The original MCS-51 designers at Intel actually decided that one port of their new microcontroller needed to be designed as "open drain" so that those port pins could pull low toward V_SS but not be having the ability to pull high.

The exact reasons why they chose to do this may never be known but a couple of things are certain:

- Many many clone and derivative 8051/8052 makers over the years have seen fit to provide devices that mimicked the original Intel design.

- Many product designers have found ways to take advantage of the open drain outputs so that they had unique capabilities or optimal circuit designs as a result.

- Some designers didn't read the data sheets carefully enough and designed their circuits without taking the open drain port into account. Then they later discovered during a debugging exercise about the open drain pins.

- Many a nine or ten pin 8x-10K ohm SIP resistor pack has been sold to provide the pullups for the open drain port on 8051/8052 designs.

Michael Karas