1

I'm designing a Z80 based computer and I want to hook up LEDs to the data, address and control busses (for added cool value). So far, the design is gonna be something like this: One board that the Z80 and RAM sits on, then that plugs into a mainboard with LEDs and slots for cards which connect to the CPU main bus for peripherals such as video or keyboard. Do I need buffers on the mainboard to drive the LEDs, expansion cards or both? What kind of current (and capacitance) can the Z80 CPU handle?

Thanks in advance

8
  • 2
    Hello and welcome to the Retrocomputing SE! I think that your question, given your context, and despite being about a CPU that was used extensively in a range of vintage computers, might belong more naturally over at electronics.stackexchange.com
    – Retrograde
    Sep 30 '20 at 20:11
  • 1
    @Retrograde yes but it's definitely on topic here also.
    – OmarL
    Sep 30 '20 at 20:37
  • @Retrograde Hmm OK I see that
    – Jachdich
    Sep 30 '20 at 20:39
  • If in doubt, refer to retrocomputing.stackexchange.com/help/on-topic which as "What should I avoid asking?" example gives: "Questions about electronics are off-topic unless they are confined to dedicated examples of existing circuitry of an existing and on-topic computer with the intention to understand its workings. For everything else, Electrical Engineering might be a good site."
    – Retrograde
    Sep 30 '20 at 20:45
  • 2
    What does the datasheet tell you? Oct 1 '20 at 6:13
8

While it's possible a very small Z80 system running at a low clock might survive the bus loading of low current LEDs, realistically you definitely want to use buffers or some sort of driver to power them.

Remember, it's not just the Z80 that drives the bus; the data lines are regularly driven by your memory devices, too and those would also have to have sufficient drive capability.

If you had a DMA controller (you probably don't...) then it's ability to drive the address lines would be a concern.

And of course your LED patterns aren't likely to be all that useful during operation at anything above a single-step or 1 Hz type clock. Though you might see a little bit about some programs on the apparent intensity of the address line LEDs.

If you had DRAM (you probably don't...) you'd likely want to use an enable on the address buffers to keep the DRAM refresh cycles from confusing the picture.

2
  • 1
    I think this answer nicely demonstrates why this question is on topic. Oct 1 '20 at 6:16
  • I see, I hadn't thought of the other devices (memory etc) driving the bus. Of course, LEDs are not gonna be useful at any real speed but I want them for 2 reasons: 1) debugging at low speeds but mostly 2) They look cool haha
    – Jachdich
    Oct 1 '20 at 16:36

Not the answer you're looking for? Browse other questions tagged or ask your own question.