Abstract: A dot line printer is releasably connected by a control circuit to the output of a monitor which senses variations or faults in the AC line voltage for a computer or the like. Each time the monitor senses any one of the five different voltage disturbances, the information is transmitted as digital data to the printer, which then produces a continuous strip chart or graphical printout of the voltage variations. The printer advances paper continuously and in small increments, while printing thereon six parallel trace lines that extend in the longitudinal or feed direction of the paper. One trace line represents time, and the remaining lines the zero ordinates for five different voltage faults. Whenever a fault is detected, the associated trace line widens to print a horizontal bar. When a power failure occurs and is thereafter restored, a header (indica line) is printed on the paper before printing of the trace lines resumes. The monitor and printer, if desired, may be operated independently of each other.