Lee:
IBM equipped their early printer adapter board with holes for these capacitors and (probably installed them on their versions of the boards). These were put there primarily to control the slew rate of the signals of the DB lines to the printer. THe LS drivers of the day were quite low output impedance parts and had reasonably fast rise/fall times. These 8 lines would/could travel down a quite lengthy printer cable and cause havoc at the printer end of the interface where considerable ringing would occur. The capacitors were one method to increase the rise/fall time of the interface bus signals to the level that they would minimize ringing at the receiver (printer) end of the cable.

As many many computer makers and adapter board cloners jumped into the market it was very common to see these capacitor holes on the boards, but no capacitors installed. I guess they figured "If IBM Then WE too..".
Well it turns out that later technology LSI chips used to implement the parallel port, in particular some of the first were things like 2P+S or 2S+P chips, used to make lower cost adapters had some difficulty driving the capacitors so they were often de-populated. I worked with several vensors in Taiwan and Samsumg in Korea sourcing the Vendex line of clone PC/XT and PC/AT computers. These had the parallel and serial ports right on the main processor board but as I recall we omitted the pads for the parallel port capacitors entirely.

Later on as the parallel ports moved directly inside the motherboard Multi I/O chip sets the parallel port entered the world of 3 flavors ... SPP, EPP, and ECP. These technologies brought bidirectional capabilities and at least one chip set I worked with had software programmable slew rate controls for the parallel port data lines much like you now see on many CPLD technologies. SO I guess today you will find some boards that do not have any parallel interface capacitors.

I am working right now with a SBC type PC board aimed at a passive backplane application. This is a '486/DX4100 board and its embedded parallel port comes vapacitors, series resistors and pullup resistors on every one of the parallel port lines!

Michael Karas