Piggyback RAM - TMM41464AP??

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Por RetroTechie

Paragon (1563)

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21-09-2012, 00:59

So you can run 256K software now on internal RAM? Nice! Big smile

Seems that MA18/KDBIR double function is interfering... To fix, you'd need a small circuit that holds S1985 /X5 pin (a keyboard return signal) low until reset line has gone from active to non-active, and doesn't interfere with /X5 after that. I don't have a ready-made / dead simple circuit for that, but shouldn't be too hard to figure something out.

To other people following this thread: suggestions are welcome... Wink

For an example: check Sony HB-G900AP service manual page 75 on how this machine does it on /X1 and /X3 signals. I suspect something similar could be done with 2 or 3 suitably connected components (resistor + capacitor + diode or something).

Por Colemu

Hero (535)

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21-09-2012, 00:51

I'm well pleased that I was able to successfully solder to them TINY PINS!! Tongue

Yeh, all tests pass at 256kb mate, well pleased Big smile

Just need a bit more assistance with the MA18 problem now... Shame to waste half the capacity oO

Por msxholder

Champion (382)

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21-09-2012, 00:57

You should find R.van Gans epifiet
He made ma18 work and KBdir. !

Por Colemu

Hero (535)

Imagen del Colemu

21-09-2012, 01:15

Looking through the datasheet posted by sd_snatcher.

Pin X5
0 Mapper address output to MA18/KBDIR PIN
1 Keyboard bus direction output to MA18/KBDIR PIN

Sure this will mean something to someone Tongue

Por RetroTechie

Paragon (1563)

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21-09-2012, 02:59

Wait..!! HB-F1XD already has a suitable circuit, and it appears very easy to extend.

Look in the service manual on the page (21) that has joystick and printer ports wiring, above (or on the right, depending on view) black marking "IC11". There's a transistor (Q5? looks a bit blurred) with base connected to RESET signal (S1985 pin 85, active-high). There's 2 diodes (D13 and D14) connected to that transistor, and the other side of each diode to a /X.. keyboard return signal. When RESET is active, the transistor pulls that common connection low, and (through those diodes) the corresponding /X.. signals. When RESET goes inactive, the transistor 'releases' those signals. The transistor + diode adds a small delay, so that /X.. line is seen as 0 when S1985 RESET signal return to normal (inactive).

So all you'd have to do is add a 3rd diode between that common point, and /X5 keyboard return signal. Optimal would be a small Schottky diode like BAT42/43/46/82/85, but possibly a plain 1N4148 works too. Connect the side that's marked with a ring to the common point / transistor pin, the non-marked side to /X5 pin. Just be careful not to solder directly to that transistor, these parts overheat easily (and diodes too, but less quickly). Solder to circuit board traces there (or diode D13/D14 / resistor array RB4). When machine works fine as before (check whether X, P, H and 5 keys work normal!), re-connect MA18 to the SRAM.

Btw: elsewhere in the service manual I see that common point would be that transistor's center pin. Don't take my word for that though, check it on the board!

Good luck!

Por Colemu

Hero (535)

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21-09-2012, 12:33

Great news, I haven't got that service manual you mention though. However, I will check the board and see if I can work out which transistor you are referring to. Ordered some BAT42's in the meantime Smile

Update: Found Q5 transistor (centre PIN 'C') and a really easy place to solder to X5, do I just need to add the diode BAT42 between these points? Question

BTW: I see that the other diodes you mention also are connected to this transistors centre PIN

Por RetroTechie

Paragon (1563)

Imagen del RetroTechie

21-09-2012, 14:22

Try this. You're an idiot for trying all this stuff without the service manual. ;) I assumed you had it all along.

Anyway, there's that common point where transistor pin & ring-marked side of D13 / D14 connect. That's where you connect ring-marked side of a 3rd diode, connect non-marked side of that to /X5 keyboard signal.

Por Colemu

Hero (535)

Imagen del Colemu

21-09-2012, 14:45

probably right about me being an idiot, I'm just not frightened of electronics and used your instructions to learn as I went along.

Managed so far, but that service manual will help Tongue

Por Colemu

Hero (535)

Imagen del Colemu

21-09-2012, 14:54

RetroTechie wrote:

Try this.
Anyway, there's that common point where transistor pin & ring-marked side of D13 / D14 connect. That's where you connect ring-marked side of a 3rd diode, connect non-marked side of that to /X5 keyboard signal.

Thats the place I thought you mean't, at least I can follow instruction well ;)

Por Colemu

Hero (535)

Imagen del Colemu

22-09-2012, 13:06

Received the diode this morning. Its works!!

You're a genious man!! Big smile

MSX2 now running with V9958, BASIC 3 and 512kb of RAM..... Tongue

I will post photos of the RAM wiring soon for anyone else wishing to try it.
Thanks to all who held my hand along the way, particularly RetroTechie.

Cheers my friend Wink

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