I've been meaning to make an '816 based project that's running at 4 MHz and I've cobbled together this MPU and timing "core" the system will use. Is using 74HC's okay for this and will the timing work?

I assume that the two inverters between CLK and PHI2 are to delay CLK slightly? What's the reasoning behind that, and where do you use CLK vs. PHI2 and why?

Am I right that you're "trimming" the active time for the data bus transceiver at both ends using the U107A NAND gate by using CLK and a further-delayed version of PHI2 as the inputs?

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Curt J. Sampson - github.com/0cjs

I'm not entirely sure what your nRD and nWR signals are being used for, but I suspect they aren't doing what you want. The way you have them derived on the schematic:

1. When the processor input clock is high, one input of each of U107B and U107C is low, and so both nRD and nWR are high, i.e. inactive. If they are meant to be Intel-style bus control signals, you want them active when the processor input clock is high, since that is when data transfer occurs.
2. When RnW is low, U107B pin 5 is low, so nWR is high, i.e. inactive. Similarly when RnW is high, nRD is inactive. U107C pin 10 should be connected to RnW directly and U107B pin 5 should be connected to inverted RnW in order to get nRD low when you're reading and nWR low when you're writing.

Also, if pin 1 of the HC245 is high, A is input and B output. This is the wrong way round since pin 1 is high when RnW is high, so the processor is reading - A needs to be output in order to drive the CPU data lines.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Since this is presumably a problem for everything in colour on your screen, why not just turn on the colour blindness compensation in your OS? (In Windows 10 this is in the Settings app under Ease of Access » Color Filters.)

Regardless of method, I'm sure you can understand that it's a lot more efficient for you to develop a technique to deal with this once than ask every new poster to replicate this work just for you. Lots of us (including me) have visual issues of various kinds, and asking all posters to compensate for all of these on the source side would be a bit ridiculous.

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Curt J. Sampson - github.com/0cjs

Ouch... that was harsh.

While I don't have any vision impairments yet, I too am not a fan of many of the new fangled schematic drawing aids. In many cases the default colors and line widths are poorly chosen. I've been drawing schematics for well over 50 years... the old days were pencil on graph paper. Back in the 80's, I customized a set of menus and created a set of components for an earlier release of AutoCAD... and always in black and white... as it's always easy to read.

Then again, there's still a good percentage of the technical world that doesn't run Windows as a workstation, who knew.

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Regards, KM
https://github.com/floobydust

Here's the monochrome version.

I'm sorry; I didn't mean it to be harsh. It just seems to me that, now that we have tools to adjust for specific impairments locally, just doing what we need ourselves is faster and easier for everyone than going back and forth and asking people again and again to make a second copy of the schematic that's adjusted for an impairment. (As I mentioned, I have some vision issues, too, and I do use local tools to help deal with postings here and elsewhere where I have issues.)


I don't use Windows as often as Linux, but Windows does seem to be the most popular desktop platform so I thought that posting instructions for that would have the best chance of being a "what to do" rather than a "search for something like this on your system." As I mentioned, other systems do support similar accessibility functions.

I should also mention that I not only have no objection to monochrome schematics; I have a slight preference for them over the default colours that KiCad produces. This isn't about colour preferences, it's about handling accessibility issues.

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Curt J. Sampson - github.com/0cjs

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Sorry. My poor brain thought that the "Blue-yellow (tritanopia)" setting in Windows might be made for people like you.


Well, if there is indeed no easy setting in Windows (because tritanomaly is substantially different from tritanopia) you can still do something similar to what I do: use image editing features to tweak things. It's the work of a couple of seconds to select an area on your screen with Windows-Shift-S, click the resulting message, click "Edit with..." and "Photos," and then drop on a filter to turn what you just selected into black and white.

Again, the real point here is that you expect dozens of people to do extra work to help just you when pretty much the same result could be achieved by only one person doing similar work. And I find it very hard to believe that if, say, a client sends you something in colour you can't see you simply dump the client if he won't conform to your needs.

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Curt J. Sampson - github.com/0cjs

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Hm. I'd not thought about it that way. Ok, that's fair.

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Curt J. Sampson - github.com/0cjs

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

(Welcome, Gamerfox! I hope you get the help you need and can at some point in the future post about your fully working system!)