Return to the RS-232 Issue: Extron Chimes In

A recent survey on AV-1 questioned technology manager’s experiences with RS-232 control of their projectors. The results revealed widespread, intermittent RS-232 problems especially with serial-controlled projectors.

90% of survey respondents indicated that at least once a month their projectors have “locked up” and stopped responding to RS-232 commands. Ill-timed, random malfunctions of this sort can result in event interruptions and loss of end user confidence in presentation systems and support services.

Translation: This is a serious issue that must be resolved.

Several respondents have pointed out that this may not be an equipment problem, per se, suggesting that a problem could manifest under a combination of conditions (specific installations, RS-232 programming, soldering, etc.). Could it be that faulty installations or programming may be contributing to these problems?

AV-1 turned to industry experts at Extron Electronics, to see if they could provide answers to some of these questions. Extron has a dedicated group that supports higher education. “We interface and coordinate issues and projects on a daily basis at Extron”, Tim Schnabel, Director of Education Programs. “If we know of an issue, we can work collectively to solve it. Being an interface company it is important to keep these channels of communications open to ensure system operation and customer satisfaction.”

Jesse Velasquez has been an Applications Engineer at Extron for over 10 years. His present capacity is serving as the Supervisor of the Control Products Support Team in Anaheim, CA. Jesse indicated that, very often, these types of issues can be traced to one of the following:

  • Improper wiring of the serial communications port specifically the hardware handshake lines (CTS/RTS pins)
  • Firmware of the video projector requiring an update and display device status polling from the control system not being handled properly

AV-1: Are there any best practices associated with writing program code?

Jesse: A moderate rate of display device polling is baked into Extron’s IPLink serial driver. We intelligently pace the polling of the display device with respect to the individual manufacture’s control protocol. For example we do not poll for display device status during the display warm up and cool down periods and delay polling of any device status queries right after a control command was sent to the projector. This practice minimizes the chances of the display locking up as a result of polling too frequently or when the display is busy.

AV-1: A number of our survey respondents suggested this very idea that over-polling might cause the projector to become overloaded with too much buffered data. What about the RS-232 wires and connections themselves?

Jesse: Using a high-quality, low-capacitance shielded communication cable is good practice. This ensures reliable serial communications between the control system and display in environments that may be noisy. Wiring distances should also be taken in consideration; a standard rule of thumb is not to exceed 50 to 100’.

Also keep in mind that as the baud rate of the serial port increases the distance you can reliably send data decreases. Leaving the projector’s baud at the factory default setting in most cases is usually fine and this is what we code our IPlink driver for. If you encounter a control issue with the display changing the baud rate setting may be a good troubleshoot tip. In some cases we have recommended that customers adjust the setting to compensate for a long cable runs or to resolve reports of intermittent communication

AV-1: We have heard projector manufacturers suggest that the projector may be getting “dirty power”. While this may seem plausible, it also is an easy answer for a company who may not want to fix a problem. Is there any fix for this, other than re-wiring a building?

Jesse: Provided that the control system and display device are properly grounded you should not require the need for a power conditioner. Communication problems can arise where the voltage between the ground pin on one end and the ground pin on the other end is not zero.”

AV-1: Are we just wasting our time with RS-232? Why not just switch to IP and not deal with any of these issues?

Jesse: While IP certainly takes away the problems with RS-232, controlling a display device over IP adds network-specific challenges such as setting up IP, subnet and gateway and making sure that certain TCP ports are allowed to pass on the network. The display control protocol over IP may also be limited, please review the specific device communication sheet or protocol manual.

AV-1: Oh, and let’s not forget that campus networks on occassion have been known to go down. Thanks, Jesse and Extron, for filling us in.

If you have additional thoughts, share your comments below or on the AV-1 list!

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