Multi-function Watchdog Timer
The Modem Monitor was first produced in 1989 to serve as an activity supervisor for industrial SCADA systems. The primary function of the unit tracked the binary level of a "Watchdog" activity pulse that indicated proper operation of the monitored system. This allowed the Modem Monitor to initiate an output pulse when the monitored system failed to toggle its normal activity output signal. A 1991 manual on the Modem monitor can be viewed here.
Arthur Zatarain conceived, designed, and developed the device to monitor and reset any device having a controllable output bit. Detailed design and per-unit production was done by Stan Taravella who modified individual configurations for each application. Although designed for typical RS-233 serial ports, the "Modem Monitor" could be configured to work with a wide variety of situations. In contrast to most devices of its era, the gadget was built with low power discrete logic devices rather than a microprocessor. This robust design minimized the influence of common mode failures due to a power spike or other interference that affected the monitored device.
The Modem Monitor could be configured in the field via jumper settings to determine the time intervals, counts, and monitored signals. The output pulse was provided via a built-in form C relay contact that could be used for any reset or signaling function. Typical applications were to reset a computer, signal an input bit, or any other momentary pulse function needed in the event that the monitored system had failed. The unit had a panel mounted switch to manually initiate the reset function. It also had an off-on switch for use during maintenance of the monitored system. The final version produced in the late 90's included a timed bypass to temporarily deactivate the modem monitor during service. That much-desired feature allowed a field technician to service a monitored system and leave the facility without concern for forgetting to re-enable the modem monitor upon departure.
The device could also monitor the state of another input that should not remain in a high (or low) state for longer than a specified time. This input was normally tied to the Receive Data (RD) input serial line from a remote system. The Modem Monitor would generate the reset pulse if the remote system sent a long "break" signal over the serial communications line. This very useful feature allowed reset of a remote system using a single keypress on a basic computer terminal, or with a personal computer running a terminal emulation program.
Another handy feature was the to ability perform the reset function if an input bit toggled more than a specified number of cycles. This feature was normally used to monitor the Ring Indicator (RI) output from a modem. The Modem Monitor would count the ring cycles of an incoming call, and perform the reset only if no activity resulted on the communications line. The ring count was cleared by the modem's Data Carrier Detect (DCD) signal that represented a successful off-hook and connection between the remote and local systems.
Typical applications included monitoring the RTS or DTR bit of a serial port on an industrial computer or PLC. Software within the monitored system would toggle the output bit to indicate proper operation. Any system problem that affected hardware or software capability to manipulate the bit would initiate the Modem Monitor's reset pulse.
Interestingly, a patent US 5,430,865 was granted toGeorge Lazik in 1995 for a "hardware remote reset circuit" with features similar to those of the modem monitor.
Copyright © 2017 Arthur Zatarain, all rights reserved. Some images are modified for confidentiality
or illustration clarity. This site should not be used as a technical or legal reference.