Re: [oc] Output loading in digital circuit

[John Sheahan <jrsheahan@optushome.com.au>]

On Tue, Apr 02, 2002 at 11:48:07AM +0800, Kenneth Hung wrote:
> Hi Steve
> 
> Ur answer might be correct in some sense in pF.
> But do u know why there's an R=500 or R=1kohm in parallel
> with the C ? I can't see no relation why there's a R and why is such
> value ? For a typical transmission line, even the termination resistor
> is 50ohm.
> 

Rl in the hundreds of ohms used to be specified for old TTL logic, 
back when it actually consumed static input current.

You need to to check DC limits to ensure you reach Vil and Vih (plus
noise margin)

I have not seen Rl spec'ed for a while.  Can you point 
me at an example?

john
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