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

[Dave Philipsen]

Generally a 4.7k resistor is used used for pull-up on 5v TTL inputs or open-collector outputs (I have seen values as high as 10k used). A pull-down is not usually needed. It’s good practice to tie any unused input either hi or lo. If you need to tie low just connect directly to ground. No resistor needed. 

Dave

> 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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