Techie RAM question.

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Womble
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Techie RAM question.

Postby Womble » Sat Nov 15, 2008 11:39 am

Hiya

Again not Atari related but techie folk hide here and I am after a second opinion on something. Recently picked up a Yamaha CX5M "music computer" as they were originally called, its an MSX machine at heart with the guts of Yamahas DX7 synth built in. Aside from the built in ROM software I have not managed to get a single thing to load on it. Popped the lid today and I may have found out why.

It originally came with 48K of RAM, the 1st bank is eight 2KB chips, the second bank is four 8KB chips. Both banks are original, but someone has been fiddling. They have cut pin 1 on all 4 of the 8KB chips and wired them together, this they have connected to a 2 way switch on the rear of the machine, flipping the switch will take pin 1on all 4 chips to either logic low or logic high, there is a current limiting resistor in the right place on the switch thankfully. Pin 1 happens to be the Output Enable (called G on the datasheet for some reason).

Datasheet:-
http://www.datasheetcatalog.org/datasheets/2300/500357_DS.pdf


Image

My question is - why on earth would anyone do this, surely OE needs to be controlled by the system itself, the ability to manually turn this on and off seems a rather pointless one. A bit like wiring a toggle switch to your dashboard so only you can control when the spark plugs fire. The mod has been very neatly done, and the current limiting resistor suggests it was done by someone who knew what they were doing, but what use is it?

There is also a reset button mounted on the front of the system, so am beginning to suspect this was used for some hardware development at some point, some project where the ability to precisely control most of the RAM would have been some use, presumably some cartridge or peripheral development scheme. The unit has quite a low serial, 1911 in fact, then again am not sure these ever took the world by storm so maybe that's not as low as it would be where it from an ST or a speccy.

Can anyone shed any light, agree/disagree with my conclusions about what this mod does?

As it stands I am pretty sure I have a 16KB system with 32KB sitting there comatose. It was neatly done and is very easily undone, am just a bit baffled as to the reasoning behind it.
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Re: Techie RAM question.

Postby joska » Sat Nov 15, 2008 12:06 pm

Womble wrote:It originally came with 48K of RAM, the 1st bank is eight 2KB chips, the second bank is four 8KB chips. Both banks are original, but someone has been fiddling. They have cut pin 1 on all 4 of the 8KB chips and wired them together, this they have connected to a 2 way switch on the rear of the machine, flipping the switch will take pin 1on all 4 chips to either logic low or logic high, there is a current limiting resistor in the right place on the switch thankfully. Pin 1 happens to be the Output Enable (called G on the datasheet for some reason).

[...]

As it stands I am pretty sure I have a 16KB system with 32KB sitting there comatose. It was neatly done and is very easily undone, am just a bit baffled as to the reasoning behind it.


I'm quite sure that you have a 0Kb system with 32Kb disabled :) The 16Kb is not a regular memory bank, it's the video RAM. The CX only have 32Kb ordinary RAM.

I have no idea why it's modified, it looks quite weird to me. As you say, it appears that the RAM wouldn't work, but then you really shouldn't be able to boot it at all. What happens when you switch it on?

The CX is quite rare, so I think it's worth fixing it. At the time it was the only home computer with MIDI, and the built-in synth was advanced for it's time. I remember seeing one in a local shop in 1984, the sound demo was quite impressive.
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Re: Techie RAM question.

Postby Womble » Sat Nov 15, 2008 12:34 pm

Actually I think you are right, oddly enough I have a collection of magazines from the early 80s in binders that show many machines stripped down and it states that this machine has 48KB, but if 16K is video RAM then that would make sense as I have seen other sources say this machine only had 32KB. Certainly only 28KB is available to BASIC, losing only 4KB to the system makes more sense than looking nearly 20KB.

I have also taken a closer look at the board, pin 1 on all chips was tied straight to the ground plane originally, and I think I took the datasheet too literally. What they call G and refer to as Output Enable is more like Chip Enable (CE) in more modern parlance, the fact "G" appears in the time slice diagrams I think is more to do with chips being enabled or disabled for the sake of paging ram in and out rather than its effect on the read/write cycle.

When the switch is in one position (presumably connecting CE to 5V) the machine will not boot so in that state I would assume the machine has zero RAM.
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Re: Techie RAM question.

Postby joska » Sat Nov 15, 2008 1:45 pm

Womble wrote:Actually I think you are right, oddly enough I have a collection of magazines from the early 80s in binders that show many machines stripped down and it states that this machine has 48KB, but if 16K is video RAM then that would make sense as I have seen other sources say this machine only had 32KB. Certainly only 28KB is available to BASIC, losing only 4KB to the system makes more sense than looking nearly 20KB.


I have a small collection of MSX1 machines (all 64Kb), and they were all advertised as "80Kb" :-)

Womble wrote:I have also taken a closer look at the board, pin 1 on all chips was tied straight to the ground plane originally, and I think I took the datasheet too literally. What they call G and refer to as Output Enable is more like Chip Enable (CE) in more modern parlance, the fact "G" appears in the time slice diagrams I think is more to do with chips being enabled or disabled for the sake of paging ram in and out rather than its effect on the read/write cycle.

When the switch is in one position (presumably connecting CE to 5V) the machine will not boot so in that state I would assume the machine has zero RAM.


Ok, then this switch disables the original RAM. Perhaps because one of the previous owners had a RAM-pack in one of the expansion ports?
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Re: Techie RAM question.

Postby Womble » Sun Nov 16, 2008 10:42 pm

Am torn between trying to upgrade the internal RAM to 64K ( I have the right chips lying around ), or trying to track down an expansion card. Trouble is the expansion cards seem to be rarer than rocking horse poo.

Would you know if a memory expansion card would be useable as RAM for games? I have read that RAM sat on the rear slot or the cart slot will appear as being in a different bank so games may try to directly address memory locations that are not present.

Have managed to find the schematic/service manual for this beast so I should be able to work out how to add a second 32KB to the main memory. Time to get my head around RAM wiring and addressing I think. Actually have found out that the custom memory controller scans all 4 slots looking for RAM, it chooses the slot with the largest memory and connects the system bus to that slot, in the case of this machine slot 0 is the onboard, slot 1 is the rear edge connector, slot 2 is the cart slot and slot 3 is the synth unit. Will be simpler to build a 64K sram chip onto the rear slot and let the system pick that slot at boot up. :D
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Re: Techie RAM question.

Postby joska » Mon Nov 17, 2008 10:09 am

Womble wrote:Am torn between trying to upgrade the internal RAM to 64K ( I have the right chips lying around ), or trying to track down an expansion card. Trouble is the expansion cards seem to be rarer than rocking horse poo.


I've never seen one in real life. I left the MSX-world around the time MSX2 came, and all my computers always had 64Kb so I never needed an expansion.

Womble wrote:Would you know if a memory expansion card would be useable as RAM for games? I have read that RAM sat on the rear slot or the cart slot will appear as being in a different bank so games may try to directly address memory locations that are not present.


Quite possible. These machines does not have a proper memory management system, so it's up to the individual game to properly detect any expansions.

Womble wrote:Have managed to find the schematic/service manual for this beast so I should be able to work out how to add a second 32KB to the main memory. Time to get my head around RAM wiring and addressing I think. Actually have found out that the custom memory controller scans all 4 slots looking for RAM, it chooses the slot with the largest memory and connects the system bus to that slot, in the case of this machine slot 0 is the onboard, slot 1 is the rear edge connector, slot 2 is the cart slot and slot 3 is the synth unit. Will be simpler to build a 64K sram chip onto the rear slot and let the system pick that slot at boot up. :D


I would recommend leaving slot 1 and 2 free in case you want to play cartridge based games (like the excellent Gradius/Nemesis games). You don't *need* both slots, but some Konami games has extra features if you combine them. E.g. with Salamander in the cart slot and Nemesis II in the rear slot you'll have a couple of extra planets to explore. Same with the Metal Gear games.

Take a look at msx.org, there's a lot of knowledge in the forums there.
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Re: Techie RAM question.

Postby Womble » Mon Nov 17, 2008 10:03 pm

joska wrote:Quite possible. These machines does not have a proper memory management system, so it's up to the individual game to properly detect any expansions.


I dont know how it compares to other MSX machines but it does seem to have a degree of intelligence, there is a custom Yamaha memory slot controller chip that scans each page (ie each slot) to look for RAM, any ROMs have a 2 byte ID at 0000H to identify themselves as a ROM, the largest amount of RAM it finds gets paged in when the machine boots. The Yamaha machine differs from other MSX machines in that slot 3 is lost to the syth module, leaving only 1 and 2 free. I can easily build an upgrade box that attaches to slot 2 (easier to get the connector for the backplane than to make a PCB with dual sided fingers to fit the cart slot). The machine will apparently find this 64KB, decide its larger than the RAM in slot 0 and use it to boot.[/quote]

I would recommend leaving slot 1 and 2 free in case you want to play cartridge based games (like the excellent Gradius/Nemesis games). You don't *need* both slots, but some Konami games has extra features if you combine them. E.g. with Salamander in the cart slot and Nemesis II in the rear slot you'll have a couple of extra planets to explore. Same with the Metal Gear games.


Theres no other way to do it really, as I cant use slot 3 and slot 0's address range is full from the ROM and the RAM I can only use slots 1 or 2 on this machine.

Take a look at msx.org, there's a lot of knowledge in the forums there.


I get the impression that the heyday of MSX love is over, the majority of the useful posts on that site point to URLs that are long dead, its a shame as you get an idea of the information that used to exist. Its a good example of why a system specific forum should host their own copies of those files, it saves the whole knowledge base from unravelling over time.
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Re: Techie RAM question.

Postby papa_november » Tue Nov 18, 2008 7:26 pm

I've heard that the CX5M is a sort of MSX1.5, as it has the ROM and RAM of an MSX1 but the VDP of an MSX2. Is this correct?

Another oddity is that the CX5M is apparently the ONLY model of MSX ever to be released in North America.

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Re: Techie RAM question.

Postby joska » Tue Nov 18, 2008 9:14 pm

papa_november wrote:I've heard that the CX5M is a sort of MSX1.5, as it has the ROM and RAM of an MSX1 but the VDP of an MSX2. Is this correct?


I don't think this is correct. IIRC the only MSX "1.5" is the Spectravideo SVI 738. The CX5 was released in 1983, which I think is too early for the VDP2.
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Re: Techie RAM question.

Postby joska » Tue Nov 18, 2008 9:15 pm

Womble wrote:I get the impression that the heyday of MSX love is over, the majority of the useful posts on that site point to URLs that are long dead, its a shame as you get an idea of the information that used to exist. Its a good example of why a system specific forum should host their own copies of those files, it saves the whole knowledge base from unravelling over time.


Take a look at http://www.msx.org/forumtopic5786.html. This thread discuss how to expand the CX5 to 64Kb.
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Re: Techie RAM question.

Postby Womble » Sun Dec 07, 2008 11:55 pm

Lots of useful info at that site, I didn't want to modify the motherboard but I did find the service manual which explains how the memory management chip works on boot up, that info plus the rear edge connector pinouts allowed me to build this puppy!

http://womblesretrorepairshack.blogspot.com/2008/12/msx-ram-upgrade.html

It even works too :) :) :)
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Re: Techie RAM question.

Postby papa_november » Thu Jan 01, 2009 8:59 pm

joska wrote:
papa_november wrote:I've heard that the CX5M is a sort of MSX1.5, as it has the ROM and RAM of an MSX1 but the VDP of an MSX2. Is this correct?


I don't think this is correct. IIRC the only MSX "1.5" is the Spectravideo SVI 738. The CX5 was released in 1983, which I think is too early for the VDP2.


Turns out I was thinking of the CX5M-II. Oops.


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