The early high speed tape readers used the clock pulse off the stepper motor (with suitable delay) to strobe the data from the photo-transistors. It was somewhat unreliable. Latter models added a ninth detector under the sprocket holes, which being smaller, neatly strobed the data in the middle of the punched data. The 11/20 I first used only had the paper tape software. The pdp-11 instruction set (at that point) was nicely orthogonal, so we often hand coded patches rather than use the assembler/editor, since it was faster.