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Evaluating Control Options

Nov 16, 2011 2:52 PM, By Patrick Barron

Making the case for IP control.

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There has been much discussion and commentary about the subject of AV and IT convergence and how the use of IP has migrated over to the world of AV and control systems. For the most part, this has been a positive occurrence and the traditional AV systems have benefitted greatly from the introduction of IP interfaces. But as often happens with new trends, there can be too much of a good thing. As great as the introduction of network and IP-based control has been, there are limitations we must recognize. As the famous comic book stated, “With great power comes great responsibility.” Audiovisual professionals need to use IP control wisely and find the applications where it will be most beneficial.

Because network and IP control has been such a hot topic, there is temptation to design systems with IP control for no reason other than just to say that you’re using IP control. Understand that IP control is just one of several viable options. All control options should be evaluated to determine which solution is best for a particular application. RS-232 is still a very reliable method for control and should be considered each time when the control method is being debated. Salespeople fall into the trap of selling systems with IP control because they saw it at a tradeshow (where everything always works perfectly, right?). Designers and engineers fall into the trap because they want to push the envelope with the newest and greatest technology. We need to remember: Just because you can doesn’t mean you should. This particularly applies to the desire to control everything possible using IP.

Recent graduates are used to doing everything over IP while programming in college. The thought of using RS-232 control seems foreign to them. They think of RS-232 like we think of the rotary telephone and 8-track tapes. To them, the question is, “Why would you ever not use IP control if it is available?” Many don’t have the years of field experience in troubleshooting systems to draw from when making design decisions. While IP connections managed from a computer may seem trivial and almost flippant, when done from a control system, it opens up a whole list of potential problems to be overcome.

The goal of any control system is to function reliably. If it isn’t reliable and doesn’t work, the method of control is irrelevant. One of the troubleshooting methods in solving system problems is to eliminate points of failure. In any system, every connection, every cable, and every piece of equipment is a potential point of failure. The most dependable system is where we have a source, a cable, and a destination. In that case, there are only three items that could fail: the source, cable, or display. When we add a switcher, we add another cable as well. Now that system has gone from three potential failure points to five. If we add a patch panel, wall plate, floor box, or other connections, we add more failure points. Large systems that have scalers, converters, and switchers that cascade into other switchers make troubleshooting extremely difficult when trying to isolate where the failure occurs.

IP vs. RS -232

The same logic applies when discussing control methods. Let’s analyze the differences in troubleshooting and maintaining an IP connection vs. an RS-232 connection, assuming that the protocol is used correctly and the code is written properly. In systems with simple RS-232, the cable essentially is the only failure point. In most cases a null modem adapter can solve 95 percent of all problems with RS-232, but when dealing with IP, the potential points of failure are multiplied greatly. We have to know the baud rate with RS-232. IP connections require that we know the IP port we are communicating through. RS-232 has a few standard baud rates that are frequently used, so even if we don’t know the baud rate, we can guess it within two or three tries. With IP connections, there are more than 10,000 valid IP ports. If the standard telnet port doesn’t work and we don’t know the port to use, we could spend a lifetime trying to guess what it might be. Programmers writing code and troubleshooting software always want to use the time available in the most efficient manner possible. With RS-232, there are very few things that might go wrong, but with IP, the list of items that can be a problem is extensive. If a programmer has to spend five minutes troubleshooting a network or IP problem, that’s five minutes too long. Trying to save a few dollars by not buying the necessary hardware to have a serial port available for control can turn into huge cost deficits if a large number of man hours are spent solving network issues that otherwise would be irrelevant.

The traditional control system is designed around an embedded processor, not a Windows or Mac machine with virtually unlimited processor speed and memory. A control system measures processor speed in hundreds of MIPS and measures memory in megabytes while the top of the line computer is approaching speeds of more than 100,000 MIPS with memory in the multiple gigabyte range. When dealing with IP we need to understand that an AV control processor has only a fraction of the capability of a typical computer. The IP connection management for a normal computer is an everyday process and thousands of manhours have gone into the development of stable IP socket connections to a multitude of different types of equipment. The socket connection management on a computer has so many layers, and there is so much processing going on behind the scenes that it would make the normal person’s head spin to comprehend how it all works. Yet we take it for granted every day when we browse to a website or stream music from the Internet. A control processor does not have the same kind of development with the IP socket connection management, and much of that management is left to the individual programmer to handle. A programmer could spend 10 years straight doing nothing but IP socket connection management and still barely scratch the surface of the amount of development that has been done to handle the IP traffic on your desktop computer. We need to realize that connecting to a piece of AV equipment over IP is not as simple as browsing to a website on your computer. There is not a magic solution available where everything instantly works. Each piece of equipment desiring to be controlled should be evaluated to see if IP control is really the best option or if there are other approaches that will work better.

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