DG680 and Video cards

[ozholmes]

Hi everyone. First let me introduce myself

I am a now retired electrical engineer who graduated just as these new fangled microprocessors appeared (in fact my work experience with Telstra for my degree was to select the preferred processor to be used for ongoing projects with Telecom Research Labs - I chose the Motorola 6800, but what would a graduate student know anyway !)

So i ordered a SYM-1 system whilst I was assigned to the Pilbara in 1979, and after an extraordinary amount of effort I managed to program it to be a clock - this was after upgrading the main RAM from 1kByte to 4 kByte (luxury!)

Returning back to Melbourne in 1980 or so, I found the ETI articles about the DG680 CPU card, and ordered the kit from Applied Technology, along with a DG640 Video card. And again after much effort I got them up and running using a modified cassette recorder (the one I had used on the SYM-1) for program storage

I then took out a personal loan (really) and purchased a S100 FDC from Morrow Designs along with a double density double sided 8 inch disk drive, stuffed all the cards into a homebrew chassis with an S100 backplane, and got CP/M up and running. And a great big thanks to whoever was handling purchasing in Applied Technology at that time, as they not only supplied the CP/M software they also patched the BIOS to run on the DG680 CPU and DG640 VDU. And they told me about a required chip replacement to make the interrupts work with the Morrow controller and the DG680 CPU

Somewhere along this journey I bought a 64k Dynamic Memory card (the 1st 16K cost around $350, the second 16K cost around $80 and the third 16K cost $16. Ah the power of mass production

I also stumbled into a group of hobbyists who also has S100 systems based on the DG680 card. We used to meet in one members house near Greensborough - there was less than 10 of us as I remember

That group morphed into the Melbourne MicroBee Users Group in Burwood after Applied Technology - I dropped out after we got over 70 people turn up to the "expressions of interest" meeting..... The first "official" meeting of the Melbourne chapter has over 250 people turn up!!!

Getting back to my S100 system. I was getting more and more frustrated with the 64x16 screen size of the DG640 VDU card, along with the "flashes" on screen whenever the CPU access the video memory. And being a poor graduate engineer I could not afford a better 80x24 card (one personal loan for my computer has been enough)

So I decided to build my own (ah the naivety of youth)

I ended up designing and prototyping a 80x24 video with an onboard Z80 CPU running in lockstep with the 6845 CRT chip. all accessed through a parallel I/O interface so as to not use up precious RAM space. The onboard Z80 allowed the video card to emulate different terminals/screens just by downloading new firmware into the VDU card. Over time I added bit mapped graphics (640x480), colour (16) and various emulations

I showed the prototype to the guys at the at the club, and several of them expressed interest in building their own version, so I got a card artwork done and had a production run of around 6 cards. These were duely delivered to the guys at the club just before it morphed to the MicroBee User Group

The card was eventually was called the "ByteWyde Systems BWS-2010 VDU

I've attached photo's of the original prototype, and the production board for your entertainment. As for the card, it got migrated to the STD buss form factor and got used in a lot of control systems in the mid to late 1980's - ByteWyde systems was my company along with my elder brother that specialized in the control system space

So after all of that, I am curious whether any of the other boards made ever got built and installed in anyone's system. If you have ever heard of this card, or even have one of the cards, i'd love to have a chat some time about thost days - it certainly was an exciting period in the computer world!

My apologies for the long post - this has obviously been bottled up inside for some time !!!

   
   
   
   

[tmuir]

I'm not familiar with your card as it was just a few years before my time, I did enjoy reading your post though, but that blue prototype board does bring back memories. I used one the same as it minus the edge connector to build myself a Z80 micorcomputer from the Z80 Microcomputer Designs projects book by William Barden whilst in my first year as an Apprentice technician working for Telstra (Or Telecom Australia as it was back then)

[ChickenMan]

Welcome to the forum

I was a member of MBUG also and I still go to their Christmas BBQ. You probably know Jim Lobley who was the Secretary and a S-100 nerd. I ended up with his system, now passed on and will appear in a Museum in the future. In the docs that came with it were a couple of your manuals which I have scanned and up on archive -
https://archive.org/details/bws-1000-z80...ers-manual
https://archive.org/details/bws-2010-vid...cal-manual

If you have any other manuals or software I would love to have them archived.

[ozholmes]

I was wondering who had uploaded those manuals to archive.org - it didn't seem like something my brother had done!

The uploaded manuals are for the STD bus version of the card, which came out about a year after the S-100 version. Most of the documentation came from the STD bus version as that was a commercial enterprise, unlike the S-100 version which was purely for hobbyists

I haven't powered on my system for over 40 years, so I very much doubt it still works. If I can find some time i'll replace all the electrolytic  and tantalum caps, and see what happens

I also have the STD bus version in a home made aluminium case using 5 1/4 FDD, but again it has not been powered up for many years.....

[someone]

Hi OzHolmes,

Absolutely fantastic stuff!
Did you manage to get your Z80 to work in lockstep with the 6845 CRTC without the use of any wait states or polling allowing the CPU to run at full speed.

If you post the contents of the board's ROMs and schematic your board may live again as an emulator or FPGA.

Cheers
Someone

[ozholmes]

Hi Someone

I got the Z80 and the 6845 working off the same clock source, so the Z80 is running at 3.38Mhz (ZCLK) with the standard dot clock of 13.5Mhz, The 6845 clock (CCLK) is half the ZCLK at 1.68Mhz, and this is also used to drive a number of TTL multiplexers so that the CRTC access the Video/Character RAM on  the first half of the CCLK and the Z80 accesses it on the second half of CCLK.

However, there is a fundamental difference in how the two devices access memory. The 6845 accesses the video memory on every clock cycle, whereas the Z80 needs between 3 and 5 ZCLK clock cycles to complete a memory read or write - the actual number of clock cycles depends on the instruction being executed. and as 3 (or 5) is not divisible by 2, there needs to be some restrictions on how the Z80 accesses video memory

So the Z80 has access to video memory when CCLK is low and the CRTC has access to video memory when CCLK is high. ZCLK is arranged for the Z80 memory read and write strobe edge to occur when CCLK is low. Video data is extracted from video memory when CCLK is high, and is latched by the LOAD* pulse at the end of the high period of CCLK for subsequent character and video generation.

When the Z80 processor requires access to memory, it generates a ZMEMRQ* signal. If this access is to video memory (bit 15 high), the clock PAL generates a ZWAIT* signal until the next time that CCLK is low. The Z80 processor then completes its memory access. The maximum delay that can occur is one half cycle of CCLK, which is 300 ns.

The end result of this (somewhat complicated) approach is that the Z80 runs at full speed for all access to the program PROM and general purpose RAM, and has a 1 Z80 clock cycle delay for 50% of the access to the Video and Character Generator RAM - this is an acceptable compromise as the Z80 is dedicated to the Video card, and has lots of spare processing power

The advantage of this approach is that video memory can be updated at any time during the CRT scan as Z80 access does not conflict with the CRTC access (and hence there are no display "flashes") without haveing to wait for line or frame flyback

I hope this all makes sense. I have attached a timing diagram that shows the interaction between the DOTC, CCLK and ZCLK.

[someone]

Hi OzHolmes,

Sorry for the delayed response because someone has been on holidays.

Thanks for the wonderful and thorough explanation.
Wait states present - so sad,    but expected as someone was wondering how to handle the Z80's variable length memory access cycles.
Interestingly what you have described is similar if not identical to how the Microbee Alpha Plus model accesses the video memory.

If someone is repeating themselves, apologies.
The difference between the DGZ80's & microbee's tape interfaces is that the microbee's is bit bashed, reliant on software delay loops &
uses an alternating '*' then space activity indicator to minimise VDU access (along with a VDU wait state disable circuit for some models) rather than
the DG680's fantastic animated wiggly LORES dot which updates more rapidly.
This is because the microbee doesn't have a Z80 CTC to generate the 1200Hz & 2400Hz tape tones (to save costs).

Cheers
Someone

[grahaml]

This has been an interesting read! I have been working through a bunch of STD cards. One, a Maple Enterprises SC180STD, uses a ByteWyde System monitor ROM. It works fine: https://www.jmpstartmicrocomputers.au/ma...-sc180std/

I found a manual for the monitor ROM and it mentioned that the monitor was more capable if a ByteWyde Hardware Debug card was installed eg breakpoints and single stepping. I was just browsing the cards and it turns out that i do have one of those as well. But what really interests me is that there were monitor ROM versions for several of the Pro-Log processor cards that i have eg the 7801 and 7803. I was wondering, Mr OzHolmes, if perhaps there are still copies of those ROMs out there. G

PS i have looked but i did not find one of your video cards in amongst my lot (either STD or S-100).

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Also now that i have my bearings wrt ByteWyde Systems i have a few manuals i should scan: BWS-2050 64180 STD Bus Processor Technical Manual, CS-64180 Mini Micro Technical Manual, BWS-1100 Command Basic. I'd love to find the command basic ROMs too.