Re: Results of Uthernet testing with Apple IIe and SN74LS245N

[mwillegal <mike@willegal.net>]

This bus can be difficult - which is one reason that Apple went bus-less on the first Macintosh.  

For what it is worth,  I had a similar problem on the Brain Board that also showed up on a couple of  IIe machines.  This is despite the fact that I basically copied Apple's Firmware Board design, gate for gate.   The first case I heard about was fixed by swapping parts.   After the second case,  I investigated further.  Timing was fine, but I  determined that better grounding on the bus driver chip was needed. 

Regards,
Mike Willegal

On Sunday, April 15, 2012 1:45:47 AM UTC-4, Michael J. Mahon wrote:
> mwillegal wrote:
> > Hi,
> > 
> > PC suffix means plastic DIP package to Fairchild and Moto and perhaps others
> > N suffix means plastic DIP package to TI and many others.
> > 
> > So difference is more likely variation between manufacturers, which should not be very significant for LS series parts.  Note that differences between the data sheets can be due to variations in manufacturers qualification process.  The package type has little bearing on chip performance.  Another option to solve the problem may be to look at substituting a different technology pin compatible part, which has more margin for this application.
> > 
> > Regards,
> > Mike Willegal
> 
> I agree, Mike--any timing differences between manufacturer's LS chips
> should be inconsequential.
> 
> The fact that it makes a difference, and that adding a super serial
> card load disturbs the bus timing enough to cause failures suggests
> that the Uthernet card has very little timing margin in a //e machine.
> 
> A proper analysis would call for using a scope to check whether the
> setup and hold times are being met by the card.
> 
> This is a problem with limited-run cards.  The card may work fine
> in most machines but not all, as a result of small timing margin
> differences.  A proper card design has sufficient timing margin
> to work not only with all specified bus tolerances, but even somewhat
> outside those tolerances, to provide good reliability.
> 
> > On Saturday, April 14, 2012 7:26:28 PM UTC-4, D Finnigan wrote:
> > 
> >>Some people may remember back in November I was testing the Uthernet on my
> >>unenhanced IIe and II Plus to see if I could get it to work reliably with
> >>Contiki and ADTPro. I found that I had to remove almost every other card in
> >>order for it to work.
> >>
> >>About a week ago, Glenn Jones told me that another Uthernet user was having
> >>a similar problem. He noticed that some cards had a 74LS245PC instead of a
> >>74LS245N. It turns out that replacing the PC with the N solves the problem.
> >>
> >>Complete notes in my blog: 
> >>
> >>Part 1- http://www.macgui.com/blogs/?e=383
> >>
> >>Part 2- http://www.macgui.com/blogs/?e=384
> >>
> >>-- 
> >>]DF$
> >>Mac GUI Vault - A source for retro Apple II and
> >>Macintosh computing.
> >>http://macgui.com/vault/
> 
> -- 
> 
> -michael
> 
> NadaNet 3.1 for Apple II parallel computing!
> Home page:  http://home.comcast.net/~mjmahon/
> 
> "The wastebasket is our most important design
> tool--and it's seriously underused."