X68000 Expert RAM Expansion and HxC Menu

[SainT]

I've just got myself an X68000 Expert. After replacing the extremely dodgy PSU hack (an uncased Jap ATX PSU dangling from zip ties) with a nice 240v micro-atx unit, it's all powering up and running. Now I need an HxC to actually get some stuff running, though, as I have no 5.25" floppies.

So, first question! Has anyone written an HxC boot loader for the X68k? So you can select the floppy image from the machine without external LCD and buttons? If not, it's something I will do, but would be handy if someone else has already done it.

And secondly, I think this is just a stock Expert with 2MB RAM. I'd like to upgrade this to 4MB for full compatibility. Can someone tell me where the memory upgrade slot is located on this machine? I've had a look, but there's plenty of shielding to remove before you can really get to anything, so it'd be good to know where I'm actually looking. The only connector I can see from PCB shots on the web is one behind the expansion header, but this is completely shielded and would require a fair bit of disassembly to get to?

The other option is a memory expansion for the expansion port, which would do. I've got 2 spare, and I'm unlikely to do anything else with them.

I'd build a memory expansion board myself (have done some before for retro hardware), and if anyone is interested I'd build a small batch for sale. Unlikely to be too expensive.

[H68k]

I don't really know of a good reliable source to get 5.25 inch floppy disks from.. other than ebay, or local charity shops that will deal with old computers.

With the HxC, it should just be a case of connect it up and go. as the X68k uses an "off the shelf" floppy disk controller chip. the 5.25 inch FDD's are pretty close to any other standard 5.25 inch drive. but they have a few additions such as: motorized eject, status LEDs and some other things that are controlled via the propitiatory IOSC chip on the IO board and not the FFD controller, so you will not have any of those features simulated on the HxC.

I'm not aware of anyone writing a controller program for the HxC on the X68k though.     


The RAM expansion header is at the top of the motherboard, close to the expansion board bay shielding, it was used by the official memory upgrades sold by sharp. you can also use 3rd party ram expansion boards (such as, the XSIMM) that slot into ether of the 2 expansion board slots to extend the system memory. the header under the RAM header is used for processor upgrade boards (sharp didn't sell any themselves, but 3rd party processor upgrades where available)

I don't know if ether of these headers where meant to be covered, they where not in my X68k (I think mine got picked clean of all it's good stuff years ago. before I got it) so this might explain why the shielding covers are missing.
   
You might also want to consider getting the I/O board (in the base) and the motherboard re-capped at some point, if you plan in seriously using the machine, doing so would greatly improve the reliability of it in the long run. nearly 20+ year old capacitors can cause all kinds of little issues and problems as well.

[SainT]

Is this the memory expansion connector?

Not sure how standard that connector is... 

There's a shit-load of shielding over the top of this connector in my machine. May be better off just going the expansion port route...

[H68k]

That has to be it, seams you have a different connector design on the expert model.

The RAM expansion header on my XVI model seams to to be a more condensed version, slightly lower down, but roughly in the same location.

It's probably a connector of Sharp's own design, or another rare and obscure connector, these machines have a few of them else ware to.


Your probably better off getting a 3rd party ram expansion board then. A variety of them frequently appear now and then. some of them have a fixed amount of memory chips fitted and others allow you to use SIMMs, but you'll have set jumpers or DIP switches to the amount of memory you've installed on it.

[SainT]

I'll build a RAM expansion board, I was just wondering the best connector to use. Looks like the expansion port is the easiest to access, and it's just a card edge, so is cheap too.

I like to try and support niche communities with new hardware where I can because I find it interesting designing hardware. And I've always wanted an X68k, of course, so it's nice to help others with the same passion.

By the looks of it the main memory is mapped from 0x000000 -> 0xC00000, and mine will have RAM upto 0x200000 being a 2MB model. So a simple PAL for address decoding and some jumpers to allow you to specify the base address should do the job. I tend to use SRAM rather than DRAM / SIMMs for memory expansions, just because it makes things really simple.

If anyone is interested, an initial guess is about £30-40 for a 2MB expansion. There will be a fair bit of room on the PCB, so I could leave room for people to add more RAM chips if they want to make it up to a 4MB expansion or possibly more.

I probably wouldn't do a SIMM board just down to the price of SIMM connectors. It's almost cheaper to just buy RAM.

[H68k]

Ahh... not to dissimilar to what other X68k owners have done in the past, at some point. someone company used to sell blank breadboard like PCBs that you could use to make your own home brew expansion boards for the X68k (home made network adaptors, ram expansion, custom interfaces of some kind etc) from time to time. a variety of these useful to somewhat mad cap.. if down right odd creations appear on the auctions.


There could certainly be a market.. if your going to make this kind of stuff, sell it for a reasonable price, and it'll sell well.. I'm sure.

As it seams your a hardware tinkerer, would it be possible to and a make a clone of the official sharp midi board, using an FPGA?

[SainT]

Yeah, certainly possible. Any idea what chip it uses? The IP core may have already been written, there's so much stuff on opencores. It tends to get a bit more pricey when you get into FPGA's, but I don't know how much midi boards sell for, so it may be worth while...

Edit: I see its the ym3802. Not something that's in opencores, but you could emulate this with a small CPLD acting as a bridge and a microcontroller to act as the midi controller. You could probably even go as far as creating a sound card which looks like the standard sharp midi controller with a microcontroller.

[H68k]

Yes, there really isn't much else to the midi board. all it is, is a bunch of logic chips and an "off-the-shelf" YM3802 midi controller IC.. (it would appear the same chip was also used in a MIDI addon board for the MSX series computers and some other yamaha electronic instruments for midi i/o)

Perhaps the source code to XM6 may give further clues as to be able to make a working clone on a micro controller? Real midi boards (ether the official ones or sacom clones) appear from time to time on the auctions. and can go from anything from reasonably priced... to silly money, depending on demand.

It would be grate to have a supply of functional clones for this kind of hardware. (ethernet boards, midi boards, ram expansion boards etc etc)

I emailed a gentleman a couple of months ago, about having some of this stuff made, in his replay it said. even though he could make it. it would not be worth his time for the amount of money he'd make from it though.