IP are familiar with its ability to function as peer-to-peer. Most PLC with Ethernet ports can function concurrently as a Modbus/TCP slave and Modbus/TCP master. So 2 PLC can very easily connect - with 2 separate Master-Slave TCP connections - and share information. One TCP connection is a Master/Slave connection with the first PLC as Master and second PLC as Slave. The other TCP connection is a Master/Slave connection with the first PLC as Slave and second PLC as Master.

Technically, this is not Report-By-Exception in the true sense of a protocol. However, since the PLC-as-Master communication events can be triggered by field inputs, it has the same effect as writing information only upon exception or when change is relevant.

Modbus/RTU as peer-to-peer
Serial Modbus/RTU is a bit harder to use this peer-to-peer trick with. A device with 2 serial ports can of course have 1 port configured as Master to issue remote reads and writes, while the 2nd port is configured as Slave to answer requests. When connected to a 2 serial port Modbus IP to Serial Bridge (such as the Digi One IAP), the 2-port serial RTU becomes a full Modbus/TCP peer, capable of operating fully peer-to-peer with other PLC and SCADA/OPC applications.

However, vendor's aren't blind to the marketing aspect of "more hardware". While adding a 2nd serial port to a one-port RTU may only cost a few dollars, most likely the 2-port RTU is a much more powerful unit, so the actual end-user cost may go up hundreds of dollars. The same is true of Ethernet; while adding an Ethernet port may only cost a few dollars, user's expectations of Web Pages and fancy functions means the Ethernet-enhanced device price may be $500 or more above that of a simple 1 serial port RTU

Fortunately, the Digi One IAP (as well as PortServer family) allow Modbus/RTU slaves to use Report-By-Exception on the serial port. As long as only one serial slave is on each port, the Digi uses configured knowledge to "split" the single serial port conversation into two traditional Modbus/TCP connections. So traditional remote Modbus/TCP masters can query the serial slave RTU, completely unaware that on occasion the serial slave RTU wakes up a acts as a Master to write data during Exceptions. Somewhere, a traditional remote Modbus/TCP slave will receive Modbus/TCP messages from the serial RTU slave, completely unaware that when not busy reporting exceptions, the remote "Master" is really a passive Modbus/RTU slave.

This feature is ideal in wide-area-network situations were bandwidth is limited or data traffic is billed on volume of bytes moved. For example, many SCADA systems only need to check on remote status every few hours ... for example lift pumps in a storm sewer system do absolutely nothing interesting for weeks or even months in the absence of rain. Even during a normal rain, checking on them every few hours is likely enough ... that is *IF* the remote life pumps can send Report-By-Exception messages during system problems.

For example, we have one customer piloting use of Modbus Report-By-Exception over cellular data network. Their eventual target is to poll the remote sites once per day. They use a simple, single-port Modbus/RTU slave which combines I/O with an LCD and push buttons to make a simple, self contained "RTU" or Remote-Terminal-Unit in the truest sense of the word. Use of Modbus in UDP/IP and Report-By-Exception allows this customer to plan for $12 per month per site bills. If forced to poll continuously with Modbus over TCP/IP, they would need to pay $50 or more per site per month. With hundreds or thousands of sites, that is a huge cost savings and opportunity for better ROI (return on investment).

More Information
Here is a general discussion of how to design Modbus/RTU serial slaves and masters to gracefully handle Report-By-Exception: