[En-Nut-Discussion] Ethernut Port Expansion issue.

Grahame M. Kelly grahame at wildpossum.com
Thu Aug 19 15:48:08 CEST 2004

Hi All.

I am developing a small board which runs some relays. I have a 74HC00 input 
chip that reads\decodes PD4 and PD5 for R/W. However its intermittent to the 
point I have check, and rechecked the Eurocard connector soldering, wiring 
etc... but I can't find anything. 

I am running a 64way flat cable with EuroCard (ab) connections both ends 
(400mm cable length) on a V2 Ethernut. I noticed that placing my scope probe 
(Tektronics 465) fixed the problem. Umm! some sort of loading required I 
thought. - So I tried a 10K across PD4 to earth then to Vcc - bummer no 
change. Same for PD5.

I then replaced the 74HC00 with other family members LS, ALS, H, and plain old
TTL considering their input loading is 4K (LS), 2K(ALS), 280(H) and plain old 
ttl is 400 ohms may provide the correct loading if that was the issue. 
Unfortunately nothing changed, so I replaced the 74HC00 with another just in 
case, but no luck - same issue..

Only the scope probe --- Why?  Maybe cable capacitance problems I thought. So 
I tried extra 10pf's to ground... and 1k/3k SCSI type cable termination again 
no joy.  I have noticed that with ALL logic families except for the HC, can't 
reach ground (they usually go only to about 1.8 - 2v for a low).

I am at "the end of a thin end of sanity" now so, any advice or better still a 
solution will be greatly appreciated.

Thanks & Cheers. Grahame

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