Designing A Z80 Computer Part1

[@iggysfriend4431]

I have just bought the book and was delighted to find there is a corresponding video course too. Awesome!!!! I will definitely be buying the 2nd book about the OS.

[@admirerofclassicalelectron2858]

I am really looking forward to the video series and even more to the future book. The beginning is very promising.

[@allunread1358]

Looking forward to the series and the book

[@vanhetgoor]

You explain it like it is easy. I would never come up with the idea of using dividers to generate video, I would be looking for a Yamaha V9958 or something or a TMS8818 when in a NTSC-country. I never understood how the ZX80 etc. made their video, something something magic ULA and the result is an image on screen. Wonderful!

[@AllElectronicsChannel]

This will be a nice series!!

[@garyjohnson4608]

Yes, I am very interested in the book you are going to release !!!!!!!!!!

[@davef3238]

Great project. I'm very much looking forward to the rest of this series.

[@MattBaker1965]

Sounds great, go for it. I am a Z80 guy with a 48k ZX Spectrum and SBCs it the 80s at Uni. :)

[@axelBr1]

Just came across your channel. My first computer was a ZX81 and a few years later I got an Amstrad CPC 464, so I have a special fondness for the Z80. As my father was an electrical engineer, I had some exposure to computer design, but always used SRAM in my fantasy designs as it's much easier to use, an advantage of not actually building something is not actually having to pay for it. After all these years will finally find out how DRAM refresh is implemented!

[@andymouse]

A great looking project that will be fascinating to follow...cheers.

[@stefanh482]

Wow! This brings back fond memories! Can't wait to see the next parts!

[@SteveRaynerMakes]

I would love to buy the book

[@derekchristenson5711]

Very intriguing! I certainly haven't seen many attempts in recent years to do video on a retro or retro-ish system without resorting to either a vintage video chip (TMS9918, etc) or something very anachronistic like an FPGA. I can think of only two at the moment, in fact (I'm sure there are others, but just settling for "connect terminal here" is certainly far, far more common). I will follow this project with interest.

[@lawrencemanning]

Very interested to follow this. The state of “modern retro” is almost exactly opposite and this is very refreshing. Haven’t watched the video in it’s entirety but am especially intrigued to hear about your mass storage and keyboard choices.

[@SteveRaynerMakes]

I'm probably a little way off from thinking about the storage devices, but I am starting to look at the keyboard.

[@axelBr1]

If "mass storage" is not to an audio cassette recorder I'm going to be very disappointed, unless it's punched tape.

[@bobvines00]

Jerry, this is fascinating and I look forward to both the rest of this series _and_ your upcoming book. Thank you for _not_ using an Arduino/Raspberry Pi (or equal) to implement your design!

[@crazyboy2006cashier]

100% behind you

[@bprosman]

@Jerry Walker, looking forward to your book. Any prediction when it is going to be available ?

[@JerryWalker001]

I am hoping to have it ready to publish by the end of September.

[@sebastian19745]

The first computer I ever built, was from a book and had the same type of video circuit (a chain of counters with some gates). The only two differences were that it have no character ROM nor dedicated RAM. The video info/ the pixels were on the main ROM and all the addresses were directed to a address interval in the main RAM. That way the computer was able to do graphics also. If I remember correctly, the main oscillator was running at 10MHz and the video signal was OIRT standard (625 lines, interlaced at 25Hz/frame). The way to get the video to an TV was trough a simple RF generator that worked on channel 3(?). The video was B&W only with video invers and blinking atributes.

[@JerryWalker001]

That sounds like a very odd design and not one I would want to repeat but if you still have it then a video would be interesting.

[@El_Croc]

Sounds similar to how sinclair zx worked, but I doubt discrete counter chips were used.

[@JerryWalker001]

@@El_Croc The ZX's produced composite video in a large scale IC. They did not use any discrete timers or counters.

[@E67Official]

Is there any preorder for the book?

[@8-bitbitsa821]

Nicely done… hopefully you’ll be going with a period correct Ascii parallel keyboard as the input device ? Although, something tells me you may go the “easier” route, scanned matrix, I/O ports ?

[@GJackie24]

This is Amazing !!!! You are a master at explanation !!! Can't wait for the book. Where did you get the breadboards from. Never saw any like that before ?

[@JerryWalker001]

The breadboards are 'SAD-01' (really). I cannot remember where I purchased them as I have had them for years but I believe they are still available from several sources (eg RS).

[@johnn0hj]

Looking forward to this series. It would be nice if you could cover your video design for those of us in the 60Hz world, but I realize your monitor is tied to 50Hz. If you could at least give an overview or an idea of what parts of the circuit would need to be addressed to deal with 60Hz. Thanks much.

[@JerryWalker001]

I will be covering such issues in later videos. The first video is not really the place to discuss variations in design as that would make the explanation very confusing. Having said that there is no real need to run at 60Hz wherever you are. 50Hz would work fine for you but I will discuss the circuits in more detail in later videos including counter values. The monitor will actually work fine at 60Hz (It came from an ADM3 which works at 50 or 60Hz).

[@johnn0hj]

@@JerryWalker001 Very good. I really didn't expect discussions on variations in the first video. Love your videos, especially ones on equipment I've owned or restored. Keep up the good work.

[@your_utube]

pure gold

[@electronictreasure4191]

I have the book, it's great but the discussion of the 74161 counting from 5 to 15 before resetting leaves me confused because output bit 3 goes high at a value of 8 not 15. what am I missing?

[@JerryWalker001]

If you look at bit 3 as a clock then it is a divide by 16 clock but if you use the falling edge of the second part of the signal then it takes 15 clock pulses (remember that the 161 is a synchronous counter).

[@electronictreasure4191]

@@JerryWalker001 Thanks for the reply! but when bit 3 goes high isnt the binRy value 1000 (8) and wouldnt this then reset the counterback to 5 before ever reaching 15? thanks!

[@JerryWalker001]

@@electronictreasure4191 Which part of the video / book are you referring to?

[@electronictreasure4191]

@@JerryWalker001 Thank you : Page 82 par 2 "Pin 11 (bit 3) is connected to the load input ...as soon as bit 3 goes high, the counter is reloaded with value set on pins 3 to 6 (which is hardwired) to value of 5" ... "so when counter reaches 15 is it reset to value 5". So this confuses me because pin 11 (bit 3) goes high when it reaches a value of 8 (binary 1000) so it would reset to 5 before being able to count to 15 (or 16), but at 8 instead? Unfortunately I don't have a 74161 in my box so I am waiting on one in the mail so I can't experiment with it. Love the book BTW read 80 pages the first day!

[@JerryWalker001]

@@electronictreasure4191 I must be miss understanding your question. This counter provides a /9 function. The counter has 4 outputs. The first is /2 then /4 then /8 and bit 3 is /16. By pre-loading with 5 it causes it to count from 5 to 16 and then reload which gives /9. It is synchronous load so it does not load until the clock input changes which avoids a very short last bit.

[@El_Croc]

If I'm understanding this correctly, if you were to double the clockspeed and timer counts then youd get a 160 col x 48 row display?

[@JerryWalker001]

You would need to also change the drive timing as well but in practice it would not actually work. The minimum pulse width for a dot on the display is around 80nS which is a maximum of around 750 pixels per row. 160 columns would need almost 1500 pixels so the display would just be a blurred mess. 80 columns is at the upper limit of what this monitor can manage. You also need to take the memory setup and access times into account. In the design I show there is around 15nS spare which is a close as you really want to go. If you double the clock speed then you would need to change the way the memory is organised.

[@El_Croc]

@@JerryWalker001 Thank-you for explaining those limits. It's always intrigued me how different resolutions were handled, character modes etc. Had an old 386 once that could go over 100 characters across in one text mode, always bugged me I couldn't get that mode on later PCs except by using graphics drawing.

[@JerryWalker001]

@@El_Croc Yes PC's used totally different techniques for video output but they came along much later.

[@pdppanelman5889]

Book that's good. PCB's ?

[@JerryWalker001]

I will be making the final pcb's available along with all the smaller project pcb's which may appear in the video's in case anyone wants to experiment with the individual circuits.