Hi folks. Hopefully my post won't get lost in all the spam I noticed in this area!
Anyhow, I want to do something fairly simple, but I want the advice of someone who actually knows more about this stuff to make sure I don't do something wrong that I'm not taking into account.
Basically I want to connect an SD card to my PC's parallel port. The biggest reason is that I want to familiarize myself with the protocol for curiosity's sake, but also because I want to eventually be able to implement an SD card reader in a different hardware project which happens to have a parallel port as well.
First of all, since my reader will be detachable from either the PC or the other device I want to eventually plug it into, I figured that using two AA batteries would be a fine power source for the SD card, with some capacitors to smooth the power. Correct me if I'm wrong.
Second, I decided that for safety's sake I would use a 74HC241 octal buffer which I have laying here already. I should be able to power this with the 3v battery power if I understand properly. I then want to connect the data out pin of the SD card into the buffer, and then from there into the parallel port. I assume this should be plenty of output voltage from the buffer when running at 3v to trigger a TTL input, right?
But here's the bigger part I'm concerned about. I then want to run lines out of the parallel port into the buffer, and from the buffer into the SD card. Now, normally, I know TTL output isn't quite high enough to trigger a CMOS input. But, since I'm powering the buffer at
3v, does this change things? Will the TTL outputs be adequate for a CMOS part running at a lower voltage? I have a chart that shows the various voltages for an HC part, but only when powered at 5v, so I don't know how that scales or anything.On the flip side, could the TTL outputs be too much for an HC part running at a lower VCC? Maybe resulting in too much current going into the SD card, perhaps?
I'd certainly appreciate any input anyone can offer on the subject. Especially any ideas on the design in the event that they've constructed something similar before.