The TG 501 is a general purpose Time Mark Generator plug-in designed to operate in a TM 500 Series Power Module. A Variable Timing Readout provides a simple but accurate means of measuring timing errors over ±7.5% minimum range. Since many specifications are given in percentages, the error readout eliminates the need for computations and, additionally, lessens parallax discrepancies. Marker selection from 5 s through 1 ns in a 1, 2, 5, sequence is available. 5 s through 10 ns markers are selected by a rotary switch. 5, 2, and 2 ns modified sine waves are selected by self-canceling push-buttons when the rotary switch is in the 521 n position. All outputs are available at the front-panel connectors, or by modification at the interface connector, located at the back of the plug-in, and the TM 500 Series Power Module. Power supplies are current limited. The 5 V supply is over-voltage protected to avoid damage to the integrated circuits.