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New Member
In an effort to extend the capabilites of my lpt port, I went out to my local electronics store and bought a 82c55a chip. After playing around with the chip, trying to get it to light up leds, I could not get anything useful working. I read through several datasheets and it appear that there are various modes that the chip has. What is the default mode and how does one go about using the chip properlly.

Any help would be appreciated.

Thanks in advance,


Active Member
By default the chip is in I/O mode i.e mode 0 with all ports as input (high impedance state). You need to set the control word register according to the desired directions of ports A,B & C i.e input or output. The register is selected using lines A0 and A1. RD\ and WR\ determines read/write operation from/to ports respectively.
Check this page which shows all the details regarding control word setting and reading/writing to ports. The page is a part of OKI semiconductor's 8255 datasheet.



New Member
After reading your post kuba, I just wanted to make sure you were aware that the 8055 is a microcontroller, which cant just be "plugged into" a cricuit and expected to run (or light up some leds). It is a small computer, which needs to be programed before it will do anything.

Slap me if you knew this.... :roll: Just trying to be helpful
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