Building a Z80 computer after completing one with 65C02.

Z80 on my project needs some pins to be tied high (INT, NMI, WAIT, BUSRQ), these are the ones that I don't need for the time being. There are many diagrams where these are directly connected to 5v VCC, and many others they are connect to VCC via a resistor of up to 1K.

The datasheet wasn't helpful in deciding whether I needed resistors or not (or perhaps my limited knowledge).

What is the correct approach here? Are the resistors definitely needed? Or is my circuit working just by sheer luck and about to fry any moment?

  • 2
    Could you provide examples where the control lines are arranged in each way. My suspicion is that where you see the resistors they are pull-ups for a signal that comes from elsewhere or off-schematic and there should be evidence of this in the schematic itself.
    – Dan
    Commented Mar 20, 2021 at 21:54
  • 1
    For pull up a resistor is not needed, but it's a good idea to restrict current. So usually it will work without, but adding a 1k is good practice. (No time for a full answer, which I feel should add information about why and what is to be done with various technologies CMOS vs NMOS)
    – Raffzahn
    Commented Mar 20, 2021 at 22:04
  • 13
    I won't answer just yet, but if you do pull up unused pins with resiators, it will be future proof and compatible when you do start using those pins. Otherwise you need to disconnect them from 5V and might need a pull up resistor anyway.
    – Justme
    Commented Mar 20, 2021 at 23:00
  • 2
    also the usual value for pull-ups these days is 2K2 ...
    – Spektre
    Commented Mar 21, 2021 at 6:59
  • Thank you all. I am really curious about @Raffzahn pending explanation. I sorted out some other issues on my setup today, and everything is working fine without resistors. So aside from future-proofing (which is a good point) what am I missing ? Shorter lifetime of components ? More prone to surges ? Or what ?
    – Charles
    Commented Mar 21, 2021 at 19:03

1 Answer 1


If your device is complete in the sense you won't do any changes or upgrades to it, you can connect inputs to 5v supply directly.

However, if you plan to have some upgrades or changes to be implemented later, you're safe with using pullups. When you need to apply a signal to the pin, you might not bother to disconnect a pullup, which is somewhat simpler to do than disconnecting input from 5v supply.

Regarding pullup size, 1k and 2k2 looks for me too small, as they source 5..2ma when applied with a logic 0. 10k looks OK.

PS: and don't forget to have strong pullup on CLK input of your Z80, if feeding clock off TTL (74xx or 74LSxx or 74ALSxx or 74Fxx, but not 74AC/HC/ACT/HCT) output. Unlike other Z80 inputs, CLK is not TTL-compliant.

  • "strong" pullup means lower resistance, correct? Good tip on understanding that CLK is different from other pins, thanks.
    – davidbak
    Commented Mar 22, 2021 at 19:42
  • @davidbak about 200 Ohms if I remember Z80 datasheet correctly.
    – lvd
    Commented Mar 22, 2021 at 19:55
  • Do not just check about currents, check the required voltage for the Z80 to register the clock input as "high" (VIH), and check the guaranteed output voltage of the clock source while it outputs high (VOH). You want some margin, i.e. VOH of the source should be well above VIH of the Z80. TTL chips are notoriously weak at providing high VOH, which is not a problem if you just use TTL devices: TTL inputs have a VIH specification of "everything above 2V is deemed high", so no need to prvide VOH higher than 2.5V for those chps. Commented Jul 10 at 23:15

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