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uPD7220 character only mode video board [message #10348]

Sat, 13 May 2023 15:26

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi
I recently asked a question on retro-comp mailing list about a uPD7220 character mode only video board. Fortunately there is an example board made for the z1013 as a GDC module. I am interested in analyzing this design to make a uPD7220 character mode video board.

https://groups.google.com/g/retro-comp/c/XlcXZVnxOdY

It turns out, uPD7220 character mode is much simpler than the 4-plane graphics board. I've spun up a new design inspired by the z1013 GDC module but updated for VGA frequencies (25.175 MHz) and expanded video SRAM for a larger text display.

The design uses only 16 ICs and could easily be adapted fit on an ECB or even a 100x100mm board. Possibly one of the larger RC2014 or RCBUS boards.

Is anyone interested in making one of these video boards? My plan is to make one for the duodyne system and merging it with the uPD7220 4-plane graphics board. One shortcoming of the graphics only board is using it for console is difficult because text is so slow on the uPD7220 in graphics mode. However, a character mode sub-system would be much faster.

Thanks, Andrew Lynch

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Re: uPD7220 character only mode video board [message #10349 is a reply to message #10348]

Sat, 13 May 2023 16:40

MissArgent
Messages: 2
Registered: January 2023

Junior Member

If anyone gets some PCBs, I'd be very interested in grabbing one. I'm a big fan of NEC's designs and I'm kind of missing my NEC APC's 7220 graphics!

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Re: uPD7220 character only mode video board [message #10350 is a reply to message #10349]

Sun, 14 May 2023 05:27

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi
I have the schematics and PCB layout for a uPD7220 graphics only board on the duodyne GitHub repository. Also, there are schematics and PCB layout for a uPD7220 character mode only board there too. This is the third generation for the graphics only board (ECB uPD7220, Z180GDC) so I am fairly confident it can be made to work again. Haven't tried the character only board design yet though. If someone wants to give it a try on ECB or RCBUS, here are where the files are located:

https://github.com/lynchaj/duodyne

Thanks, Andrew Lynch

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Re: uPD7220 character only mode video board [message #10353 is a reply to message #10348]

Mon, 15 May 2023 05:19

e2k
Messages: 33
Registered: June 2021

Member

So, how "slow" is the drawing of characters on the 7220 in graphics mode?
You have any numbers available?

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Re: uPD7220 character only mode video board [message #10354 is a reply to message #10353]

Mon, 15 May 2023 10:03

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi
No numbers but noticeably slow. It wasn't absolutely terrible but nothing you'd want to use for any period of time.

This is based on our experiences with Z180GDC. Maybe some of the other testers can chime in on their experiences.

For doing static graphics, full screen, it seemed to be OK drawing patterns. Maybe the graphical pseudo-character mode could be sped up somewhat with software improvements. However, it had several RWM operations for each character which limited throughput.

I think a true character mode would be much faster. Once there is a working character mode board then the next challenge would be merging the designs into a single board or at least a graphics board plus character mezzanine.

The Z180GDC lessons learned are heavily influencing the duodyne design. Such as the larger PCB planform and the decision to support dual DMA channels on the processor board and bus.

I am designing/converting multiple boards and for most the PCBs are the right size. In some cases, they are larger than necessary but that is much nicer than being too cramped.

Thanks, Andrew Lynch

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Re: uPD7220 character only mode video board [message #10355 is a reply to message #10354]

Mon, 15 May 2023 10:41

plasmo
Messages: 916
Registered: March 2017
Location: New Mexico, USA

Senior Member

I have found scrolling as the longest operation. Z80all does not have hardware scroll function and takes about 5 mS to scroll 64x48 text screen with 25Mhz Z80. With 7.37Mhz Z80 the slowdown becomes noticeable so I find it useful to implement hardware scroll for VGARC text display board.
Bill

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Re: uPD7220 character only mode video board [message #10356 is a reply to message #10355]

Mon, 15 May 2023 23:47

e2k
Messages: 33
Registered: June 2021

Member

Was the DEC Rainbow really that bad? Don't remember it being so bad?

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Re: uPD7220 character only mode video board [message #10357 is a reply to message #10356]

Tue, 16 May 2023 03:25

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi
I don't know for sure with the DEC Rainbow but I've seen schematics for other systems with uPD7220 chips. A lot of them have a dedicated character mode hardware in addition to the graphics mode. Some even have two uPD7220s for both graphics and character mode.

Thanks, Andrew Lynch

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