Re: IEEE-1284 problem

Tim Wescott wrote:

Are you terminating the signals properly? Is your add-in board terminating the signals properly?

Poorly terminated signals could cause considerable ringing on the strobe pins, which would cause double writes. Termination matters, but things often work when only one side is terminated; it may be that your board isn't terminated, the motherboard is, and your add-in board isn't.

Careful digging into manufacturer's data sheets for their EPP chips don't detail anything about termination, I rather suspect they don't deal with this. Some boards have no sign of resistors, either, others do. These particular Par Port cards do have what looks like 33 Ohm series resistors on a bunch of the pins.
I'd go over the IEEE-1284 spec carefully for the expected termination on the printer side and make sure I was compliant. Then I'd consider terminations anyway, just to make sure.


I threw a 100 Ohm series resistor on the nWAIT line right out of the driver on my board, and it appeared to make things WORSE!
Also, the nature of the ringing on that line made it appear there is no receiving termination on the PC plug-in card. It appears to have a small cap to ground and a 33 Ohm R in series going to the chip on the par port card. Assuming that that chip's pin is programmed as an input when the par port card is the master, such a resistor would not help the situation.
Well, maybe the fact that adding a series resistor to nWAIT CHANGED things is an important clue, and I just have to find the
magic combination that makes this work. Raising the impedance of a line subject to a lot of crosstalk is not the best approach. I've played arond with the FPGA state machine on my board to change the timing relationships, giving the data more setup time, etc, without much help.

I'm hoping to make my board work with commercial, off the shelf PC parallel ports, despite their flaws and cut corners. Of course, I've found a couple that have such serious flaws as to be totally unusable, and that goes for some on the motherboard par ports, too.

Thanks for your comments.

Jon
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