Further Thoughts
Besides that 8051 was connected to the serial port of a PC, it was also successfully connected to the parallel (GPIO) port of a CerfBoard. Namely pin 0 of the port 2 of 8051 connects to the pin 0 of the CerfBoard's GPIO port; similarly pin 7 connects to pin 7, and so do all the intermediate pins. Lower 8 pins of the CerfBoard's GPIO port must be set to input mode. thumboard.c is the source code of the corresponding kernel driver for the CerfBoard. main.c is the source code of the user level test program. This Makefile could be used to build the executables thumboard.o and main. The scripts install-thumboard reinstall-thumboard delete-thumboard are helpful for automatic (re/un)installation of the thumboard module.
There was also attempt to connect 8051 to the PS/2 (i.e. keyboard) port of a PC, on the assumption that the latter is in essence a serial port. However the documentation obtained from the web site http://www.arne.si/~mauricio/Kbdfaq.htm did not seem to be clear enough especially on the part of the handshaking protocols. This topic as well as the one of connecting 8051 to the serial port of a CerfBoard are to be further investigated.