Programming an 8255 in Assembly Language

jdmaxey used Ask the Experts™
I'm looking for any good references that will help me program an 8255 in Assembly. My actual assignment is to program the CPU to send binary numbers through an 8255 to a quad OR gate IC to verify that all the OR gates are operating properly. I have the address of the 8255 and I know that I ned to operate pins PC4-PC7. Is there a site or anything else that might shed some light on the assembly language portion of the problem?
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actually you don't 'program 8255' - there is no CPU inside ;-), you just have to set proper signals in proper sequence on proper pins of the chip ;-), this can be done by hardware design around the 8255 with extra logic ICs, or by external CPU/MCU which will send proper signals...

so it's all depends on your schematics... try to explain what kind of device you have and depending on that your problem can be solved...

get some datasheet for 8255, there are all modes of operation described...

like this one:
you could try this site:


I checked out both of your sites and they were a huge help. I realized I couldn't program the 8255 directly, just a failure on my part to communicate effectively. I think I have the 8255 figured out now. Thanks for your help. Do they still make this thing, or is there a newer version that works better?
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I don't know about any new versions, so probably the same part still exist, but actually nowdays there are so many sofisticated microcontroller units, that those good old parts like 8255, 8251, 8253 etc are now obsolete... you can have one single chip that includes all of this plus much more ;-), very efficient and simple...

if you are really looking to electronic engineering - read about MCUs

The 8255 is still advertised for sale within the pages of Everyday Practical Electronics, a monthly amateur mag.

try sx28
I won't recomment thinking of using a 8255. Its is an old device (I guess the design was in the beginning 80th) and it wonders me, that this is still available. Compared to modern chips, those designs waste a lot of silicon and manufactorers do not earn much money (thats the reason, why most of those chips are already out of production).
If you already have a micro, just think of using an adress-decoder and a 74HC273 for output or if you havn't, there are a lot of micros with flash, port-pins, uart and often also the clock-generator inside (for example atmels AVR-family). They can be programmed with an inexpesive interface directly from the PC and a lot of users are offering help (AVR-Forum).

Good luck
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