| Posted By
Hypex
on 2019-09-29
22:38:07
|  Can PLA assist in memory expansion?
Hello.
As we know the C16 memory setup is a point of contention amongst others where the internal wiring doesn't allow for easy RAM expansion nor does the cartridge layout support it.
I'm wondering if the PLA can assist in this regard? There are a few PLA replacements by now.
I suspect a rerouted PLA may not help with C16 memory expansions but just thought I'd ask. 
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Posted By
MCes
on 2019-09-30
09:36:37
 | Re: Can PLA assist in memory expansion?
unfortunately the logic that decides whether the present address corresponds to the RAM bank or not is inside the TED, it is not in the PLA ......
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Posted By
Hypex
on 2019-09-30
12:25:20
| Re: Can PLA assist in memory expansion?
Well, a TED replacement is next on the list, but that's harder than a CPU replacement.
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Posted By
MMS
on 2019-09-30
15:19:01
 | Re: Can PLA assist in memory expansion?
There are some old C16 memory expansions confirmed to work.
AFAIK they required no inner modifications...
This one even states: "connect and ready", "very easy usage"
http://plus4world.powweb.com/hardware/64K-RAM_Speichererweiterung
eg. Jurek and Jucing 64K expansions look very similar to each other.
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Posted By
Hypex
on 2019-10-01
12:38:30
| Re: Can PLA assist in memory expansion?
@MMS
Well, I wish the working design was manufactured. I bought one example off eBay but when I tested it in a working machine it was flaky. 16K one power on, 64K the next. And when 64K came up I tested a Plus/4 game and it broke. Memory was corrupted.
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Posted By
MCes
on 2019-10-02
12:34:12
| Re: Can PLA assist in memory expansion?
@MMS
I'm sorry: the 16k ram bank of the C16 CANNOT be expanded with a cartridge without modifying the C16, if you look at the last link image it is indicated where to cut to exclude the internal RAM.
It follows that the C16 remains without internal RAM and to function the RAM cartridge must be inserted and therefore it will not be possible to insert any other cartridge!
At this regard, I propose something that I posted on FB:
"
Somebody told that the original project of C16/PLUS4 had a pin on cartridge for deselecting the internal RAM memory,and this make possible to add a 64k RAM expansion on cartridge without cuts/desolder anything....
I found a PLUS4 schematic that indicate "Z" pin as "NC/(RAMEN)"
but the final revision don't have it, and add RAMs is more complex or with more electronics.
"RAMEN" could be interesting because it can deselect the internal RAM only when a ram cartridge is plugged, this let the 16K internal RAM operative when no cartridge is plugged: cut traces o removing RAM chips imply that C16 can't work without the ram expansion on cartridge...
Why Commodore didn't connect "RAMEN" is a mystery....
I made a theoretical schematic that implement "RAMEN" on C16 without new chips, but only using a pair of unused gates....
If somebody is a owner of a64k expansion ram cartridge could try this little mod on C16 (see PIC), on cartridge the pin "Z" has to be connected with GND: if C16 is without cartridge it will work as a common C16, but plugging the cartridge it will be see 64k (without bus conflict...)
"
@Hypex,
Would you like to try the diagram in the figure and test it for yourself and for the forum?
----------------------------------------------------------------------------------------------------------------------------------
Currently the only memory expansions that do not require HW modifications have to be installed inside the C16 between the TED chip and its socket.
These expansions work because they intercept the RAM enabling signal (/CAS) and do not propagate it to the C16 internal RAM but divert it to the 64k bank RAM present into the expansion pcb.
You can find here 2 of this solutions:
https://plus4world.powweb.com/hardware/KC16BLAST
https://plus4world.powweb.com/hardware/64KxC16
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Posted By
MMS
on 2019-10-02
16:01:43
| Re: Can PLA assist in memory expansion?
@MCes
Thank you for your detailed answer.
I do not want to questionmark your knowledge, and I am far from that (1) no intention 2) not enough HW knowledge for that).
It annoys me that so many seemes to be sold, and we do not understand why and how.
-I had a friend who had this 64K expansion, but was rather expensive, so my parents --decided to build in for the 60% of the price (so in fact my C16 had cut traces). I pushed my parents because we played nice Karate King on that expanded C16. It worked.
-Pigmy just recently confirmed, that he had a C16 with a 64K external expansion, most of his 64K conversions made with that. It was unstable, because overloaded the C16 weak PSU, but otherwise it was OK, and he still owns that C16 (his conversions seems to be the confirmation to that).
-Also, an INTERNAL 1MB expansion used that RAMEN connector, but with +5V.
http://solder-synergy.de/plus4/hardware/makers/1meg.gif
Certainly I cannot be sure that all those 64K expansions listed in the HW section all REALLY worked.
The only (branded) 64K memory expansion was Kingsoft, it was INTERNAL and connected directly to the TED IC place, and certainly Kingsoft was approaching a really working and safe solution.
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Posted By
MCes
on 2019-10-02
16:39:06
| Re: Can PLA assist in memory expansion?
@MMS
I can't analyze what I haven't on my lab, but I can analyze the C16 system machine.... if a RAM access is requested then the TED will lower the /CAS line, the TED /CAS pin is DIRECTLY connected with the /CAS pins of internal RAM, so the internal RAM will occupy the bus ALWAYS (during any ram access).
This make physically and logically not possible any external expansion if the internal RAM is working: you have before to switch off the internal RAM!
Remember: you have to disable the internal RAM or the expansion can't work properly.
The kingsoft one work intercepting the /CAS line from TED (as described in my previous post).
The first link that I already posted is a modern reproduction of kingsoft one, the second link is the same expansion but done with static memory and the possibility to choose if use a 16K or 64K system.
The 1M expansion that you linked use the name "RAMEN" but it's not the signal on "Z" pin of the cartridge connector: same name, different signal....
The 1M expansion is a heavy modify that need a lot of glue-logic and is based on 8chips D-RAM based bank: immagine a +4 and remove the 64Kbit rams chips, replace them with 8chips 256Kbit (they are pin compatible..): they are the 1°256Kbyte bank.
Now solder another 8 chip over them ( piggyback ), all pins but not the /CAS pins that will be soldered together to create the line /CAS2 (the 2° 256k bank).
piggyback another bank and you will have /CAS3.
piggyback another bank and you will have /CAS4.
I don't remember how it could be mechanically compatible with the space inside a +4, but it shure that this system absorb a lot of current!
Only using modern static ram it will possible to make less invasive expansion (huge D-RAM need more than the 256 refresc cycles that TED can perform)
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Posted By
Hypex
on 2019-10-26
11:39:27
| Re: Can PLA assist in memory expansion?
@MCes
"Would you like to try the diagram in the figure and test it for yourself and for the forum?"
Sure, can you send me a PCB of the mod? 
It's unclear what parts are needed.
Does "Cut" mean I need to cut a trace? 
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Posted By
MCes
on 2019-10-27
06:29:22
| Re: Can PLA assist in memory expansion?
@Hypex
the diagram that I proposed use a pair of unused gates that are inside the chips on motherboard, the mod consist in cutting a trace, applying a pair of pull-up resistors and making some wired connections between points on motherboard.
The second half of the mod consist in connecting on "64k external ram expansion cartridge" the "Z" plug connection to ground.
Without cartridges the internal ram bank will work, but inserting the external RAM expansion the internal RAM bank will be disabled cancelling the bus conflict between internal and external ram, and external ram will work (I hope... )
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Posted By
Hypex
on 2019-10-29
10:02:22
| Re: Can PLA assist in memory expansion?
@MCes
Okay I can understand now. I could see what it was doing. Just was was unclear on where and what lines it did it with.
In that case, if it involves cutting traces, I would prefer not to. Sorry, letting the team down here I guess. 
Right now, there are solutions, internally, to deal with this cleanly. By putting a board between the TED or CPU and socket to intervene the lines. This could be another way to actvate a working expansion. But, if a board is going to be sandwiched between them, then it might as well contain memory. To which you have already addressed this.
Another way to tackle it, I thought, is for the expansion ram to leave the first 16K alone and just add another 48K. That would avoid the internal and external conflict, since both would be wired to work together. However, if the internal lines are programmed for 16K only, and to mirror the lower 16K range into all 16K ranges, then that won't work either.
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