1.Port Programming

This is more on the lines of a data sheet rather than a tutorial. The programs are the only saving grace. Two sorts of ports on a PC that offer a world of experimentation to the casual electronics buff are the parallel port and the game port. The serial port requires some more programming skill but is not that difficult. First I will describe the easier ones and gradually we may even go inside our computer and have fun with the ISA slot (if there are still any left around).

i) Parallel port

The parallel port aka printer port is specified on Windows machines usually as LPT1 or LPT2 while on Linux machines it may be usually either /dev/lp0, /dev/lp1. Whatever the name, on PCs its common base address is 0x378 assuming you have only one printer port. Somewhere I read that the addresses 0x3BC and 0x278 are also allotted for printer ports. If in Windows check System Information to find out the address on your machine. For our purpose I will assume it to be 0x378..since that's the one on my machine ;-)

The only parallel port that I got to study was the 8255 programmable port. However I could find no relationship between that and the one available on my PC. Frankly, it is the research that followed that has led me to write this. The printer port on your system would more likely be a 25 pin female D-connector. Here is the pin details:

PIN	DIR	SIGNAL
1	IO	STROBE : goes "low" -data read in
2 - 9	IO	D0 - D7 : data lines
10	I	ACK : approx 5micro s pulse -"low" printer has received data and ready for next
11	I	BUSY : "high" - printer can't receive data
12	I	PE : "high" -printer out of paper
13	I	SLCT : indicates printer is in selected state
14	O	AUTO_FEED_XT : "low"-auto feed of paper after every line printed
15	I	ERROR : "low" in paper end,offline or error states
16	O	INIT : "low" resets printer controller
17	O	SLCT_IN : Data entry to printer possible only when pin "low"
18 - 25		Ground pins

A low state corresponds to 0V and high to 5V. According to IBM specifications pins 1,14,16 and 17 (control pins) have open collector drivers pulled to 5V through 4.7k resistors (sink 20 mA, source 0.55 mA, high-level output 5.0 V minus pullup). The others sink 24 mA, source 15 mA, and their high level output is min. 2.4 V. The max. low state volt for both is 0.5 V.

For each port we need to know three addresses

BASE+0 : Data Port
  A write on this port latches data on to the pins and a read takes in the last values latched on to the pins

BASE+1 : Status Word
  Read only.

B0	B1	B2	B3	B4	B5	B6	B7
Reserved		INT status	ERROR	SLCT	PE	ACK	BUSY

BASE+2 : Control Word
  Write only

B0	B1	B2	B3	B4	B5	B6	B7
STROBE	AUTO_FD_XT	INIT	SLCT_IN	INT ENABLE	Extended Mode Direction	Reserved

Now that we have mentioned the basic hardware interface of the parallel port we can move ahead to making some use of all this info. We will try to write some programs utilizing the port in the next tutorial.