| Z80 MC6845 video board examples [message #8430] |
Wed, 31 March 2021 04:36  |
lynchaj
Messages: 1080
Registered: June 2016
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Senior Member |
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Does anyone have examples of Z80 MC6845 shared memory video board designs (not transparent mode)? I recall seeing some years ago but am not finding them with some searching. The only example I can come up with is the Dick Smith Super80 VDU Expansion Board (VDUEB) which is a modification to the Super80.
The reason I am asking is the Z80 MBC project allows for external bus memory and sometime down the road I'd like to experiment with it. The legacy Z80 SBC is IO only on the ECB and does not allow for external memory peripherals. So it can't use bus memory boards or shared memory video boards. Instead it uses designs like SY6545 transparent mode and TMS9918 for video solutions.
The IBM CGA and MDA are MC6845 shared memory designs but they are way too complex for what I am considering. Just would like to see what examples are available. Much appreciated, Thanks
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| Re: Z80 MC6845 video board examples [message #9860 is a reply to message #9859] |
Tue, 12 April 2022 09:50  |
norwestrzh
Messages: 196
Registered: November 2015
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Senior Member |
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Andrew --
I built the original ZRT-80, and thought it was a good approach to a video terminal. Unfortunately, I found that mine worked well for a short time, but it hung up periodically. It had to be reset constantly. I don't know if the problem was my limited build skill, or if there was a problem with the design. I do remember (I think?) that the build instructions that came with the device specified that some of the components (speed?) were critical for proper operation. Maybe it would be prudent to evaluate the original design to be sure that this hang-up/reset problem doesn't persist in a newer version?
Roger
[Updated on: Tue, 12 April 2022 10:40] by Moderator Report message to a moderator |
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| Re: Z80 MC6845 video board examples [message #9862 is a reply to message #9861] |
Tue, 12 April 2022 10:54 |
lynchaj
Messages: 1080
Registered: June 2016
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Senior Member |
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Well what do you know? I *do* have one in the basement.  An original it seems complete with documentation, a board at least partially (or nearly fully) constructed plus misc parts and an 8" floppy disk in an original box no less! The schematic is the same one as the ZIP file but twice as big and much clearer so easier to read.
I have an ASCII parallel keyboard that would be perfect for this project but I've always thought those keyboards are really delicate compared to a PS/2 keyboard. Also PS/2 keyboards are readily replaceable on eBay whereas a parallel ASCII keyboard is practically a historical artifact of long-lost era.
[Updated on: Tue, 12 April 2022 10:58] Report message to a moderator |
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| Re: Z80 MC6845 video board examples [message #9930 is a reply to message #9881] |
Tue, 17 May 2022 06:09 |
etchedpixels
Messages: 333
Registered: October 2015
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Senior Member |
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There's also the keep writing the last thing on the dead cycle approach where your video memory is write only.
CPU address and data to the video RAM are latched when they happen and the video side runs with the 6845 access half of the cycle reading, then the other half of the cycle sets write on the ram for most of the cycle, so that the last byte buffered keeps getting written into the RAM when the video memory is free. Write only but no wait states and the CPU is usually unable to write to video fast enough to need nops between updates. And of course you can shadow it with RAM if you need to.
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. With regard to the ASCII keyboard .... I got a tiny device a while ago ( forgot where 
