FS-A1 RAM upgrade

Champion (328)

17-01-2017, 21:16

Sorry if I'm rehashing old stuff, but I couldn't find an answer anywhere else. Is there some documentation on how to upgrade an FS-A1 to 128kB RAM? I found this discussion: https://www.msx.org/forum/msx-talk/hardware/fs-a1-ram-upgrade , but there doesn't seem to be an answer. Fair warning: my knowledge of hardware is fairly rudimentary

By Protoman

Resident (44)

20-01-2017, 14:27

Hello uberjack! I upgraded my FS-A1 with a 512K SRAM chip similar in method to the Sony HB-F1XV RAM upgrade instructions listed on the Tabajara Labs website. Both the Sony and this Panasonic have the same S-1985 MSX2 Engine which controls the RAM addressing. However, I had to add an additional timing delay circuit to enable the complete 512K of RAM at boot, otherwise the system will only see 256K. Let me know if you need assistance! :)

By l_oliveira

Hero (534)

26-01-2017, 20:39

Timing delay, no it's a circuit with a diode which inputs a value on a specific pin on the S1985 at reset, tells it to enable 512KB mapper. Otherwise the pin is used as an extra keyboard control line.

By Protoman

Resident (44)

25-02-2017, 13:48

Actually, the additional diode you're speaking of is used to connect the ground output from an existing transistor on the Sony HBF1-XV to the keyboard control pin of the S-1985 at boot. The diode is used for isolation because the transistor is connected to an existing circuit. I'm saying that I built a timing delay circuit which will ground the keyboard pin at boot up and release the ground signal after a set time, to allow the 512K to be addressed as well as let the keyboard function. This was required on the FS-A1 since I didn't find an existing transistor circuit like on the Sony HBF1-XV. I tested all the transistors on the FS-A1 and none were viable for activating the 512K RAM and then turning off to allow the keyboard to function.

By l_oliveira

Hero (534)

03-03-2017, 05:18

You don't need to do that timing delay thing. All you need to do is tie that diode to the S1985 RESET line as the RESET input clocks the latch which stores the chip configuration (that is described on the chip datasheet by the way). The chip finding the input line at logic low level (driven low by the presence of the diode) will put the chip into 512KB mapper mode. If the line is left unconnected an internal pullup resistor on the chip will make the latch store a "1".

The diode is there to prevent a current return flow from the control line into the reset pin, which would cause one to interfere with the other.

By daniel683104

Paragon (1263)

10-10-2019, 22:42

Bringing this up again...

So after getting the 256K RAM (SRAM upgrade) working how we get the 512K to work. You say to get a diode but what will be the conections on the S1985??

Thank you guys

By Sebbeug

Champion (405)

03-02-2020, 11:34

@daniel683104
Finally, what's the way to show 512k during boot ?
Thx