This article is from the alt.comp.hardware.homebuilt Frequently Asked Questions. It is maintained by Mark Sokos (msokos1@gl.umbc.edu) with many contributions from others.
http://cal003109.student.utwente.nl/stefan has info on various ports (parallel, serial, keyboard, etc.)
The comp.sys.ibm.pc.hardware.* FAQ (posted on news.answers and comp.sys.ibm.pc.hardware.misc, available via ftp from ftp://rtfm.mit.edu/pub/usenet/comp.sys.ibm.pc.hardware.misc) contains connector/bus pinouts and other useful information.
http://www.hut.fi/~then/electronics/computer.html#pchardware IBM, Technical Reference, Personal Computer AT, 1984
"The IBM Personal Computer, from the Inside Out" by Sargent and Shoemaker (published by Addison-Wesley) provides a lot of information about interfacing an IBM type PC to various things.
Talking to a floppy control from NC4000: ftp://ftp.hal.com/pub/elvey/floppy.txt
Using a non-DOS machine to talk to a DOS disk ftp://ftp.hal.com/pub/elvey/disk.txt
http://www-techdoc.intel.com and www.national.com Look up the datasheets for their super-IO parts, which have Paralell Port, Serial Port, RTC, Floppy Controller, all in one chip.
http://www.doc.ic.ac.uk/~ih/doc/adc_dac/
16- and 32-bit DLLs for port I/O may be found at: http://www.lvr.com
WINio.Dll and WIN32io for Windows 95 http://www.ping.be/~ping0751
Accessing PC I/O ports in Windows http://www.rain.org/~pra
EDE300 Application Note #1: Device Control via the PC http://www.netins.net/showcase/elab In the 'Integrated Circuits' section under 'EDE300'
Also, see each sub-section below.
The section "Recommended books/magazines" lists several books that may also be helpful.
 
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