Posted by Mike Agri on 1/26/2012 to
Today, wireless systems for telephone, alarms, security, and other applications are all the rage. Here is a cautionary tale.
- At one time, IBM, and later Simplex Time Recorder Company, manufactured electronic, self-correcting clock systems that employed a high power frequency generator wired in parallel across the electrical service. The last minute of every hour, the frequency generator would be turned on for a fixed period of time, and insert one of several available frequencies on the entire AC distribution system in the entire building. The secondary clocks, which plugged into any available single phase, 2-wire 120V outlet, each contained a signal amplifier which turned the frequency into a voltage that would energize a solenoid operated correction mechanism in each clock for system-wide synchronization. It was discovered that If two relatively proximate facilities running on the same utility feed just happened to be set to correct using the same frequencies, it would sometimes result in cross talk between building clock systems which might have one or both facilities with a slightly different Master Clock time setting cause an adjacent facility to also go into correction, with the usual result of each or both systems advancing, every hour.