11/23 clock issue

Pete Turnbull pete at dunnington.plus.com
Fri Feb 6 06:09:32 CST 2015

On 06/02/2015 07:44, Holm Tiffe wrote:
> You don't ned no pullup for +5. All open TTL inputs are reading High w/o
> any pullup.

Yeah.  So someone at Commodore thought when they designed one version of 
the PET.  We had a few that erratically misbehaved.  It turned out that 
one input on a 74LS00 (I think it was) was floating, and switching noise 
made it erratic.  Floating inputs place the internal circuitry in an 
intermediate state, can cause increased current draw, typically slow the 
device down by increasing switching times, and can cause misbehaviour.

TTL is supposed to have a 1K pullup (to limit possible transients); 
LSTTL can be directly connected to Vcc.


Pete Turnbull

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