Lowering SIMM MB amount? \ VOGONS

Lowering SIMM MB amount?

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First post, by relo999

Posted on 2024-07-20, 13:21

relo999 Offline

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Joined: 2017-06-24, 22:46
Location: Netherlands

Recently got a FM Towns and installed added RAM but that slows down the system. From what I've read the optimal amount of RAM needed in the FM Towns for games is 8MB. My FM Towns has 4MB soldered on the motherboard. But the only SIMM sticks I have that work on the FM Towns are 16MB and 32MB.

So my question is: Is there some way to lower the amount of RAM on a SIMM? Maybe desoldering some of the chips? I'd rather ask before desoldering 6 of the 8 chips on a given 16MB stick.

Reply 1 of 2, by mkarcher

Posted on 2024-07-20, 14:14

mkarcher Offline

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The FM Towns uses a different scheme to indicate module size than most other systems. See https://www.pixsoriginadventures.co.uk/fm-tow … memory-upgrade/ . Maybe you can make a 4MB module compatible by adjusting the solder bridges / 0 ohm resistor for size indication. Maybe you can manage the FM Towns to treat your 16MB module as 4MB module by setting the size indication appropriately. To convert a 16MB SIMM (4 mega-words of 32 bits each) into a 4MB SIMM (1 mega-word of 32 bits each), you can not just remove chips. Each of the 8 chips on a 16MB SIMM has 4 mega-words of 4 bits, so after removing 6 chips, you have an 8-bit memory module of 4 mega-words (actually mega-bytes, as the word size has been reduced to 8 bits) instead of a 32-bit memory module of 1 mega-word. Instead, you would need to disconnect the address pin A10 from the contacts (e.g. paint it with nail polish), and pull it to either ground or 5V, preferably through a resistor, on the module.

Reply 2 of 2, by relo999

Posted on 2024-07-20, 15:02

relo999 Offline

Rank Newbie

Rank: Newbie
Posts: 59
Joined: 2017-06-24, 22:46
Location: Netherlands

mkarcher wrote on 2024-07-20, 14:14:

The FM Towns uses a different scheme to indicate module size than most other systems. See https://www.pixsoriginadventures.co.uk/fm-tow … memory-upgrade/ . Maybe you can make a 4MB module compatible by adjusting the solder bridges / 0 ohm resistor for size indication. Maybe you can manage the FM Towns to treat your 16MB module as 4MB module by setting the size indication appropriately. To convert a 16MB SIMM (4 mega-words of 32 bits each) into a 4MB SIMM (1 mega-word of 32 bits each), you can not just remove chips. Each of the 8 chips on a 16MB SIMM has 4 mega-words of 4 bits, so after removing 6 chips, you have an 8-bit memory module of 4 mega-words (actually mega-bytes, as the word size has been reduced to 8 bits) instead of a 32-bit memory module of 1 mega-word. Instead, you would need to disconnect the address pin A10 from the contacts (e.g. paint it with nail polish), and pull it to either ground or 5V, preferably through a resistor, on the module.

Thanks!

Jumping pin 69 and 70 worked for size indication. Now the 16MB stick is recognized as 4MB.

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