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Todd's ELF-ish gets some more Memory #ELF #Homebrew

taf123
 

The ELF-ish so far has 4k of RAM, partially decoded on the main board using a CDP1859 to decode the four 1k blocks, with the first 1k further decoded to four 256 byte pages.

The CDP1859 !EN is enabled by gating together !MA15, to ensure we're in the lower 32k reserved for RAM, and the so-called !4K0 (the enable like for the first 4k block).



The !4K0 line is simply tied LO using a pull-down resister connected to the !4K0 line on the memory second expansion port.  The idea is that any expanded memory board plugged into the port would more fully decode the address space, and raise this line HI for addressing memory above the 0000 - 0FFF range.  There's a matching !R4K0 line which does the same function for the ROM area above 8000, which we've seen connected to the decode for the memory-mapped I/O space in upper memory.


My plans were to build a memory expansion board at this time.  But, before I did this, I managed to get a hold of something special, which I had to try first.

Back in the day, RCA offered several CDP1802-based development platforms.  These were modular, using what they called their Microboard line, all connected together through a 44-pin motherboard.

The manual for the complete line can be found on-line - I found it on bitsavers at http://www.textfiles.com/bitsavers/pdf/rca/cosmac/1982_RCA_Microboards.pdf

Well, one day while looking around eBay, I spotted a CDP18S623 8k Microboard for sale at only $54.98, so I snapped it up.

And this is what arrived.



It uses a pair of 4050's to buffer the Address lines and signals, and a pair of CDP1856 bus buffers for the data bus - just like I did on the expansion chassis.

The DIP switches allow the board to be put in any of the available eight 8k blocks within the 64k CDP1802 memory space,

With four CDP1866 chips doing address latching and decoding to provide the various !CS lines.

Then the rest is a matrix of 32 RCA 89052 512 x 4-bit chips, which are predecessors to the MWS5114 1k x 4-bit chips.

 Because of differences in the motherboards between some of the RCA development platforms, they provided a jumper area to change come of pin uses.

This was convenient for the ELF-ish.  Remember, the expansion memory is responsible for de-asserting !4K0 to disable the low 4k RAM.  By adding a small jumper-wire here, I was able to present the board's enable line back to an unused pin on the 44-pin edge connector.  This I used to disable the main board RAM.



I then just needed to wire up a 44-pin card edge connector to a 26-way IDC cable and I was ready for testing.



The good news is that the board was still basically functional - it was correctly selected in the 8k block I had set in the dip switches, and I could read and write to various memory locations.

Yay.  So I did a simple write/read test to all 16 of the 512-byte pages of the 8k board.

The bad news - two of the chips were DOA.  The worse news - one of these was in the first 512-byte page, so there would be hole in the memory if I tried to use this board as-is.

Although I've never seen RCA 89052 anywhere, the RCA manual for this Microboard says that the replacement part is the MWS5114.  Looking at the supplied schematic, the chips are pin-compatible.  The 5114 obviously has one more address-line, but that pin is tied LO on this board, so the 5114 would just function like a 512 x 4-bit memory.

I hope to attempt a repair of this board in the near future - updates to this forum when that happens.

taf123
 

I wanted to bring the ELF-ish up to 16k of RAM, but didn't want to keep playing around in such small increments, so I decided to go with the RCA CDM6116 2k x 8-bit CMOS SRAM.

Putting 6 of these on an expansion board would add 12k, to bring the total up to 16k.

This time, I used a pair of 74HC244 octal buffers for the address bus and signals since I had a couple more signals involved.  Again, I used a CDP1875 output port as the upper address latch.


I also included two more 26-way memory expansion connectors.  Memory3 was intended for ROM expansion, so the !R4K0 line was buffered and passed through to it.

The Memory4 port was intended to let me expand the RAM further, so I could eventually fill the lower memory to a full 32k of RAM.  Of course, now that I own an RCA 8k Microboard, it will go here instead, assuming I can fix it.

To supply the required !CS lines I used a 74HC138, 3-to-8 line decoder, inverting.

I used A14 and A15 as !CS for the '138, placing this in the lower 16k of memory space.  Of course, the decoder will also generate the !CS lines for the lower two 2k blocks already decoded on the main board.  But by gating these together, I generate the !4K0 line.  If either of these lines are LO, then the !4K0 remains LO.  But if both are HI, the !4K0 line goes HI, disabling the lower 4k of RAM.  The other six !CS lines go to the six CDM6116's.

Of course I'm addicted to battery backup, so I had to include it here as well. By using a different mode setting, the DS1321 just passes the !CS lines through to the RAMs, rather than doing further decoding as was done with the CDP1822's in the first 1k of memory.

Each DS1321 can only handle four !CS lines, so I used two, both supplied by the same battery.



For the data bus buffering, I again used a 74HC245 bidirectional octal buffer.  The enable of this was made by again gating the A14 and A15 together to make sure we're in the lower 16k.  But of course, we don't want to enable this buffer if the lower 4k of RAM on the main board is active, so we gate this with the !4K0 line.

Finally, I was getting concerned about the length of the un-buffered side of the data bus, so I decided I would present the buffered data bus to the Memory3 and Memory4 expansion ports.  I included a !MEM3_EN and !MEM4_EN, which those expansion boards would assert when they are selected, to enable the buffer.  This was all OR-ed together using diode/resistor logic.



One final little detail.  The signal that the 8k Microboard asserts when it is selected is active HI, but I was intending to use active LO for the !MEM3_EN and !MEM4_EN lines, so I included a little jumper and gate to select wether to use the !MEM4_EN line directly, for one of my expansion boards, or to invert it for the RCA board.


And here are the connections for those future expansion boards.


Phew!  All of that fits nicely on one of the matrix boards I've been using.





And wired up for testing.



And, of course, it works.

Welcome to the 16k version of the ELF-ish.

Terry Gray
 

It would appear that at the rate you are adding functionality and the fact that you aren's stacking the boards that you may have to add a room to your house just for your Elf!  Way back in the 70's I used to wirewrap like that...found it very cathartic.  Need to see if I still have the patience.

Lee Hart
 

taf123 wrote:
Back in the day, RCA offered several CDP1802-based development
platforms... their Microboard line...
The good news is that the board was still basically functional...
The bad news - two of the chips were DOA.
Although I've never seen RCA 89052 anywhere, the RCA manual for this
Microboard says that the replacement part is the MWS5114. Looking at
the supplied schematic, the chips are pin-compatible. The 5114
obviously has one more address-line, but that pin is tied LO on this
board, so the 5114 would just function like a 512 x 4-bit memory.
I suspect that the 89052's were MW5114's that had a bad bit. Rather than throw them out (RAMs were expensive back then!), they used them as 512-byte RAMs.

I hope to attempt a repair of this board in the near future - updates to
this forum when that happens.
I have a tube of RCA MW5114's if you (or others) need any. I also have a couple tubes of 5101's (some RCA, some NEC). :-)

--
Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com

taf123
 

On Sat, May 11, 2019 at 03:35 PM, Lee Hart wrote:
taf123 wrote:
Although I've never seen RCA 89052 anywhere, the RCA manual for this
Microboard says that the replacement part is the MWS5114. Looking at
the supplied schematic, the chips are pin-compatible. The 5114
obviously has one more address-line, but that pin is tied LO on this
board, so the 5114 would just function like a 512 x 4-bit memory.
I suspect that the 89052's were MW5114's that had a bad bit. Rather than
throw them out (RAMs were expensive back then!), they used them as
512-byte RAMs.
That makes a lot of sense - I wouldn't be at all surprised.

I hope to attempt a repair of this board in the near future - updates to
this forum when that happens.
I have a tube of RCA MW5114's if you (or others) need any. I also have a
couple tubes of 5101's (some RCA, some NEC). :-)
Thanks for the offer, but as I wrote that up, I did an eBay search for some more MWS5114's and already placed an order.

Bad timing ;-)  Sometimes, this Internet thing can be too efficient.

Best regards,
Todd

taf123
 

On Sat, May 11, 2019 at 03:35 PM, Lee Hart wrote:
www.sunrise-ev.com
P.S. - I love the VIP2k from your website...

Gregg Levine
 

Hello!
In my case, I happen to like the entire site.
And I actually bumped into someone wearing a 6502 badge at this years
VCF East show.

However,,, IMHO Lee I believe you are right regarding homebuilt EV
designs and this country.

Incidentally Todd, how'd you manage all of that with a bored domestic
beside you? (Think of Hobbes and you're there.)
-----
Gregg C Levine gregg.drwho8@...
"This signature fought the Time Wars, time and again."

On Sat, May 11, 2019 at 9:31 PM taf123 <todd.ferguson@...> wrote:

On Sat, May 11, 2019 at 03:35 PM, Lee Hart wrote:

www.sunrise-ev.com

P.S. - I love the VIP2k from your website...

Lee Hart
 

Gregg Levine wrote:
In my case, I happen to like the entire site.
And I actually bumped into someone wearing a 6502 badge at this years
VCF East show.
Hey, that's pretty cool! BTW, we're working on a new 6502 Badge for this year's VCFMW in Sept 2019. There is a preview of it at <http://sunrise-ev.com/6502.htm> near the bottom of the page. :-)

However,,, IMHO Lee I believe you are right regarding homebuilt EV
designs and this country.
Sad; but true. The problem of "nobody wants to build anything" is common in many parts of the hobby world. It's so darn easy to just buy it all...

Incidentally Todd, how'd you manage all of that with a bored domestic
beside you? (Think of Hobbes and you're there.)
I too am flabbergasted at how fast Todd's project is progressing. My spouse would never let me get away with that!

--
Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com

taf123
 


Incidentally Todd, how'd you manage all of that with a bored domestic
beside you? (Think of Hobbes and you're there.)
I too am flabbergasted at how fast Todd's project is progressing. My
spouse would never let me get away with that!
Just to be clear, the stuff I've been posting over the past few days covers pictures I've taken of my build project over at least 1.5 years.  Someone asked to see pictures and the flood gates opened ;-)

We're almost caught up to the current build state.

But yes, spot the single guy ;-)

Cheers,
Todd

taf123
 

Hi Terry -

On Sat, May 11, 2019 at 09:49 PM, Terry Gray wrote:
It would appear that at the rate you are adding functionality and the fact that you aren's stacking the boards that you may have to add a room to your house just for your Elf! 
It'll soon be a problem, I'm sure ;-)

Way back in the 70's I used to wirewrap like that...found it very cathartic.  Need to see if I still have the patience.

My previous wire-wrap was at uni back in the late '80s.  I'll never know how I managed to use one of those pencil tools back then, but this project was almost a non-starter when I began with one of those.

Finding that used electric wrapping gun on eBay is at least 75% responsible for the sprawl - can't imagine having got this far otherwise.

Cheers,
Todd