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Grasping serial comms: A hitchhiker's guide

Posted: 10 Jul 2013     Print Version  Bookmark and Share

Keywords:USB  RS-422  RS-485  programmable logic controllers  TCP/IP 

It is a popular phenomenon that the inhabitants of Planet Earth are fond of creating technological gaps between what has just been invented (and can be readily purchased), and what kind of equipment is actually installed in their hospitals, factories and other industries. Like the rules of their game of cricket and their lingering love affair with polka music, this is something humans have never been able to explain to fellow sentient creatures. It has, in fact, called into question their right to be regarded as fellow sentients at all.

Many humans have huge investments in legacy serial equipment. So the natural human response was to invent an entirely different universal bus called USB, and to equip new computers for USB while eliminating the serial port. (Never mind the fact that the serial protocol is still so useful to their species that the number of deployed serial devices is expected to keep growing for years to come.) Fortunately, in spite of everything the humans have tried to do to prevent it, it's actually quite easy to get their serial devices to talk to their networks. You can even get their serial devices to talk to their smart phones and tablets. Grab your towel and your notepad, and we'll tell you all about it.

Here, in the form of a video, is a fun, but educational look at serial communications and the transition to USB and Ethernet. Serial devices can now communicate anywhere and everywhere. Solutions target the industrial communication technology gap.

The human objection to serial communications
Serial communications based on RS-422 and RS-485 communicates digital information over twisted pair wire from transmitters to receivers. RS-422/485 systems can communicate at rates up to 10Mbit/s (though most systems operate at lower bit rates). Both systems utilise balanced outputs and differential inputs, which provide better noise immunity than single-ended systems. RS-485 is used as the basis for many commercial and industrial data communications systems, like Profibus, Interbus and Modbus.

Serial communications are commonly used to link programmable logic controllers (PLC), supervisory control and data acquisition (SCADA) systems, remote terminal units (RTU) and other equipment in custom networked systems. The RS-422 and RS-485 standards do not define protocols. They are simply physical layer standards (and even then, do not specify connectors or pin-outs). This means that RS-422 and RS-485 can be implemented in many systems and applications. This was all working very well, which is why humans invented USB to replace it.

USB uses completely different protocols, completely different cabling and completely different ports. Better still, it has an effective range of only 15m. Granted, that can be extended up to 30m using USB hubs like B&B Electronics' UHR204. But it's still just a fraction of the 1219.2m (4000 ft) range achieved by RS-485, which—to humans – was a key selling point. Additionally, USB connectors don't grip the cables very firmly, unless you're using high retention ports like those, for example, in B&B Electronics' USOPTL4. As USB carries 5 V DC power, this creates an opportunity for fire when vibration shakes a cable loose. Humans love fires, and they report them on the news whenever they think they've had a good one. Serial connectors generally employ thumbscrews to hold them in place. This reduced ability to create newsworthy conflagrations was one reason for trying to move away from the serial standard.

Serial communications and the expansion port
Humans equip their desktop computers with card slots that allow computers to be configured in numerous ways and for numerous purposes. As new generations of computers are designed and built, the nature of this bus is continually reconfigured in order to keep older cards from functioning in newer machines. Humans love to recycle, and they are enchanted by the fact that an ISA card that once sold for $175 can be broken down into its constituent elements and recycled for a total profit of three cents.

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