Re: Convert 68008 Outputs to Both /UDS & /LDS

[Harry S. Speer]

=====================================================   

    

   15:47, 3:47 PM, Wednesday, July 29th, 2026    

   Re: Convert 68008 Outputs to Both /UDS & /LDS 

   

   All -  

   I came up with an idea of how to create both the 

   /UDS & /LDS on the 68008 CPU Card for the RCBus-68k.

   Using a 74LS04 Inverter and a 74LS32 2-input OR gate, 

   and using the 68008 Address Line A0 and the single 

   /DS Data Strobe Output, we get:  

   ------------------------------------------------

   For generating /UDS:  

   (NOT A0) OR /DS == /UDS  

   and 

   For generating /LDS: 

   /UDS == A0 OR /DS === /LDS

   ------------------------------------------------

   I sent these ideas to Mark Durham & he correctly

   changed my NOR gates to OR gates. 

   

   In this manner, the A0 line does not need to 

   leave the 68008 CPU Card, at all, and we get both   

   of the /UDS and /LDS to go out onto the bus.    

   Thus the 68008 CPU is able to access both memory chips, 

   Both Upper and Lower bytes, of Mark's 16-bit ROM+RAM 

   Cards that he  created for his 16-bit 68000 CPU cards.

   

   I will be implementing this on my 68008 card and 

   I feel certain that Mark will do so soon. He is 

   also working on a 68008 CPU card at the moment,

   as well as other cards.

   Harry

   cc: self      

===================================================

[Mark Durham]

Harry,

I think you might be able to generate /UDS and /LDS using just one chip. If you use one half of a 74LS139 with address line A0 connected to the A0 input and /DS connected to the A1 input and /AS connected to the /E input, then I think output O0 = /LDS and output O1 = /UDS.

Mark.

[Alan Cox]

That ought to work at least at low speed. Or just use existing 8bit wide cards. One nice thing about 68008 is you can just use standard existing 40pin boards 

Probably the only positive given the lack of async fetch and stores the 8088 has.

--
You received this message because you are subscribed to the Google Groups "retro-comp" group.
To unsubscribe from this group and stop receiving emails from it, send an email to retro-comp+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/retro-comp/8d149edc-40be-41f2-8022-54c22eea3dccn%40googlegroups.com.

[Andrew Lynch]

Hi, I recognize people want to preserve as much RC80 as possible but since signals are missing already and it looks like substantive changes would be required, please take another look at Duodyne bus.  

It was designed with 8-bit and 16-bit CPU signals in mind plus full 32-bit address and data busses.  Here is the signal layout:

https://github.com/lynchaj/duodyne/blob/main/duodyne.bus.pdf

I believe signals dedicated for a specific CPU but not present in the system *may* be available for repurposing as needed 

Merovingian: Choice is an illusion created between those with power and those without.

--

You received this message because you are subscribed to the Google Groups "retro-comp" group.
To unsubscribe from this group and stop receiving emails from it, send an email to retro-comp+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/retro-comp/9ec9ff9c-8cf3-40cb-92c2-2fd887adcd3e%40gmail.com.

[j...@nmysbh.co.uk]

But that's a completely different physical configuration- let alone electrical. Isn't the point of this about supporting 68k CPUs on RC-Bus?

Jon

On 30 Jul 2026, at 14:55, 'Andrew Lynch' via retro-comp <retro...@googlegroups.com> wrote:



To view this discussion visit https://groups.google.com/d/msgid/retro-comp/998603671.1954005.1785419745612%40mail.yahoo.com.

[Alan Cox]

On Thu, 30 Jul 2026 at 16:03, j...@nmysbh.co.uk <j...@nmysbh.co.uk> wrote:

But that's a completely different physical configuration- let alone electrical. Isn't the point of this about supporting 68k CPUs on RC-Bus?

It's drifting so far away from RC-Bus in the need or new components like 16bit wide RAM boards (or 32bit for 68020+) that I am not sure it matters. If you are trying to support 8bit wide RCbus peripherals that already exist then it makes sense but if you are trying to bussed 68K systems especially at higher speeds the problem is quite different and the the question whether rcbus is the right starting point versus a well designed buffered bus is a good one.

Build what you want anyway.

[Jon Jones]

In which case looking at the Rosco kits (Now sold by Solder Demon) or the work Steve Crompton is doing over at Mega Micros. Both doing 68K kits with expansion slots.

J

[Greg Holdren]

Or you can source the parts and get the gerbers from Rosco git repo: https://github.com/rosco-m68k/rosco_m68k

Greg

[Harry Speer]

23:17,  11:17 PM, Friday, July 31st, 2026 

Yes, as for existing designs for 68k CPU cards with a Bus for 

add-in cards, the Mega-Micros stuff here: 

https://www.mega-micros.co.uk/index.htm 

Looks very good but I continued my search after checking it 

out for a while.  The builder and I did not "hit it off" well and

that is a really serious problem for me.  Not that he had bad

hardware.  This is a "Brain-Birth-Defect" kind of thing - my 

problem not his. 

 

And I hesitate for a few reasons on the ROSCO from 

Solder Demon here: 

https://solderdemon.com/store/rosco-m68k-through-hole-kit

The main card has just the one 64-pin set of male pins for a 

Bus interface, and then it looks like a 64-pin ribbon-cable connects

that main board to a backplane full of slots. 

Also I don't see much in the way of cards to plug into said Backplane.

The one thing that I DO like is that he offers an assembled version 

for $140 USD - assembled and tested.  That is VERY Attractive to Me 

because of my seriously impaired eye-sight and how hard it is for me

to do soldering with my poor eyes. 

 

I need to send solder-demon a message asking him how the main-board 

and the Backplane connect.  I did not see that on his 

solderdemon.com

webpages. 

Maybe that info is on the GitHub webpage. Or learn how to use 

Discord, I think it was.

https://github.com/rosco-m68k/rosco_m68k

Problem is that I have already bought a lot of stuff for heading 

down the RCBus-68k Path.  Switching to some other platform

might be a bit of a problem.  I suppose I could offer it up for 

sale online.  Hmmmm.....

Harry   HSS