Parity Generator Question

PaulB · Post by **PaulB** » Thu Nov 18, 2004 9:38 am

I hope one of you electrical gurus out there can help me. I've been wanting to post this since the forum went offline.

I wish to make this parity generator and can understand all of it except for pin 4 on the chip. It says 'input' but what does this mean? Does it mean +5V or something else?

Paul.

Post by **Greenious** » Thu Nov 18, 2004 2:38 pm

I belive that it is parity input if you want to use the chip as a parity checker instead of a parity generator.

PaulB · Post by **PaulB** » Thu Nov 18, 2004 6:29 pm

Cheers Greenious, so if i just want it to be a parity generator then pin 4 is left unconnected?

Paul.

Post by **Greenious** » Thu Nov 18, 2004 7:45 pm

To be honest I have never done a parity generator mod, so I am no definitive authority on this.

Also, I must admit that I don't know what chip you're using so I might be wrong for that reason aswell.

But yes, I think you should leave it not connected, but doesn't the schematics you got show that?

PaulB · Post by **PaulB** » Thu Nov 18, 2004 9:39 pm

I hope you can see the picture in my first post, i've a server on my pc and it's loading from that. If you can see it, that's all that was provided on the site i got it from http://atari.nvg.org/parity/parity_gen.html. The chip in question is a 9 bit parity generator Phillips 74HC280. The picture looks pretty straight forward and easy to put together but i didn't want to start building it without knowing what the 'input' pin was for.

Paul.

Post by **Greenious** » Thu Nov 18, 2004 10:12 pm

Ah, ok, now I think I understand.

Input looks like it should be connected to ground from what I can see.

It seems to be controlling whether the chip is used as a parity generator, or a parity checker.

You best bet is to find a datasheet for a 74HC280. That should tell you for sure what to do.

belboz · Post by **belboz** » Fri Nov 19, 2004 8:12 pm

The 74HC280 is a 9bit parity generator/checker.

Pin 4 on the chip is the 9th bit of the 9bits of data input.

You only need 8 bits since your taking the 8-bits of SCSI data and feeding them into the first 8 bits on the 74HC280. Since the 9bit isn't used and we want to use the "odd" output on the HC280 we tie that 9th bit low.

In reality the chip description on the site you linked to should call pin 4 D8 instead of input.