How to for Data Logging Applications and Remote Sites

An Introduction to Data Logging Application

Data logging is an offshoot of data monitoring. Often, the same data being monitored by the HMI is also sent to a Data historian for logging. This works great except for remote sites or when the data is very important. In these cases, you want to have a local datalogger. The protocol that is used the most in these applications is Modbus.

The reasons why Modbus is so popular for this application are the same as noted before. Modbus is:

• Easy to implement in the device.
• Does not take very much CPU power or resources.
• Data can be pulled at long intervals.
• Universally supported.

We will now look at two applications.

Data Logging Application Example 1:

A customer has a gas analyzer that needs to have its values logged every minute. It is in a remote part of the plant that is inspected once per month. The gas analyzer is old and only supports Modbus RTU over RS-485.  

Solution:

Use an EasyLog by 4next to collect and store the data. This family of devices can pull data from either a Modbus TCP slave over ethernet or a Modbus RTU slave over RS-485. The data is stored in either Comma-Separated-Variable (CSV) or JSON format on a micro-SD RAM card in EasyLog.

The logs can be accessed via FTP, HTTP REST, MQTT or reading the micro-SD RAM card directly.

The setup is easy:

• Wire the gas analyzer directly to the EasyLog via RS-485.
• Using a web browser, set the Modbus RTU parameters to match the analyzer.
• Set up what variables you want to pull from the gas analyzer and how often.
• Select how you want to get the files or have them sent to you.

In this case, the site has to be inspected once a month. The service technician will connect to the EasyLog with an Ethernet cable and using the web browser download the month log during these visits.

Data Logging Application Example 2:

In this application example, you have a site with three power meters that have Modbus TCP onboard. You need to log several parameters from each meter every minute. The site is away from the main location. You have a satellite Ethernet connection to the site, but sometimes, dropouts occur. The customer therefore wanted a backup system on site in case of interruptions.

Solution:  

For this application, we will use an EasyLog by 4next wired into a Helmholz unmanaged switch. Also plugged into the switch are the three power meters. We then set up the EasyLog to pull the data from each power meter via Modbus TCP.

The EasyLog can be used as a backup or as a data concentrator in this case. As a backup, it simply logs all the data. As a data concentrator, it would collect all the data from the three meters and then send this up to a MQTT Broker which would then send it to the Human Machine Interface (HMI) and data historian. Using the QoS feature of MQTT will ensure that the data gets there.

Conclusion

Data logging is easy using Modbus and the EasyLog by 4next. One of the best aspects of this product is that it is also very cost-effective.

If you have a data logging applications that you want to discuss with us, please contact us.

How do you learn about Modbus?

Modbus.org has a lot of free information on Modbus. In general, there are many websites with Modbus information, and there are also many books on this topic.

As for formal training, there is only one that we know of, which is our course called Modbus Troubleshooting and Maintenance. Like our other Troubleshooting and maintenance courses, this course is focused on the practical aspects of the protocol. It has many hands-on exercises to show you how the protocol works and what issues you may run into. 

Read our blog posts exploring Modbus:
1. Yes, we still use Modbus. An enduing legacy you need to know.
2. Simple and cost effective tools for monitoring applications
3. Data Logging Application
4. Controlling Slow Application (coming up)

This is a special post from our colleague James Powell, P.Eng of JCOM Automation.