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Modbus-to-SNMP Converter: How to Choose and Buy

By Andrew Erickson

October 30, 2022

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If you're looking to connect Modbus devices to an SNMP network, you'll need a Modbus-to-SNMP converter. These devices are designed to bridge the gap between the two protocols, allowing you to monitor and manage Modbus protocol devices using SNMP.

How to choose the right converter for your application

When choosing a Modbus-to-SNMP converter, there are a few things to keep in mind.

First, you'll need to make sure that the converter is compatible with the Modbus devices you're using.

Second, you'll need to decide which features are important to you. Some converters come with additional features like web interfaces and support for multiple protocols.

Finally, you also need to ensure that your converter supports the version of SNMP you need. This is typical SNMP v1, SNMP v2c, or (for security-conscious applications) SNMP v3.

Understand the fundamental difference between polled Modbus registers and asynchronous SNMP traps

One of the most important things to understand when connecting Modbus devices to an SNMP network is the difference between polled Modbus registers and asynchronous SNMP traps.

Modbus registers are typically read at regular intervals by a Modbus master. This means that the data in the registers is only as up-to-date as the last time they were read. In the Modbus protocol standard, this is simply how things are done. New changes wait until the next polling cycle to be reported.

SNMP traps, on the other hand, are asynchronous messages that are sent by an SNMP agent to an SNMP manager in response to a change in state. This means that the data carried via SNMP traps is always up-to-date within the SNMP manager. When a condition changes, an SNMP agent typically sends out a trap within milliseconds.

This presents an interesting challenge for any "converter". You're not just modifying incoming Modbus messages and "converting" them to SNMP. You have to actively poll a Modbus device, store values, then send them as SNMP traps.

To avoid a flood of SNMP traps repeatedly containing the same unchanging values (ex. 82 degrees, 82 degrees, 82 degrees), your converter should probably perform some logic to send SNMP traps only when something important has changed.

SNMP RTUs are a great starting point, since they already detect events and report via the SNMP protocol

If you're looking for a Modbus-to-SNMP converter, it's worth considering an SNMP RTU instead.

SNMP RTUs are devices that already detect events and report them via the SNMP protocol. In other words, they already do much of the "heavy lifting" required by a Modbus-to-SNMP converter.

Although SNMP RTUs might cost more than you expected to spend on a "converter", they also bring to the table high reliability and the ability to accept non-Modbus inputs like contact closures.

Simply look for an RTU that accepts Modbus as an input type

Because SNMP RTUs are "almost there" in terms of your required functionality for your project, you just need to find one that can poll Modbus registers.

At DPS, we started developing RTUs with this capability roughly five years ago in response to client requests. The most common need was for collection of no more than perhaps 32 or 64 analog registers. One government client came to us with a requirement to monitor over 1000 Modbus registers, so we tackled that with a more powerful hardware platform.

Because virtually all of our NetGuardian and TempDefender RTUs support SNMP (v1, v2c, & v3), that renders the handful that support "Modbus Alarms" in firmware to effectively be rack-mounted "Modbus-to-SNMP converters" in the way I've described above.

Find a partner who can help you sort out the finer details

If there's one thing we've learned by gradually improving our Modbus implementation over the years, it's that there's always something new that might be necessary in a deployment.

Initially, I would say that it was our engineers who were catching up to the commonly used Modbus standards. While we initially support only one "Function Code" (register format), we now support many.

More recently, however, we've had to adjust our approach to compensate for the various details of other manufacturers...

Little details like head and tail times can be critical

Just last week, I visited a client with a big-name generator that communicates using tens of thousands of Modbus registers (no joke!). I was able to troubleshoot my client's desires for specific registers and nuisance alarm suppression by configuring our TempDefender G2 RTU at the site.

One weird problem required a different approach. We were seeing a persistent, intermittent failure of Modbus communication between our RTU and the generator. About 4% of all polls would return a garbled result.

What we discovered was that the generator was responding just a little bit too quickly. Because this was Modbus via RS-485 half-duplex, communication can only go in one direction at a time. If you've ever used a walkie talkie, you know what I mean.

In rare cases (about 4% of the time), our RTU would not hear the very first piece of the Modbus response from the generator. The solution here was to have the generator tech dial in a longer "head time" so that the generator will wait just a split second before responding.

Speak with an engineer now so you can plan and purchase a perfect-fit Modbus-to-SNMP converter

Now that you understand a bit more about the SNMP RTU as an "almost there" Modbus-to-SNMP converter solution, I invite you to speak with one of our engineers.

We can help you determine if an SNMP RTU is the right fit for your needs. If not, we can help you understand what other options might be available. We can also help you configure the perfect SNMP RTU for your application.

For example, if you need to support a lot of Modbus registers, we can recommend one of our more powerful platforms like the MODBX (pronounced "Modbox" or "Modbus box"). If you have a smaller application, our TempDefender G2 RTU might be a better fit.

Call me at 1-800-693-0351 or email me at sales@dpstele.com

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Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 15 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...

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