Beeb Issue 7 with Rom mod...

[Multiwizard]

Hi,

anybody know how this (switched stacked roms) was intended to work?...

Thanks in advance...

[Ukwebb]

If that's a 74ls04 ic on top, I'm going to guess that they're two 8k ram chips to make a 16k ram bank ( times two)
But you need to see what designation is on the 28pi. Chips really to confirm

[vanekp]

It was common to use 2x8kb ram chips stacked and with an inverter chip to select the upper and lower half of ram, and the toggle switch will be to write protect them.

[Multiwizard]

If that's a 74ls04 ic on top, I'm going to guess that they're two 8k ram chips to make a 16k ram bank ( times two)
But you need to see what designation is on the 28pi. Chips really to confirm

See pic/attachment...

[vanekp]

MB8464-15L Description: Standard SRAM, 8KX8, 150ns, MIXMOS, CQCC32

[Ukwebb]

Yep, that's an 8k SRAM chip , the one underneath is probably the same, so yes, 16k SRAM 'module' for a 16k bank of sideways ram.. The wire from pin27 will be for the write protect through the switch and onto ic77 pin 8.

[maniacminer]

I guess the switch write protects one bank or the other? That's a bit of a pain.

[vanekp]

I guess the switch write protects one bank or the other? That's a bit of a pain.

surely not as its a double pole switch., they are both protected or they are not, could be an issue if you only want to protect one bank.

[Multiwizard]

So to remove it

I just have to de-solder the wire at pin 1 of IC72 (SN74LS245N)?...

[vanekp]

So to remove it

I just have to de-solder the wire at pin 1 of IC72 (SN74LS245N)?...

Yes.

[Multiwizard]

Thanks Peter...


After powering up I also noticed a difference,

see pics...

It looks it works in another modus... (bottom pic)

[maniacminer]

I guess the switch write protects one bank or the other? That's a bit of a pain.

surely not as its a double pole switch., they are both protected or they are not, could be an issue if you only want to protect one bank.

Looking at the wiring, it's either A or B is connected to the R/nW line.