[Coco] What do you use for pull-up and pull-down TTL resistor values? (LS series) (coco related project but possibly OT)

[Ed Snider]

Hi James,

It’s always best to refer to the datasheet for a particular device if possible, but in general,
tie unused inputs high or low, outputs can be left unconnected.

It will always be safe to tie them high or low through a pulp/pulldown (within the chip voltage ratings).
Usually it can be tied to ground without a resistor, though that probably isn’t best practice.

I generally use 10K for the value, though the value here isn’t critical.

- Ed



> On Apr 22, 2019, at 11:38 AM, James Ross <jrosslist at outlook.com> wrote:
> 
> When you need to hardwire a TTL input (NOT using a switch) to either high or low (for example let’s say pin1 (DIR) and pin19 (/OE) of the 74LS245 chip – but any chip really) what value of a pull-up (high) or pull-down (low) resistor do you use?  Or do you use one at all?
> 
> There is *SO* much information on the internet about pull-up and pull-downs I get confused on how to know when one is necessary or not and if so, what value to use.  
> 
> Do you just go w/ a known good value that works everywhere? like 4.7k seems to be such a value for both pull-up and pull-down? Or do you have two different values, one for pull-up and one for pull-down?
> 
> Or do you re-calculate for each use and try to find the optimal value in each case based on the chips specs?  
> 
> Also different scenario but related -- do you tie all unused TTL inputs to either high or low? And do you use a pull-up or pull-down to do it?  (w/ outputs left unconnected)
> 
> Thanks
> James
> 
> 
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