Finding the source of data loss between instruments using Modbus communication protocol can often be tough. There are multiple tools within the data controller that can help you troubleshoot.
It is important to determine which instrument is acting as the Modbus Master when going through the troubleshooting process as it will direct you to the correct instrument that is causing the error. If the data controller is the Slave, then all you can do is make sure that you are using the correct bit or register address from the Modbus map (customer login req'd), and that you are writing to a Q-type (Modbus) controller channel. Should that be correct, then the source of the error is coming from the instrument acting as the Master. If the logger is the Master, then you can use the tools below to help identify the problem.
When the logger is acting as a Master, data must first be transferred to the Client Table before it is either written to a channel or sent to another instrument. The Client Table registry is also called the holding registers, as it is essentially the holding place for data before it is assigned to a location. The Client Table can be accessed by remoting into the data controller and choosing S (Status Menu) > V (View Modbus Master Status) > T (View Modbus Client Table).
The holding register location is specified within the Modbus SERVER.CFG file configured in the data controller, and each holding register can be viewed in the Client Table to see the data being assigned in real time. Note that all data represented in the Client Table is in hexadecimal.
When reading data from an instrument to the data controller, it is always a good idea to check the Client Table to see if the data is getting to the controller. In addition, the Client Table also acts as the list of data transmission errors which can be accessed by looking at register 6000 within the Client Table.
Register 6010 corresponds with the error message for the first line in the SERVER.CFG file, with every register after that corresponding with the next consecutive line. Each different kind of error is represented as a hexadecimal number defined in the Modbus Exception Codes (customer login req'd). The most common Modbus error is ‘FFDF’, which indicates that the data controller and the instrument cannot communicate.
The Server Table is the list of registers directly corresponding to a particular channel or set of digital lines. Using the data controller’s Modbus Map, these registers can be determined and used as an indication of the final placement of the data. If the holding register has been verified and has the correct number but the channel is still not reading correctly, then the incorrect register address may be referenced within the SERVER.CFG file.
In the upcoming 8864 controller firmware upgrade 5.02, the Modbus TCPdump feature has been added as a more direct way of seeing what kind of Modbus data is being transmitted to and from the controller. Choosing this option in the Modbus Master Status Menu pulls up the stream of Modbus going into and out of the data controller.
Each line shows the IP address of the instrument transmitting the data, the direction the data is transferred, the type of data (bit or register) that is being transferred, and the hexadecimal version of the data strings being transferred. The data shown on this screen can be written to a file within the logger and accessed over an FTP connection. This allows users to capture a large period of data being transferred over Modbus to help pinpoint irregular periods. Note that this is a new feature, and will not be available in an 8832 or an 8864 with firmware older than v5.02.
Using the tools mentioned above can help find sources of error, but other action will need to be taken to fix any of those errors. Many issues arise due to a SERVER.CFG file that is configured incorrectly, but there are multiple sources of communication errors between instruments. Some examples of common issues are described below.
Clint Anderson, System Implementation Engineer, ESC Published: 10/19/2016 12:04:17 PM
Receive notices of new articles and free webinars, of training classes being offered, and regular anouncements and offers.
We'd love to answer your questions.
Check out our helpful animation that teaches the basics of