Written by Steve Claypool, Caltrans
Bottom line: if you are purchasing a 170 system, or new or replacement 210 conflict monitors for the 170 system, specify a 1.5 second watchdog timer. Caltrans began specifying 1.5 second monitors in 1989, New York State DOT began specifying them in 1991. Some of the newest monitors are available with switch selectable 1 or 1.5 second watchdog monitors, and many older monitors can be modified to 1.5 seconds. This does not affect the green conflict timing or 24 volt power loss timing, both of which remain at .5 seconds.
The problem with the 1 second w/d timers is that, in some cases, they cause recurring “false” calls, putting an intersection into w/d flash when there is no hardware or software malfunction. Ninety to ninety five percent of 170 systems rarely, if ever, experience this problem, even with the 1 second w/d monitor. The “problem” intersections will go on w/d flash intermittently, sometimes with predictable regularity, from as little as about once a year, to as much as once or more per week. We know that sometimes the problem is caused by utility failures; a few problem intersections have been cured when a loose or corroded neutral was found and repaired. We suspect other sources of utility caused w/d flashes, but are not sure of the exact nature of the failure. Caltrans labs tried to simulate various line distortions, but could not duplicate the w/d failure. We also know that the problem is sometimes in the cabinet wiring. Several instances of recurring false w/d problems have been cured by replacing the output file or the entire cabinet. It would be logical to assume that some recurring problems are caused by a combination of the two causes.
In several cases where output file or cabinet replacement solved a recurring problem, the components were inspected and no apparent malfunction was found. In most cases where line problems were suspected, no identifiable problem was found. Various line filters have been tried over the years with no clear benefit. Several coincidences have been noted; one is that an intersection may develop a recurring “false” w/d problem when there is medium to heavy construction in the immediate area. This may be a factor of the power tools and equipment causing line disturbances, but many times the w/d problem occurs in the late evening or early morning, indicating it may be somehow related to the nature of the temporary service supplying the site. Another coincidence is that a rash of false w/d calls often occur after a widespread power failure. This is particularly bothersome because it often occurs during stormy weather when most signal techs are already busier than normal. *Note: large scale power outages often cause “real” watchdog failures also, either because of bad batteries in the 170 or because of temporary or permanent damage to the processor caused by power surges.
The watchdog monitor monitors a pulse put out by the 170. The “pulse” is written into the program, and changes state every 100 milliseconds. This pulse is output through one of the NPN transistors on the I/O board of the 170. A watchdog fault occurs if there is greater than 1.1 seconds between signal changes (greater than 1.6 seconds in a CALTRANS type monitor). Less than +4 VDC is a LO, and greater than 12 VDC or open circuit is a HI. The extra half second apparently gives the system a little more time to recover from whatever transient problem is causing the false w/d flash.
In the East Bay Area, once we began putting the 1.5 second monitors in the problem intersections, false w/d calls dropped dramatically, from two or three per month to about two or three per year: almost invariably in intersections which still have 1 second monitors. (Even though the standard has changed, 1.5 second monitors are only being introduced in new cabinets or as replacements for defective monitors, so the 1.5 second monitors represent only about 10% of all monitors and were more or less randomly distributed around the area. Once we understood the advantage of the newer monitors, we began to move them to the problem intersections, accounting for the reduced trouble calls.