Discretes, analogs, and controls are great for non-protocol communication. They're all industry standards for monitoring and controlling your remote equipment. Their simplicity makes them much easier to troubleshoot.
These basic input/output types do have limits, though. You can't build a complex management interface using binary switches. Sometimes, you need to have menus and commands.
One common example on older equipment is the serial interface (RS232/RS485). A serial port in this case is commonly referred to as the Craft port, and it's used to access a terminal interface via a simple serial connection.
There's nothing inherently wrong with serial communications for device management, but they're becoming more difficult to justify in the modern era. Leasing dedicated circuits for serial links used to be commonplace. Nowadays, companies are wrestling with keeping (increasingly expensive) circuits online for dwindling amounts of gear.
Of course, you could theoretically drive out to a remote site with a six-foot serial cable every time you wanted to access a device, but that creates even more truck-roll and labor-time waste.
In today's world, you really need a LAN-based bridge to access the serial equipment you still have in your network. You need to connect your equipment to a simple serial-to-LAN box, which routes the traffic over LAN back to your central office.
Because it's serving access to terminal connections, that device is called a "terminal server". Plenty of manufacturers produce standalone terminal servers today. You have a lot of options to choose from, but don't miss an opportunity to combine multiple functions into one remote management device.
If you need a terminal server, by definition, you're dealing with a remote location. If the serial device was on your desk, you wouldn't need remote-access equipment to reach it.
At locations where you need a terminal server, you probably also need an RTU. A complete monitoring solution incorporates discrete inputs, analog sensors, control relays to manipulate equipment, and serial/terminal interface access.
Take advantage of this double need by choosing an RTU with a built-in terminal server. Think about how much better it is to use a single box. You only have to buy from one vendor, you're only paying for one chassis and one power supply, you only have to install one device, tech support has only one phone number, and your RTU and terminal server manufacturers can't play the dreaded "blame game" when something doesn't integrate properly.
Not all manufacturers offer terminal servers on their RTUs, as many are content to ignore legacy functionality and focus only selling new gear. They'd rather force you into throwing out a big chunk of your monitoring system and buying something new.
Fortunately, some of the better RTU manufacturers understand that making you happy by supporting your existing gear is the best way to increase long-term sales. One technical way that they do this is by offering integrated terminal servers.
The NetGuardian 832A G5 & NetGuardian 420 remotes have terminal servers (8 ports and 4 ports, respectively). In fact, most of the NetGuardian models (there are enough variations to make my head spin) have at least a single serial port that you can use as a terminal server.
This can be a fair point, certainly. If you just installed an RTU within the last few years, it's probably not the best idea to replace it just to gain an integrated terminal server. Although the value of integration is high, the cost in a scenario like this one might just be higher.
Consider these options when you already have an RTU at a site:
Now that we've covered the basic concepts, your next step is to review specific examples:
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