PLC Programming Handbooks
Modbus Plus Traffic Issues in Bridge Plus Devices
Courtesy of: Delta Automation, Inc.
It is important to keep in mind when deciding the routing paths for your data transmissions which utilize more than one Modicon Bridge Plus device, the amount of traffic which may attempt to flow through it.
While a "backbone" of bridges on the main trunk of a History | Specifications | Modbus Organization">Modbus Plus network is generally the most efficient design for larger systems, problems can arise when data is requested from nodes that route over more than one bridge. A bridge plus device has eight master and eight slave paths for utilization of data transmissions. Additionally, there are paths reserved for programming use only. When all of the data paths are busy in the bridge, the incoming transaction will be queued in the first bridge plus device. Once a path becomes available, the queued transaction is removed from the queue and given a path. A response is only returned to the origin when a complete path is made to the destination. Since a bridge must open ports on both sides and maintain the proper timing, a transaction will only be queued in the first bridge and not in subsequent bridges for transactions that utilize multiple bridges. This will result in lost data or data not getting to or from the proper nodes, with no logical indications.
Utilizing the MBPSTAT program supplied with your computer Modbus Plus network card, monitor the traffic on each side of each bridge located on your network, to determine how many paths are being utilized at one time. If traffic is indicated on all eight paths, it is possible that some messages may be queued for transmission. In fact, it has been the experience of Delta's Engineers, that if the traffic exceeds an average of about 75% (six paths), that this bridge is beginning to get overloaded and may pose future problems. The ultimate solution is to add an additional bridge in "parallel" (with different addressing of course), and re route some of the traffic over the new bridge.
These are the types of issues and problems that can be avoided by utilizing the network services of Delta Automation Inc. for all of your network design and support needs. A complete written report is generated with a network certification which includes distances, token rotation time, cable resistance, and a graphic display. All of this information is important for future planning for expansion, maintenance and servicing. Delta Automation Inc. certifies Modbus Plus, Modbus 11 and Remote I/0 networks in metallic and fiber optics.