Controlled Decision: RS-232 vs.IP
Look at the back of any major-brand AV control processor these days and you'll probably see a snapshot of an industry in transition.
No Passing Fad
IP-based control is not a passing fad, but those serial ports on the other side of the control module are not likely to go away any time soon, either. A protocol for transferring serial binary data that was first defined in 1962 by the Electronic Industries Association as a standard for connecting electromagnetic typewriters to modems, RS-232 remains the most common way of controlling pro AV equipment in signal management systems.
"With RS-232, you have a dedicated pipeline, a two-way communication between two devices, and there's nothing to interfere with it except the wire itself," explains Rich Sasson, director of technical services for Crestron.
Blaine Brown of Sensory Technologies will build a dedicated IP network for his client's AV systems to enhance control.
Credit: AJ Mast/WPN
There are still plenty of devices that can't live on the network–DVD players and consumer audio equipment, for example. And call them Luddite, but plenty of AV pros out there still appreciate the reliable simplicity of controller and equipment communicating through just one serial cable.
"We predominantly use RS-232 in the systems we build," says Dan Walter, senior programmer for Phoenix-based ExhibitOne Corp. "We just find it a lot more reliable. Anytime you put something like a network between two devices, you run the risk of something failing or not performing as it should. And with an RS-232-based system, there's one less thing you have to check. I'll put it this way: Everyone has used Ethernet at some point in their life, and everyone has lost Ethernet connectivity at some point, as well," he explains.
"If you're a more traditional AV dealer, you're probably more comfortable with serial because you've been doing it so long," Noble admits. "Everything is localized, and you're in control. When you move onto a network, you have to realize that somewhere there's an IT manager who owns it."
So which control standard is right for your next project?
Pro AV breaks down some of the key advantages and disadvantages of both schemes.
Flexibility And Performance
Serial communication between two devices is defined by fairly stringent limits, with any increase in cable length mandating a direct and proportional decrease in the amount of data that can be transferred quickly. For that reason, RS-232 cable runs should extend no further than 50 feet as a general guideline, and AV equipment can never stray too far from its control module.
The number of devices controlled is also constrained, of course, by the number of RS-232 ports on the given control module, meaning that the addition of more system components can dramatically increase the cost of the controller.
These limits do not apply when the controller and the devices are on the network. Control spans to any addressable device and can extend as far as the network itself. This allows, for example, a technician on one side of a college campus to control a projector in an auditorium that's half a mile away.
IP protocol enables greater expansion, too, with the number of equipment pieces that can be added to an AV system limited by network bandwidth but not by the number of ports on a processor.
Meanwhile, high bandwidth (the IT gods willing) enables faster communication between controller and device–gigabytes per second compared to RS-232's maximum transfer rate of 19.2 megabytes per second–enabling device control to be far more granular than merely "on" and "off."
And while RS-232 configurations require extensive wiring, existing IP infrastructure can now be found in most corporate, education, and government environments–even a lot of residential ones–enabling an AV integrator in many cases to leverage a network backbone that has already been established.