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Designing a portable RAM card

techknight

Well-known member
So 5120 can only go to 8MB total and 5126 to 9MB total.. You are testing on a 5120.. You should test on a 5126 board and bring it up… Perhaps my 5126 board.. heeee….
Backlit needs the /AS jumper. Without it it only sees 5MB. with it I can get all 8MB.

However there is an odd bug. When an application goes into expanded RAM the backlight flickers on and off rapidly with RAM accesses.

If I dont use the /AS wire and cap it at 5mb its perfectly fine.

But my /AS wire was a horrendously long clip lead plus I didnt have /DTACK hooked up either, so that could have been an issue.

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trag

Well-known member
Wow! Excellently cool progress, Techknight.

What does /AS do? Is that Address Strobe? Still don't know what it does, even if that's the correct name. It seems bizarre that putting a load on that wire could cause the backlight to switch off and on.

 

techknight

Well-known member
the /AS is the address strobe, yes...

Starting with the Backlit portable, they removed the /AS in the RAM connector and replaced it with a signal, known as /RAM.CS 

This signal is controlled by an external address decoder network, which actually sits between the ROM socket, and the RAM itself. a series of 74 series gates. but I digress... 

This /RAM.CS will only assert within the first 5MB of address space. After that, the signal no longer gets asserted. Therefore, on the M5126, you have a RAM ceiling of 5MB in the RAM slot. PDS is an exception though because you have access to the entire bus so these limitations dont exist. 

but since I didnt feel like dealing with the Eurocard connectors, or the PDS, I designed the card to fit in the existing RAM slot. 

So.... In order to gain full addressing on the backlit portable via the RAM slot, you have to pick off the /AS address strobe signal either from the PDS, or from the ROM slot. Since ROM is still attached directly to the bus, and not isolated through an external decoder. 

The catch is, after 5+MB the RAM decoder network no longer generates a /DTACK signal to tell the processor that the data transfer was successful. 

So, I am thinking that since I didnt hook the DTACK line up, it was causing a ton of bus errors even though the system information was showing the full 8MB of RAM. 

If I use the /RAM.CS signal instead of /AS, everything works great. But only sees 5MB as expected. If I remove the jumper that connects to /RAM.CS and replace it with a wire that goes to /AS instead, It shows the full 8MB of RAM. But every time an application launches and uses the extended RAM (RAM past 1MB), the backlight flickers on and off extremely rapidly, at the same pace as the RAM accessing occurs. 

so i think the DTACK connection not being made is an issue, OR, since my /AS wire clip lead was so long and coiled, it was distorting the signal to the RAM card causing issues with data transfers in and out of RAM. 

Funny thing though... no bombs, or system errors/sad macs. Just kept right on chugging with backlight flickering. Soon as I exited microsoft word 5.1, it quit. backlight stayed on. 

 

techknight

Well-known member
you can try it, but i doubt itll work.

So what I did was setup 2 CPLD firmwares, one for 9MB, and one for 8MB. I built my 2nd card, and flashed it with 9MB. 

But its giving me a chime loop so I think one of the RAM ICs trag sent is bad. my connections are ok. 

So once I get that fixed, I can dick around and see if I can max the backlit model to 9MB. the non-backlit cannot exceed 8MB ever. unless ROM is moved.

Trying to do this without using the PDS slot, because my next goal is to attach a 68020 to the PDS.  

 
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Macdrone

Well-known member
A 68020 with math co pro? Both memory cards I have are PDS so put me on the list for a couple of these I think.

 

techknight

Well-known member
Soon as I learn how to do it.. cant find any reading materials out there for how to tie two processors together and busmaster them, which is how accelerators work.

Not to mention the bus incompatibilities. 68000 is a 16bit cpu with 16bit bus while 020+ is 32bit. So how do you run a 32 bit proc on a 16 bit bus? Im sure designers figured all this out 20+ years ago. But I havent.

 
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techknight

Well-known member
I fixed my 2nd RAM card. One of the solder joints didnt take.

I ran the /AS line to the 2nd card on the backlit. Worked perfect no flickering.

My 1st card still flickered. Ironically this is the same card giving me problems in the non-backlit acessing 9MB. I think one of the RAM ics is bad.

Ill try my 2nd card tonight in the non backlit.

 

techknight

Well-known member
I couldn't wait. I grabbed haplain's non backlit and tried out my 2nd card.

Worked perfectly. All 9MB.

That tells me I have bad RAM on my first card.

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max1zzz

Well-known member
Very nice!  Looks like my portable is going to be costing me yet more money.... :)

Also, surely the bad chip has to be the last chip on the card? As it works when you have it set to 7mb and that's wasting half the last chip, or have i misunderstood something here?

 
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