Piggyback RAM - TMM41464AP??

By Colemu

Hero (544)

19-09-2012, 16:07

Is it possible to piggyback two TMM41464AP 32k memory chips?

Here is the link to the datasheet: https://vstuff.virginmedia.com/n/50-17/uid/Pkc6VAlpRP2NthQr/...

My MSX2 has two in already, but I do have another 2 spares. I was hoping I could piggyback the existing two....

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By luppie

Paladin (869)

19-09-2012, 16:53

This depends on your MSX. You can piggback these chips, but iir you have to connect pin 16 to it's own addressing signal.

By Colemu

Hero (544)

19-09-2012, 17:01

Would this signal come directly from the S1985 in my MSX2

Model is HB-F1XD by the way

By RetroTechie

Paragon (1563)

19-09-2012, 19:31

1) Can you run a mapper test to confirm that the 64K main RAM is actually a mapper? That's not always the case, for example in my Sony HB-F1 mkII there's 64K, but it's not a mapper (despite some Wikipedia references to the contrary).

2) IIRC the S1985 uses CAS-before-RAS refresh (which is nice), a side effect is that splitting DRAM control signals works through the DRAM's /WE and /OE signals.The /RAS and /CAS signals are common for all DRAMs, unlike in machines based on S3527 engine. See for example HB-F9P schematic for how that's wired. You'd need something like a 74HCT139 for this, and pick up MA16 signal from the S1985.

3) Personally I wouldn't bother with 128K but go directly for 512K as that's what the S1985 supports. And a 512K * 8 static RAM would be the easier way to wire that up (cheap too).

By Colemu

Hero (544)

19-09-2012, 20:36

OK, luckily I already have one mate. Found it in the cupboard (its AS6C4008).

What do I do with it?

Looked through you're old post, you said to connect to the top of the ROM.

Do I connect the corresponding PINS (i.e. A1 of ROM to A1 of RAM, D7 of ROM to D7 of RAM etc)?

Not a clue which PIN's are required from S1985, and what to connect them to

By RetroTechie

Paragon (1563)

19-09-2012, 20:51

Piggyback it on top of a ROM - so I'd first make sure you don't need to change those anymore (full 2+ ROM set present & working as intended?)

You'd bend pins 3 (A14), 22 (/CE), 24 (/OE), 29 (/WE) and 30 (A17) sideways (pins 1,2,31 and 32 not touching anything when going on top of a 28-pin EPROM, right?). Not 100% sure, but likely when you wire
Vcc (pin 32) to +5V
/CS to an unused /SLTSL signal (from S1985)
/WE to Z80 /WR line, and
/OE to Z80 /RD line
MA14 and MA15 (from S1985) each to a different remaining SRAM address line (pins 1,2,3,30 or 31)
Leftover address lines temporarily to GND or +5V (doesn't matter which)
the SRAM should work. Possibly the /OE pin can even be tied to GND, tying to /RD might be safer. (when either doesn't work 100%, try the other option)

Then run mapper test program(s) to confirm correct function as a 64K mapper.
When that works, proceed with MA16/17/18. For example one by one, run mapper test(s) again, hook up next address line. The order of address line connections doesn't matter.

By RetroTechie

Paragon (1563)

19-09-2012, 21:12

P.S. Run MSXMEM program to determine current memory layout & find out which slot(s?) 3-x would be available to put the mapper in. Schematic indicates slot 3-1 and 3-2 are unused so far?

If you'd want it to go in slot 3-3 like original mapper (I assume?), you'd have to disable the built in RAM first so you'd have a non-working machine in case the SRAM doesn't do its job. When you'd use a free slot, you'd temporarily have RAM in 2 slots which is fine. There are mapper test programs around that will test all mappers they find (and MSX2+ memory check will count multiple mappers too, so you'd see it on the boot screen).

By Colemu

Hero (544)

19-09-2012, 22:13

Original ram = slot 3.3

If I take out that ram, can I use some of their signals for the new RAM?

By RetroTechie

Paragon (1563)

19-09-2012, 22:31

Yeah the D0-D7, and slot 3-3 select signal, nothing else (slot select not connected to the DRAM chips directly btw). If you want to re-use DRAM multiplexed address signals, you'd need DRAM for the expansion. 4 pcs. 256K*4 bit (44256 type) in this case. More expensive, old chips, hard to obtain, much more difficult to construct a working circuit with, and I have no schematic / instructions / circuit board layout for use in the HB-F1XD.

You could use /SLT33 signal to control the SRAM, but not with the DRAMs in place. Remove/disable those, and you'd have a non-functional machine until you get the SRAM to work. Using a free slot is much easier, if the SRAM doesn't work 100% you could even run tests from BASIC to find out what's going on. Once the RAM expansion works, it's trivial to move the mapper from one slot to another.

By Colemu

Hero (544)

19-09-2012, 23:23

everything connected apart from -
CE - think this is the slot select (you said to unused signal on s1985 - where do I find this?)
A16
A17
A18
Where on the S1985 would these connect?

Update: after checking above - I gather that a16,17,18 connect to ground. I may have incorrectly connected A14,15 to the ROM (using a wire to get to the right PIN)

No idea where to get the Slot Select signal....

By Colemu

Hero (544)

20-09-2012, 01:20

I took a chance and removed the 2 x 32kb DRAM.

Used it's slot select signal as I couldn't find one on the S1985.

I haven't moved A14 & 15 as yet (they're still joined to the corresponding PIN's on the ROM.

It works, but still as 64kb

Do I now need to disconnect A14 & 15 and connect them to S1985 (if so, which PIN's on the S1985 do they need to connect to) ?

I'm just releaved that it's still working after removing the DRAM

Update: boots but nothing loads. Obviously adjustments required. If you can advise RT, it would be appreciated.