What does USB stand for? Universal serial bus. Universal means everything. Bus means a group, but what does the word “serial” mean?
In the world of computers, there are basically two ways to send information, serially or in parallel.
You might remember that before network printers and USB printers there were parallel printers. A lot of information was sent over several pins in a fairly wide connector all at the same time, or in parallel.
Imagine it as the difference between the number of cars that can travel on a six-lane highway and the number that can travel on a two-lane road. If speed is equal, three times more cars (three in each direction) can travel in the same time as on the two-lane highway.
USB, USB 2, and USB 3 increased the “speed limit” so that more “cars” could travel in series.
That’s the history, now let’s talk about serial control of devices.
Until the days of Windows XP, it was quite normal to find a “serial port” on all Windows computers. This port has nine pins arranged with five in one row and four in the next.
Before USB and PS-2, these ports were used for all sorts of things, but especially for mouses and modems.
After the introduction of USB, these ports were to most people relics of the past.
It’s really sad because serial communication provides some advantages that IR and RF don’t. When it comes to automation, the chief advantage of serial is two-way communication.
Imagine that you’re controlling an FM receiver or a thermostat. Wouldn’t it be great to be able to send out a command and get back information that the command was received and acted on? Certain serial protocols could do just that.
When I say, “certain protocols,” you should know that there are various types of serial control. RS-232, RS-422, RS-485, are all variations on a basic theme. Happily, each can be converted to the other with the right adapter. I’ve had good luck with adapters from B & B Electronics in the past, but there are other suppliers.
I probably know the most about 232, so I’ll talk about that. RS232 can provide both feedback and control of attached devices. Depending on the type of device and manufacturer, it’s possible to hook up the piece to a computer, fire up software like hyperterminal (which came with Windows XP and before), type in a few words, phrases, and/or numbers and control the device.
The connection wasn’t as easy to accomplish as you might imagine. First, you needed to know what type of connection the hardware required. Since, a computer had a 9-pin connector, you’d need to connect to a 9-pin jack. If you had two plugs or two jacks, that wouldn’t work, so you’d need to adapt one of the two. Often, a simple change in “gender” from male to female or female to male would cause a problem where the transmit pin of one end would connect to the transmit pin of the other, causing a sort of “head-on collision” of data.
To fix this, you’d use a special cable or adapter called a “null modem.” All that adapter would do is swap the transmit and receive pins, so transmit from one end would go to receive on the other and vice versa.
With a good physical connection, other settings would need to be correct. You’d ask yourself all about “databits,” “stop bits,” and “transfer speed.” If any of those settings (or one of a couple more) was wrong, nothing would happen.
One last thing could cause problems — cable length. The faster the data flowed, the shorter a distance it could go without problems.
I once installed Sony Pan/Tilt cameras in a church that wanted them just a little farther from the control location that RS232 would allow. I solved the problem by converting 232 to 422 because that protocol didn’t have quite the same distance limitation.
Serial control is not nearly as common as it once was, but it’s worth knowing a bit about it, should you ever come across it. Just remember that each instance will be a little different and have its own unique challenges.
If you need to do serial control from a computer today, by the way, you’ll be hard-pressed to find a current computer with a serial port. That’s why I’ve included a USB adapter link to Amazon in the picture at the top of the article. These things can be flakey, so you might have to try different brands to get it to work right with your product, but that’s a good place to start.
In some ways, serial control is the best of the control technologies we’ve discussed so far, but there is another way to control things that’s better in most ways. Next week we’ll talk about IP control.