Hello all, I'm building a JTAG interface w\ a buffer that connects to my PC parallel port. The schematics indicate a 74__244, which is an octal (8) buffer chip. The directions say that "avoid 74LS244s because they don't work with the wide range of voltages put out by some parallel ports" This seems bunk to me, a lot of 74LS series chips I've seen take some pretty darn high input voltages without vaporizing themselves. The only reason I could see for going to an 74HC is to be able to handle a 12V input. As far as the 74F, isn't that just a 'Fast' chip? IIRC, the PC parallel port uses 0-5V signaling, so the 74LS244 should be fine. Frankly, if I vaporize the parallel port on my ancient P166 laptop, I'm not going to be upset. I'll just pull one of my 8 P233MMX integrated everything boards out of the closet and fry it to So is there any logic to the argument that a 74HC244 would be better than a 74LS244 interfaced to a pc parallel port?
The lpt port produces a nice 5v TTL level signal. FACT! Thats the standard for ECP,ESP etc. anyhow. I would just get either a TTL buffer, or a TTL level enabled CMOS buffer it should work fine. The only problem with the parport is that some can only source 2ma, but that shouldn't be a problem, just remeber if you go for TTL enabled cmos, nice 10k pullups! Happy LPT killing
Alright, that's pretty much what I figured. I was aware of the low current limitation, which is why it is unwise to use an unbuffered JTAG. Here is the schematic, and since 5/2.2mA =~ 2.2K, it should be fine.
okay, just again TTL enabled CMOS is a must, HC often isn't HCT is i think. Check the datasheet probably best, i have mostly given up descrete stuff.
From what Ive read and remember: 74LS is for low-power/battery operated designs. 74HC is for high current output (handles 20mA on the output, better for driving things universally). It also consumes the most supply current. 74F is for really fast designs. Handles 3.3V systems very nicely. I mainly stick to 74HC series. Their cheap and easy to find. And they drive any transistor. I had a point where I had to buffer the output of a 74LS NAND gate because it wouldnt drive a ULN2803 input (which is a darlington transistor array. Very sad that the LS couldn't give enough output current to drive that). On the other hand, Ive seen a few JTAG schematics that list the 74LS series chips. I would say, if the design says NOT to use a LS, then use the HC. They wouldnt have mentioned it unless there was a reason.........
after giving to the almighty one that is google, i found this on phillips semi. 74HC244 5V Octal Buffer/Line Driver; Non-Inverting (3-State) 74HCT244 5V Octal Buffer/Line Driver; Non-Inverting; TTL Enabled (3-State) Not sure if thats used by other logic gate manifactures, but like i said lpt is TTL level, so best get TTL enabled CMOS.
Well I suppse I'll put together the digikey order then. I was trying to avoid having to pay their silly $5 for <$25 order fee. Oh well. Thanks for the advice guys!
