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How to Monitor Your Rectifiers via SNMP & Alarm Dialer

By Andrew Erickson

September 7,

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I just had an interesting call with a DPS client who needs to monitor rectifier major alarms (2 per rectifier). This application has a few interesting elements, so let's walk through it as a learning exercise now. This should help you as you plan your next remote monitoring purchase.

The Goal: Route "Major" rectifier alarms to an existing alarm dialer

This client, a company in the Midwest, has an existing DPS alarm system.

The monitoring system already in place consists of two devices:

1. The NetGuardian 832A G4, which came bundled the Ribbon C15 softswitch (formerly Genband)
2. A SiteDialer, which gives the NetGuardian the ability to dial out over a POTS line and deliver automated voice alerts

This system was only ever intended to monitor the C15 softswitch at that one site. The NetGuardian came bundled by Genband (now Ribbon) for expressly that purpose.

The SiteDialer, an accessory added later via a direct purchase from DPS, allows the NetGuardian to dial out via a POTS line (mostly routed over VoIP in modern networks) when an alarm occurs. Voice messages delivered directly to your technical staff work wonders for important alarms that cannot wait.

The new requirement required a thoughtful extension to the existing system:

1. At each of 5 additional sites, an AC-powered DC rectifier had a pair of important "failure" alarms to collect.
2. These alarms were important enough to warrant immediate voice dial-out, as an transformer/rectifier failure will inevitably lead to depletion of the backup batteries if not detected and resolved. As these 2 alarm points are "summary alarms", they also capture problems that are not complete failures, such as out-of-spec rectifier output voltage. Even major events like lightning damage could be reported via these alarm outputs.

The solution to this client scenario is the diagram shown above: NetGuardian LT G2 RTUs monitoring each of the 5 rectifiers, then sending SNMP traps to the single head-end NetGuardian 832A G5 - which can then trigger dial-out alerts.

The solution to this client scenario is the diagram shown above: NetGuardian LT G2 RTUs monitoring each of the 5 rectifiers, then sending SNMP traps to the single head-end NetGuardian 832A G5 - which can then trigger dial-out alerts.

The Problem: The existing monitoring system needed a cost-effective extension

As I noted above, the original monitoring system was designed specifically to monitor the C15 softswitch at one site. There was no question that additional equipment would be needed, but what was the most cost-effective way to do it?

During and after my call with this client, I considered one obvious possibility first:

We could deploy 5 small NetGuardian LT RTUs, then route alarms via LAN back to the existing central SiteDialer device, which supports up to 255 total alarm points.

Unfortunately, after consulting with DPS Engineering, I learned that the SiteDialer pairs up with only one NetGuardian RTU. My plan wouldn't work, at least not as originally conceived.

I then considered using point-to-point pairs of alarm-echo devices. These are simple devices that take in a contact closure on one side, send the alarm state it via LAN to the companion unit, and trigger a relay closure in the companion unit. In this way, a simple contact closure is effectively relocated from one facility to another.

With just 10 of the 32 discrete inputs on the existing NetGuardian 832A (which were unused), we'd be able to collect those 10 contact closures.

Although this would be effective, it had a certain ugly inefficiency. To collect alarms at 5 sites, you'd need 10 total boxes (5 "alarm bridge" pairs). This would require extra purchasing and be a waste of space - especially at the head end.

The Solution: An SNMP link between two RTUs

After considering a straightforward RTU-to-LAN option that wasn't supported by the SiteDialer, then a bridged "echo" system that required excessive hardware, I reconsidered how to make an RTU-to-LAN setup work.

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It was then that I realized that I had overlooked one of the newer NetGuardian 832A features: SNMP Alarms.

This is different than the usual RTU handling of SNMP traps. Like all DPS NetGuardians, the 832A model is able to send SNMP traps whenever an important event happens.

"SNMP Alarms" is a more advanced function. This enables the NetGuardian to act as a kind of "mini SNMP manager". With it, you can receive traps from a limited list (several dozen are supported simultaneously) of pre-databased OIDs. You can't take in EVERY trap your device(s) might generate, but you can selectively choose to watch for quite a few important traps.

This was the key to my solution. Because the NetGuardian 832A can process SNMP traps (when equipped with the proper option), and EVERY NetGuardian can send SNMP traps, I had a way to establish a cost-effective LAN-based link with minimal hardware.

The NetGuardian 832A G4 would need an upgrade to the G5 model to enable this feature, but that could take advantage of our 30% trade-in discount program.

Every project must have a solid business case to get approved

My client now has my proposal. I like the solution we assembled together.

Like any new purchase, the quote requires consideration. Even important ROI - like protecting site uptime via the monitoring of rectifier alarms - must be justified to management.

Although I'm not always privy to the inner workings of client decisionmakers, I have been told on several occasions that my proposal detail helps.

Rather than just sending a simple price sheet, I (and all others in DPS Sales) send PDFs that are complete with an explanatory letter, product drawings, and text descriptions of the components proposed.

Act now - Before your next preventable service outage

Truthfully, the explanatory letter - which is always right behind the cover page - is a great aid for DPS as much as it is for clients. Especially within larger client organizations, decisions take time.

Sometimes, the initial interest in a project wanes after the quote - but before a purchase decision. No one intends for this to happen, but other emergencies crop up and steal the team's focus. Inevitably (although sometimes years later), an unresolved problem returns with a vengeance.

Whenever this happens, my old proposal PDFs get dusted off (sometimes literally, if they were printed). I get a somewhat frantic call. The project moves quickly.

When that happens, I take a look at the letter that I wrote (some number of months ago). I get quickly reoriented, verify that the solution still works, and help you justify the purchase internally.

When you work with me, it's my sincere hope that we can ride the first wave of urgency and drag the project across the finish line. Otherwise, you'll have to suffer through the surprise second wave of urgency whenever it arrives.

To help you get everything done in one shot, I'll ask you a lot of questions about the pain and trouble you're currently experiencing. If you can quantify it in dollars and cents, even better. That allows me to put together a proposal that's more likely to resonate with you and your management.

Just tell me what you're trying to accomplish

Maybe you don't need to monitor rectifier alarms. Maybe you have a solar array. Maybe you have entire comm sites at a power utility (ex. substations) or trackside at a railroad company.

Whatever you need to monitor, just tell me what problems you currently face. I'll help you build a customized solution like I did here. It might just be a collection of off-the-shelf equipment. DPS might engineer you a new box. Whatever it is, I promise that it will fit your requirement perfectly.

To get started now, just give me a call at 559-454- or me at

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 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...

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