View Full Version : Do some of the Z80 signals need a pullup?

February 28th, 2018, 03:46 PM
I saw a note on the RC2014 page about a 10K pull-up on some signals. Is this to strengthen an already internal pullup? Best practice type of thing?

February 28th, 2018, 03:49 PM
While I'm asking this - also - is the method to control these signals to pull them down or go HiZ, or to drive them high/low? If driven high/low and you have multiple devices wanting to drive the BUSREQ line for example, did they have to be joined then with logic like an AND gate?

February 28th, 2018, 04:51 PM
Typically BUSREQ drivers would be wire-ORed, like from open-collector outputs. That signal requires a pull-up if whatever is driving it does not provide sufficient pull-up. Of course, open-collector provides no pull-up at all. Similar for INT and probably WAIT, NMI. I don't know which, if any, of the Z80 inputs have internal pull-ups. If you dug into the datasheet it might tell you, but 10K pull-up sounds OK, although I see 1K on those inputs in the H89. In that case, I see INT driven by a standard totem-pole TTL output with no pull-up. I'm sure there are other views out there. If you do drive it with tri-state outputs, you'll still need the pull-up.