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Which RTU Capacity Do You Need?

By Andrew Erickson

December 3, 2019


The Remote Terminal Unit, Remote Telemetry Unit, or "RTU", is an electronic device that will be installed at your remote location. This device will be the starting point of your monitoring system.

Your RTU will collect alarm data from your equipment and sensors, encode this information into a format that is transmittable, and transport the data back to your central "master station" (or just send you an email/text directly).

Shopping for the right RTU is easy once you have a good list of your actual requirements

RTU capacity varies widely, so you need to choose carefully

RTUs are equipped with input channels ("alarm points") for sensing, output channnels for control, audio/visual indications for alarms, and communications port(s).

If you don't have enough capacity to cover everything you want to monitor, you won't have good network visibility. That's why it's so important to properly calculate how many RTU alarm points you'll need.

Your first step will be to determine how many discrete alarm points you need at your site. These are simple binary on/off inputs that you can use for monitoring just about anything.

Discrete alarm points are known by many different names, including:

  • discrete alarms ("discrete" on/off, as opposed to analogs)
  • alarm points (each is a data "point" representing one "alarm" condition)
  • contact closures (many devices will close an electrical contact relay to report an event)
  • digital inputs ("digital" on/off, as opposed to analogs)
  • discretes (shortening of "discrete alarms")

Finding your perfect RTU capacity is a simple process

To help you get a perfect-fit RTU, I'll break the selection process down into a few straightforward steps.

So, how many discrete alarms do you really need? It all comes down to two things: Equipment alarms and discrete sensors.

Count up all of your equipment alarms

Counting up all of the equipment alarms you want to collect
Your first step is to build a list of all of the discrete-output equipment alarms you want to collect.

Equipment alarms are contact closures (relays) that your equipment can latch to report various conditions. These can include events like:

Add extra alarm points for any discrete sensors you plan to use

Discrete sensors provide a contact closure when one isn't already present. Sensors can tell you when:

  • Your site door is open (simple magnetic door contact)
  • Someone is moving around in your site (passive infrared motion sensor)
  • Your site temperature exceeds a pre-selected "too high" level (discrete temperature sensor)
  • Your site humidity has exceeded a pre-specified value (discrete humidity sensor)
  • Your generator is vibrating (discrete vibration sensor, a substitute for "running" if your generator doesn't have a built-in alarm)

Simply add up all of the equipment alarms and discrete sensors you want to monitor at your site. This total is your estimated discrete alarms required.

Keep in mind that electrical relays are relatively primitive. Although that does have some distinct advantages in the modern world, many pieces of gear no longer include relays and communicate using a protocol (ex. Modbus or SNMP) instead. This industry trend probably affects how many discrete alarm points you need.

Don't forget to allow room for future growth

Imagine buying clothes for your growing child. It's not enough - or even desirable - to buy that child clothes that fit perfectly today. You have to allow room for future growth.

In many ways, your network is like a growing child. You have to allow some room for future growth (within reason).

Therefore, it's essential that your accurately measure your real-world needs, then add the correct amount to anticipate future growth. You don't want to end up with less than you need. You also don't want to waste budget money on capacity you'll never use.

For many of my clients, the right allowance for future growth is an extra 15% on top of their calculated present need.

Allow at least 15% extra for future growth when choosing your RTU capacity
Be sure to add extra discrete alarm inputs to allow for future growth, commonly about 15% of your calculated immediate need.

Remember that - in some cases - analog inputs are superior

Discrete alarm inputs are binary. They only tell you "yes or no," "on or off", "open or closed," etc.

For many values, especially temperature and humidity you'll get a much clearer picture of your remote site if you use an analog input instead.

Sure, it's good to know that your site temperature is "exceeding 80 deg F", but are you at 81 degrees or 181 degrees? An analog input can answer that question, while a discrete alarm input cannot.

Analog inputs are more expensive components, so only you can find the right balance for your company. The important thing is - as you count up how many discrete alarm inputs you need - to remember that analogs might be a better way to monitor certain things.

As you calculate your required inputs for your different sites, don't forget that standardization has value

You'll probably end up with widely varying totals at each of your unique remote sites. One might need 37 alarm inputs, while another might only need 12.

Keep in mind, though, that a perfect fit at EVERY site is not your goal. You need to balance good capacity at each site with the benefits of standardization.

The more RTUs you purchase, the more training you have to conduct with your staff. Imagine the difference of training your team on just 1 web interface vs. 5 different web interfaces.

Sparing has similar economics. If you have 100 RTUs of a single model, you can achieve a 10% sparing target with 10 units of that single model. If you have 3 of one RTU model, 7 of another, 19 of a third, etc., your sparing plan is much harder to execute. You'll end up needing more spares to make sure you have enough RTUs during inevitable emergencies, including:

  • Natural disasters
  • Lightning strikes
  • Techician misuse (ex. Frying a power supply wired backward)
  • Miscellaneous RTU failures

For that reason, there is a cost triggered by too much customization when choosing RTUs.

Try to limit yourself to 3 different RTU models or less

After calculating your required inputs for each site, try to sort them into groups. Most people can end up with something like "small", "medium", and "large" sites.

This way, each group can use one RTU model, so you won't have to deal with too many different units for training and sparing.

Aim for no more than 3 RTUs. Two is even better. Total standarization on one RTU would be awesome if all of your sites are fairly similar.

Standardization of this type will mean that you have some RTUs that are "too big" for some sites. Still, it's better to tolerate this than to deal with too many different models.

RTU expansion shelves are a great "hack" to achieve both customization and standardization

RTU expansion shelves are one way to expand your customization envelope without negatively impacting standardization. These look very similar to an RTU, but they attach to a base unit rather than being a complete RTU themselves.

3 RTU expansion shelves that quadruple the capacity of the base RTU
In this expansion-shelf example, 3 NetGuardian DX expansion shelves are expanding a base NetGuardian 832A RTU.

Expansion shelves increase your base RTU's alarm inputs (and analogs & controls) without changing the RTU itself. You won't have 2 different IP addresses or 2 configuration screens, so you'll still manage your information in the same interface as usual.

This allows you to effectively deploy a "bigger" RTU at sites that need it without disrupting your training and sparing standardization.

Even better, you can decide to buy and install expansion shelves even after your initial roll-out. This makes it easier to handle unexpected growth at a site.

1-pin vs. 2-pin connectorization: Your chance to double your alarm inputs without any additional connectors

Traditionally, discrete alarm inputs are physically composed of 2 electrical pins: one for the alarm (ALM) and one for the return/ground (RTN/GND) path.

Dry contact closure and contact-to-ground discrete input
These two circuit diagrams show the difference between 2-pin "dry contact" inputs and 1-pin "contact to ground" inputs.

In this typical configuration, the RTU provides a small electrical current on its ALM pin, then looks to see if that current is returned to the RTN/GND pin (this is called a "continuity test" in electrical terms).

If your equipment/sensor has "latched" its relay (the equivalent of lowering a drawbridge to provide a connection), then your RTU will sense flowing electricity and know the current state of that alarm point.

So, we need two electrical pins for every discrete alarm input we want. These add up quickly, and they all have to physically fit on the back (or front) panel of our RTU.Some RTU manufacturers offer an alternative "contact to ground" pinout that requires only 1 pin per discrete alarm.

You can double your number of discrete alarm inputs by choosing this configuration. While there are some technical concessions you have to make (beyond the scope of this article), it's generally a safe choice if you're not certain you need 2 pins per alarm.

Here's how a 50-pin Amphenol connector is laid out in a 1-pin-per-alarm configuration:

2 pinouts for a 50-pin amphenol connector: dry-contact and contact-to-ground

The left example 50-pin "amphenol" connector uses a dry-contact configuration with 2 pins per alarm, so only 24 inputs fit on the connector (2 pins are dedicated GND).

The right example from the NetGuardian 864A RTU uses contact-to-ground with only 1 pin per alarm, so 48 inputs fit on the connector.

When should you choose a 1-pin configuration vs. adding expansion shelves?

The economics of alarm-expansion options might surprise you.

The RTUs that I've worked with generally cost only about 5% more when you choose a "contact to ground" configuration. That compares favorably to buying expansion shelves for your RTUs later, which might cost about 50% of the original RTU price.

Both double your alarm-input capacity, but one costs 10x as much.

This 10x price differential has big implications. If you'll end up buying an expansion shelf (to double your capacity) at just 10% of your sites later, you're better off financially to just purchase a double-capacity "1 pin per alarm" RTU at every site.

You'll spend the same amount (or less, in all likelihood), and you get doubled alarm capacity at all of your sites.

Counting Up How Many Alarm Points You'll Need: A Practical Example

As an example to bring all of these principles together, lets work through our process on a hypothetical network.

Our hypothetical example network has two different site sizes

Let's say that our network contains:

  1. (15) "small" sites with 8 equipment alarms and 4 discrete sensors to be monitored (12 total)
  2. (5) "large" sites with 18 equipment alarms and 6 discrete sensors to be monitored (24 total)

Start by establishing target RTU capacities - including future growth capacity

With our base requirements tabulated, we'll sum them up and add a bit for future growth:

  1. (15) "small" sites need 12 discrete alarms total + 15% for growth = 14 discrete alarm inputs required
  2. (5) "large" sites need 24 discrete alarms + 15% for growth = 28 discrete alarm inputs required

Match your final capacity requirements to commercially available RTUs

Now that you know you need one RTU with at least 14 discrete alarms and another RTU with at least 28, you can go shopping.

With your precise requirements in hand, shopping for your RTU becomes much easier. You'll be able to ignore the bad fits and choose the exact RTU you need.

Talk to RTU manufacturers about what they have to offer. Expect that you might have to make minor adjustments to your alarm counts to match what's commercially available.

For example, your 14-alarm sites would match well with a 16-point RTU like the NetGuardian 216.

Your 28-alarm sites could be well served by a 32-point RTU like the NetGuardian 832A.

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